Cytology Pp Histo 1

• ORGANIZATION OF CELLS☞ NUCLEUS: Genetic material, Nuclear envelope☞ CYTOPLASM: Limited by cell membrane

• Contains three structural components– Organelles – metabolocally active– Cytoskeleton – structural framework (fibrillar elements)– Inclusions – cell products, metabolites (metabolically inactive)

• CYTOSOL – fluid component (cytoplasmic matrix, hyaloplasm, intracellular fluid)

• Exhibits dynamic functional interactions among certain organelles.

– Uptake & release of materials by the cell– Protein synthesis– Intracellular digestion

➲ Structural & functional units of life (and of disease processes) in all tissues, organs & organ systems.

Two basic cell types 1. Prokaryotes

✹ lack nuclear envelope, histones & memb. organelles

2. Eukaryotes✹ multicellular organisms

with nucleus

The major intracellular compartments of an animal cell. The cytosol (gray), endoplasmic reticulum, Golgi apparatus, nucleus, mitochondrion, endosome, lysosome, and peroxisome are distinct compartments isolated from the rest of the cell by at least one selectively permeable membrane.

✪ nucleus o ✹ nuclear membrane o ✹ euchromatin o ✹ heterochromatin o ✹ nucleolus

  ✪ plasma membrane / cell surface

o ✹ endocytotic vesicles o ✹ coated pits o ✹ glycocalyx o ✹ cell junctions

➽ tight (occluding) junctions ➽ adhering junctions ➽ gap (communicating) junctions

o ✹ microvilli o ✹ cilia (inc. primary cilium)

  ✪ cytosol ✪ inclusions

o ✹ pigment granuleso ✹ lipido ✹ glycogen

✪ membranous organelles o ✹ mitochondria o ✹ rough endoplasmic reticulum o ✹ smooth endoplasmic reticulum o ✹ Golgi apparatus o ✹ vesicles

➽ transport vesicles ➽ secretory vesicles ➽ lysosomes, phagosomes ➽ peroxisomes (microbodies)

  ✪ non-membranous organelles

o ✹ ribosomes o ✹ cytoskeleton

➽ microfilaments intermediate filaments actin filaments

➽ microtubules ➽ mitotic spindle

o ✹ centriole / basal body

✪ movement o ✹ internal transport o ✹ contractility o ✹ amoeboid locomotion o ✹ ciliary action

✪ absorption / secretion o ✹ phagocytosis o ✹ pinocytosis o ✹ exocytosis o ✹ transport acrosso membranes o ✹ membrane recycling

✪ cell-cell attachment ✪ sensation and transduction

✪ energy production ✪ energy storage ✪ biosynthesis

o ✹ proteins o ✹ lipids o ✹ carbohydrates o ✹ nucleic acids

✪ intracellular digestion ✪ structural integrity ✪ cell division

o ✹ mitosis o ✹ meiosis

✪ apoptosis

Other cellular processesActive transport and Passive transport

- Movement of molecules into and out of cells. Autophagy

- The process whereby cells "eat" their own internal components or microbial invaders.

Adhesion - Holding together cells and tissues. Cell division

- a eukaryotic cell process resulting in the formationof daughter cells; there are two major types, that of mitosis and meiosis, asexual reproduction and sexual reproduction, respectively.

Cell movement: Chemotaxis, Contraction, cilia and flagella. Cell signaling

- Regulation of cell behavior by signals from outside. DNA repair and Cell death Metabolism: Glycolysis, respiration, Photosynthesis Transcription and mRNA splicing - gene expression.

. Relative Volumes Occupied by the Major Intracellular Compartments in a Liver Cell (Hepatocyte)

© 2002 by Bruce Alberts, Alexander Johnson, Julian Lewis,

INTRACELLULAR COMPARTMENT

PERCENTAGE OF TOTAL

CELL VOLUME

Cytosol 54

Mitochondria 22

Rough ER cisternae 9

Smooth ER cisternae plus Golgi cisternae

6

Nucleus 6

Peroxisomes 1

Lysosome 1

Endosome 1

Relative Amounts of Membrane Types in Two Kinds of Eucaryotic Cells

* These two cells are of very different sizes: the average hepatocyte has a volume of about 5000 μm3 compared with 1000 μm3 for the pancreatic exocrine cell. Total cell membrane areas are estimated at about 110,000 μm2 and 13,000 μm2, respectively.

MEMBRANE TYPE PERCENTAGE OF TOTAL CELL MEMBRANE

LIVER HEPATOCYTE* PANCREATIC EXOCRINE CELL

Plasma membrane 2 5

Rough ER membrane 35 60

Smooth ER membrane 16 <1

Golgi apparatus membrane 7 10

Mitochondria

Outer membrane 7 4

Inner membrane 32 17

Nucleus

Inner membrane 0.2 0.7

Secretory vesicle membrane not determined 3

Lysosome membrane 0.4 not determined

Peroxisome membrane 0.4 not determined

Endosome membrane 0.4 not determined

I). Cell Structure A). Cytoplasm: 

All of the cellular material between the plasma membrane & the nucleus i). cytosol:  ii). cytoplasmic organelles:   iii). Inclusions B). Mitochondria ATP production 

C). Ribosomes i). Small granules that are made of proteins and RNA called ribosomal RNA or rRNA. ii). sites of protein synthe

D). Endoplasmic Reticulum (er) i). rough er  (RER) ii). smooth er (SER) 

E). Golgi Apparatus i).  modify, concentrate, and package proteins. ii). Transport vesicles bud of and move to the plasma membrane and discharge the contents.

F). Lysosomes Spherical membranous bags containing digestive enzymes

G). Cytoskeleton give shape and general movement to the cell

i). Microtubules  ii). Microfilaments 

iii). Intermediate Fibers                    

H). Centrosomes and Centrioles During mitosis the centrosomes line up on the opposite ends of the cell and organize the tubules

that pull the chromosomes apart.

I). Cilia & Flagellai). Cilia 

ii). Flagella                     J). Nucleus

The nucleus contains the genetic material i). Nuclear Envelope            It regulates the entry and exit of large particles ii). Nucleoli            This is the site of ribosome production iii). Chromatin Chromatin:  DNA & histone proteins which package the DNA  Cell divides & the chromatin coils into chromosomes

A: The fluid mosaic model of membrane structure. The membrane consists of a phospholipid double layer with proteins inserted in it (integral proteins) or bound to the cytoplasmic surface (peripheral proteins). Integral membrane proteins are firmly embedded in the lipid layers. Some of these proteins completely span the bilayer and are called transmembrane proteins, whereas others are embedded in either the outer or inner leaflet of the lipid bilayer. The dotted line in the integral membrane protein is the region where hydrophobic amino acids interact with the hydrophobic portions of the membrane. Many of

the proteins and lipids have externally exposed oligosaccharide chains. B: Membrane cleavage occurs when a cell is frozen and fractured (cryofracture). Most of the membrane particles (1) are proteins or aggregates of proteins that remain attached to the half of the membrane adjacent to the cytoplasm (P, or protoplasmic, face of the membrane). Fewer particles are found attached to the outer half of the membrane (E, or extracellular, face). For every protein particle that bulges on one surface, a corresponding depression (2) appears in the opposite surface. Membrane splitting occurs along the line of weakness formed by the fatty acid tails of membrane phospholipids, since only weak hydrophobic interactions bind the halves of the membrane along this line. (Modified and reproduced, with permission, from Krstíc RV: Ultrastructure of the Mammalian Cell. Springer-Verlag, 1979.)

Three views of a cell membrane. (A) An electron micrograph of a plasma membrane (of a human red blood cell) seen in cross section. (B and C) These drawings show two-dimensional and three-dimensional views of a cell membrane. (A, courtesy of Daniel S. Friend.)

MEMBRANE PROTEINS1. Integral proteins 2. Peripheral proteins

. dissolved in lipid bilayer, amphipathic . do not extend into lipid bilayer

. transmemb. proteins (receptors/transport) . inner leaflet

. are often glycoproteins . bond to polar groups or integral

. attached to P-face, ext. surface of inner memb. proteins. leaflet, seldom in E-face. . Function part of cytoskeleton or

intracellular secondary messenger

Schematic drawing of the molecular structure of the plasma membrane. Note the one-pass and multipass transmembrane proteins. The drawing shows a peripheral protein in the external face of the membrane, but the proteins are present mainly in the cytoplasmic face.

The ultrastructure and molecular organization (right) of the cell membrane. The dark lines at the left represent the two dense layers observed in the electron microscope; these are caused by the deposit of osmium in the hydrophilic portions of the phospholipid molecules.

. Composed of lipid bilayer & associated proteins

. Envelops the cell

. Semipermeable memb

. Sensory device

. Inner & outer leaflets

. Trilaminar structure

Experiment demonstrating the fluid nature of proteins within the cell membrane. The plasmalemma is shown as 2 parallel lines (representing the lipid portion) in which proteins are embedded. In this experiment, 2 types of cells derived from tissue cultures (one with a fluorescent marker [right] and one without) are fused (A –>B) through the action of the Sendai virus. Minutes after the fusion of the membranes, the fluorescent marker of the labeled cell spreads to the entire surface of the fused cells (C). However, in many cells, most transmembrane proteins are stabilized in place by anchoring to the cytoskeleton.

LIPID BILAYER. Freely permeable to small nonpolar lipid-soluble molecules.. Impermeable to charged ions.

➲ Molecular structure 1. Phospholipids – amphipathic polar head (hydrophilic) & 2 nonpolar

fatty acyl tails (hydrophobic) 2. Glycolipids . in outer leaflet only . form glycocalyx 3. Cholesterol (2% of plasmalemma lipids) . in both leaflets . structural integrity ➲ Fluidity ✹ exocytosis, endocytosis, memb trafficking

& memb biogenesis ✹ ↑ rise in temp & unsaturation of hydrocarbon

(fatty acyl) tails ✹↓ by ↑ in memb’s cholesterol content.

The proteins of the plasmalemma are synthesized in the RER and then transported in vesicles to the Golgi complex, where they may be modified and transferred to the cell membrane. This example shows the synthesis and transport of a glycoprotein, which is an integral protein of the membrane.

✹ Phospholipids ➽ Phosphatidylcholine

➽ Sphingomyeline ➽ Phosphatidylserine

➽ Phosphatidylethanol amine ✹ Glycolipids

➽ Galactocerebroside ➽ Gangliosides

Internalization of low-density lipoproteins (LDL) is important to keep the concentration of LDL in body fluids low. LDL, which is rich in cholesterol, binds with high affinity to its receptors in the cell membranes. This binding activates the formation of pinocytotic vesicles from coated pits. The vesicles soon lose their coating, which is returned to the inner surface of the plasmalemma: the uncoated vesicles fuse with endosomes. In the next step, the LDL is transferred to lysosomes for digestion and separation of their components to be utilized by the cell.

Cells respond to chemical signals according to the library of receptors they have. In this schematic representation, 3 cells appear with different receptors, and the extracellular environment contains several ligands which will interact with the appropriate receptors. Considering that the extracellular environment contains a multitude of molecules, it is important that ligands and the respective receptors exhibit complementary morphology and great affinity.

Diagram illustrating how G proteins switch effectors on and off. (Modified and reprinted, with permission, from Linder M, Gilman AG: G proteins. Sci Am 1992;267:56.)

Functional characteristics of membrane proteins 1. Ratio of lipid to protein – 1:1 (by weight) in most cells to 4:1 in myelin 2. Some diffuse laterally in the lipid bilayer, others are immobile (held in

place by interactions with cytoskeletal constituents).

GLYCOCALYX• Sugar coat in outer leaflet, thickness = 50 nm.

• Consists of polar oligosaccharide side chains linked covalently to most protein & some lipid (glycolipid).

• Consists cell-surface proteoglycans to which are bound glycosaminoglycans.

• Functions– Cellular attachment to extracellular matrix

components.– Binding of antigens & enzymes to the cell surface.– Facilitate cell-cell recognition & interaction (e.g.,

sperm – egg adhesion).

1. Selective permeability - Separate int. & ext. environment of a cell or an organelle, prevent intrusion of harmful substances, dispersion of macromolecules,

dilution of enzymes & substrates. - Homeostatic mechanism (maintain optimal intracellular conc. of ions,

H2O, enzymes & substrates – passive diffusion, facilitated diffusion,

active transport). 2. Signal transduction

- Receptor proteins vs signal molecules (ligands) . Ion channel-linked receptors

. Enzyme-linkedreceptors . G protein-linked receptors

. Nuclear receptors (steroids, retinoids, Vit. D, thyroid hormones 3. Endocytosis – phagocytosis (cell-eating): insoluble substances.

- pinocytosis(cell-drinking): fluid with solutes. - receptor-mediated endocytosis: ligands & surface receptors.

4. Exocytosis 5. Compartmentalization

. Spatial temporal organization of metabolic processes. 6. Storage, transport & secretion

CELL MEMBRANE : FUNCTIONS

Diagram illustrating (A) the concept that cells synthesizing proteins (represented here by spirals) that are to remain within the cytoplasm possess (free) polyribosomes (ie, nonadherent to the endoplasmic reticulum). In B, where the proteins are segregated in the endoplasmic reticulum and may eventually be extruded from the cytoplasm (export proteins), not only do the polyribosomes adhere to the membranes of RER, but the proteins produced by them are injected into the interior of the organelle across its membrane. In this way, the proteins, especially enzymes such as ribonucleases and proteases, which could have undesirable effects on the cytoplasm, are separated from it.

Role of the RibosomeThe route from the DNA code to the protein.

Before cell division, the DNA in our chromosomes replicates so each daughter cell has an identical set of chromosome.  In addition, the DNA is responsible for coding for all proteins.  Each amino acid is designated by one or more set of triplet nucleotides. The code is produced from one strand of the DNA by a process called "transcription". This produces mRNA which then is sent out of the nucleus where the message is translated into proteins.  This can be done in the cytoplasm on clusters of ribosomes, called "polyribosomes".  Or it can be done on the membranes of the rough endoplasmic reticulum.  The cartoon to the left shows the basic sequence of transcription and translational events.

Initiation

The cartoon shows the initiation of this process. It begins with the small subunit of the ribosome bound to the mRNA.  An initiator tRNA is attracted to the region (carrying a methionine.  It binds to the triplet code AUG.This then attracts the large ribosomal subunit which will bind to the small subunit. Note that it has an A site and a P site.  These are different binding sites for the tRNAs.  The cartoon below describes the next phase in the process.  Elongation

RIBOSOMES. 12-nm wide & 25 nm long. Consist of small & large subunits (composed of rRNA & proteins). Free (in cytosol) or bound to membs (RER or outer nuclear memb). Cluster along single strand of mRNA to form polyribosomes. Sites of where mRNA is translated into proteins. Consists roughly of 60% RNA % 40% protein (small mRNA, tRNA & large subunits of AA polypeptide chains - ribonucleoproteins . Intensely basophilic – protein synthesis - Ergastoplasm (glandular cells) - Nissl bodies (neorons) - Basophilic bodies (all other cells)

The left hand view of this cartoon shows the free polyribosomes connected by the mRNA. They are arranged in rosettes and these can be seen in the cytoplasm in conventional electron micrographs. The right hand view shows the arrangement of polyribosomes on the rough endoplasmic reticulum. Note that the growing polypeptide chain (which projects down from the large subunit) is inserted through the membrane and into the cisterna of the rough endoplasmic reticulum

The endoplasmic reticulum is an anastomosing network of intercommunicating channels and sacs formed by a continuous membrane. Note that the SER (foreground) is devoid of ribosomes, the small dark dots that are present in the RER (background). The cisternae of the SER are tubular, whereas in the RER they are flat sacs.

Smooth endoplasmic reticulum: a multipurpose organelle.

Smooth endoplasmic reticulum is found in a variety of cell types and it serves different functions in each. It consists of tubules and vesicles that branch forming a network. In some cells there are dilated areas like the sacs of RER. The network of SER allows increased surface area for the action or storage of key enzymes and the products of these enzymes. In the case of SER in muscle cells, the vesicles and tubules serve as a store of calcium which is released as one step in the contraction process. Calcium pumps serve to move the calcium.

Smooth Endoplasmic Reticulum (SER). Irregilar network of memb-bounded channels, lacks ribosomes. Usually appears as branching anastomosing tubules, or vesicles.. Less common than SER, predominates in cells synthesizing steroids, triglycerides, & cholesterol.. SER membranes arise from RER membranes.

FUNCTIONS. Steroid hormone synthesis : Leydig cells, Zona fasiculta (Suprarenal gland). Drug detoxification : Hepatocytes (Oxidation, Conjugation & methylation). Muscle contraction & relaxation : Skeletal m. (sarcoplasmic reticulum, release & recapture of Ca++)- Metabolism of lipid & cholesterol- Glycogen synthesis, storage & breakdown in the liver

Schematic representation of a small portion of the rough endoplasmic reticulum to show the shape of its cisternae and the presence of numerous ribosomes which are part of polyribosomes. It should be kept in mind that the cisternae appear separated in sections made for electron microscopy, but they form a continuous tunnel in the cytoplasm.

The cartoon in this figure shows the rough endoplasmic reticulum with a bridge adjoining two sacs. In this way, the sacs communicate and proteins fill the spaces all over the cell. They even communicate with the inside of the nuclear envelope. The outside membrane of the nuclear envelope is studded with ribosomes and is part of the rough endoplasmic reticulum. An immunocytochemical labeling protocol, such as that found in this figure, will delineate the reticulum filled with the newly synthesized proteins.

Rough Endoplasmic Reticulum (RER)• Site where noncytosolic proteins are synthesized (secretory, plasma

memb, lysosomal proteins).• System of sacs bounded by membs (outer surface studded with ribosomes , interior region called cisterna).• Memb. may be continuous with outer nuclear memb (peinuclear cisterna).• Abundant in cells synthesizing secretory proteins

– Pancreatic acinar cells (digestive enzymes)– Plasma cells (immunoglobulins)– Fibroblasts (collagen)

• has receptors (ribophorins)

• FUNCTIONS : Combination workshop & shipping depot– Segregate proteins for export or intracellular use– Initial (core) glycosylation of glycoproteins– Synthesis of phospholipids– Assembly of multichain proteins – Certain posttransitional modifications of newly

formed polypeptides

The transport of proteins across the membrane of the RER. The ribosomes bind to mRNA, and the signal peptide is initially bound to a signal-recognition particle (SRP). Ribosomes bind to the RER by interacting with the SRP and a ribosomal receptor. The signal peptide is then removed by a signal peptidase (not shown). These interactions cause the opening of a pore through which the protein is extruded into the RER.

The ultrastructure of a cell that synthesizes (but does not secrete) proteins on free polyribosomes (A); a cell that synthesizes, segregates, and stores proteins in organelles (B); a cell that synthesizes, segregates, and directly exports proteins (C);and a cell that synthesizes, segregates, stores in supranuclear granules, and exports proteins (D).

Schematic representation of a phospholipid-transporting amphipathic protein. Phospholipid molecules are transported from lipid-rich (SER) to lipid-poor membranes.

Morphology . Rod-shaped (0.2 um wide, up to 7 um long).. Membs. (outer, inner).. Compartments (intermemb, inner matrix).. Granules within matrix (bind divalent cations Mg2+ & Ca2+.. Proliferate by fission of preexisting mitoch.. Lifespan = 10 days.

Enzymes & Genetic apparatus. Contains all enzymes of Krebs (TCA) cycle in the matrix (except succinate dehydrogenase, in inner memb).. Elementary particles (ATPase synthase, coupling oxidation to phosphorylation of ADP to form ATP). Possess (in matrix) their own genetic apparatus composed of DNA (circular), mRNA, tRNA & rRNA (with limited coding capacity)Photomicrograph of the stomach inner covering. The large cells show many round and elongated mitochondria in the cytoplasm. The central nuclei are also clearly seen. High magnification.

Three-dimensional representation of a mitochondrion with its cristae penetrating the matrix space. Note that 2 membranes delimiting an intermembrane space form the wall of the mitochondrion. The cristae are covered with globular units that participate in the formation of ATP.

❇ Condensed mitoch. – in brown fat cells (produce HEAT rather than ATP). ❇ Mitochondria are maternally - derived

❇ One liver cell has ~ 800 mitoch.

FUNCTIONS ✹Energy production ☞ Raw materials

O2

Pyruvate FAs

☞ Products CO2

ATP ✹Respiratory chain

Structural lability of mitochondria. A: Electron micrograph of a section of rat pancreas. A mitochondrion with its membranes, cristae (C), and matrix (M) is seen in the center. Numerous flattened cisternae of RER with ribosomes on their cytoplasmic surfaces are also visible. x50,000. B: Electron micrograph of striated muscle from a patient with mitochondrial myopathy. The mitochondria (arrows) are profoundly modified, showing marked swelling of the matrix.

☞ Mitochondria accumulate where metabolic activity is intensive

. Apical ends of ciliated cells . Middle piece os spermatozoa

. Bases of ion-transferring cells in kidneys

The chemiosmotic theory of mitochondrial energy transduction. Middle: The flux of protons is directed from the matrix to the intermembranous space promoted at the expense of energy derived from the electron transport system in the inner membrane. Left: Half the energy derived from proton reflux produces ATP; the remaining energy produces heat. Right: The protein thermogenin, present in multilocular adipose tissue, forms a shunt for reflux of protons. This reflux, which dissipates energy as heat, does not produce ATP.

GOLGI COMPLEX (APPARATUS)• Consist of several disc-shaped cisternae (saccules) arranged in stack.• Cisternae are slightly curved with flat centers & dilated rims.• Distinct polarity across the stack. • REGIONS

– Cis (entry) face• Forming (outer, convex) cisternae at side of the stack facing the RER.

– Trans (exit) face• Maturing (inner, concave) cisternae at side of the stack facing vacuoles

& secretory granules. – Medial compartment

• Few cisternae between the cis & trans faces.– Trans Golgi network (TGN)

• Lies apart from the last cisternaat the trans face ans is separated from the Golgi stack

• Corresponds to a tubular reticulum formerly called GERL (Golgi-associated endoplasmic reticulum from which lysosomes originate).

• FUNCTIONS– Processing of noncytosolic proteins synthesized in RER.– Memb. retrieval, recycling & redistribution – Glycosylation, sulfation & phosphorylation– Initiation of packing, concentration & storage of secretory products– Limited proteolysis of proteins

Three-dimensional representation of a Golgi complex. Through transport vesicles that fuse with the Golgi cis face, the complex receives several types of molecules produced in the (RER). After Golgi processing, these molecules are released from the Golgi trans face in larger vesicles to constitute secretory vesicles, lysosomes, or other cytoplasmic components.

GOLGI APPARATUS Dorsal root ganglion cells 

Guinea pig, Kopsch's method, 612x.This organelle was discovered by the Italian histologist Camillo Golgi in 1896. The method used here and modifications of it are the best available for revealing this important cell organelle. Nucleus: Vesicular. Prominent nucleolus. Cytoplasm: The Golgi apparatus is the only cytoplasmic organelle seen in this preparation. Golgi apparatus: Reticulated appearance and highly developed in a perinuclear network. The Golgi apparatus is known to play an important role in packaging and concentrating protein-rich secretory products elaborated by glandular cells; in general, it does not appear to play a role in the synthetic mechanism. The Golgi apparatus is also found in cells that are not secretory, and its function in these cells is uncertain. It is stained black by this method.

Electron micrograph of a Golgi complex

Electron micrograph of a Golgi complex of a mucous cell. To the right is a cisterna (arrow) of the rough endoplasmic reticulum containing granular material. Close to it are small vesicles containing this material. This is the cis face of the complex. In the center are flattened and stacked cisternae of the Golgi complex. Dilatations can be observed extending from the ends of the cisternae. These dilatations gradually detach themselves from the cisternae and fuse, forming the secretory granules (1, 2, and 3). This is the trans face. Near the plasma membrane of two neighboring cells is endoplasmic reticulum with a smooth section (SER) and a rough section (RER). x30,000. Inset: The Golgi complex as seen in 1-micrometer sections of epididymis cells impregnated with silver. x1200.

Golgi apparatus (A) of a spermatid.

The above drawing shows an actual interface between the ER and the Golgi complex.  The "Export complex" is seen at the top of the drawing.  Note that the vesicle are moving to contribute to the cis-Golgi network of vesicles and cisternae

Transport of material in and out of the Golgi complex involves budding and fusion of vesicles. This cartoon shows that the membranes of each join and align themselves during the process so that the inside face remains in the lumen and the outside face remains towards the cytoplasm.

What types of secretion are controlled by the Golgi complex?

• The Golgi complex controls trafficking of different types of proteins. Some are destined for secretion. Others are destined for the extracellular matrix. Finally, other proteins, such as lysosomal enzymes, may need to be sorted and sequestered from the remaining constituents because of their potential destructive effects. This figure shows the two types of secretory pathways. The regulated secretory pathway, as its name implies, is a pathway for proteins that requires a stimulus or trigger to elicit secretion. Some stimuli regulate synthesis of the protein as well as its release. The constitutive pathway allows for secretion of proteins that are needed outside the cell, like in the extracellular matrix. It does not require stimuli, although growth factors may enhance the process.

How does the Golgi complex regulate the insertion of plasma membrane proteins?

The protein sequence is coded for membrane insert start and stop sites. This directed the insertion and alignment points. Those that are multipass proteins have multiple start and stop sites. The important role of the Golgi Complex is to make certain the plasma membrane proteins reach their destination. This figure shows the route. Note that the orientation of the protein is maintained so that the region destined to project outside the cell (a receptor binding site, for example), ends up in that place. In order to do this, it must be placed so that it faces inside the vesicle.

Main events occurring during trafficking and sorting of proteins through the Golgi complex. Numbered at the left are the main molecular processes that take place in the compartments indicated. Note that the labeling of lysosomal enzymes starts early in the cis Golgi network. In the trans Golgi network, the glycoproteins combine with specific receptors that guide them to their destination. On the left side of the drawing is the returning flux of membrane, from the Golgi to the endoplasmic reticulum.

A simplified “roadmap” of protein traffic. Proteins can move from one compartment to another by gated transport (red), transmembrane transport (blue), or vesicular transport (green). The signals that direct a given protein's movement through the system, and thereby determine its eventual location in the cell, are contained in each protein's amino acid sequence. The journey begins with the synthesis of a protein on a ribosome in the cytosol and terminates when the final destination is reached. At each intermediate station (boxes), a decision is made as to whether the protein is to be retained in that compartment or transported further. In principle, a signal could be required for either retention in or exit from a compartment. We shall use this figure repeatedly as a guide throughout this chapter and the next, highlighting in color the particular pathway being discussed.

Vesicle budding and fusion during vesicular transport. Transport vesicles bud from one compartment (donor) and fuse with another (target) compartment. In the process, soluble components (red dots) are transferred from lumen to lumen. Note that membrane is also transferred, and that the original orientation of both proteins and lipids in the donor-compartment membrane is preserved in the target-compartment membrane. Thus, membrane proteins retain their asymmetric orientation, with the same domains always facing the cytosol.

COATED VESICLES• Characterized by a visible cytoplasmic surface coat.

• 1. Clathrin-coated vesicles– Coated with clathrin (consists of three large and three small polypeptide

chains that form a triskelion, three-legged structure). Thirty-six clathrin triskelions associate to form a polyhedral cage- like network around the vesicle.

– Are formed during receptor-mediated uptake (endocytosis) of specific molecules by the cell, but lose their coat quickly, permitting clathrin to recycle back to th plasma membrane.

– Are associated with the regulated signal-directed transport of proteins from the trans-Golgi network ultimately to lysosomes or to secretory granules.

• 2. Non-clathrin-coated vesicles ➽ involved in the transport of proteins from the RER to the Golgi

complex, from one Golgi cicterna to another, and from the Golgi complex to the plasma membrane.

➽ Are associated with contitutive (unregulated) protein transport (bulk flow).

➽ Have coats that are not yet fully characterized but primarily consist of a protein called β -COP, which does not form a cage-like network around vesicles and has a different appearance than clathrin.

Schematic representation of the

endocytic pathway and membrane trafficking. Ligands, such as hormones and growth factors, bind to specific surface receptors and are internalized in pinocytotic vesicles coated with clathrin and other proteins. After the liberation of the coating molecules, the pinocytotic vesicles fuse with the endosomal compartment, where the low pH causes the separation of the ligands from their receptors. Membrane with receptors is returned to the cell surface to be reused. The ligands typically are transferred to lysosomes. The cytoskeleton with motor proteins is responsible for all vesicle movements described.

Photomicrograph of a kidney tubule whose lumen appears in the center as a long slit. The numerous dark-stained cytoplasmic granules are

lysosomes (L), organelles abundant in these kidney cells. The cell nuclei (N), some showing a nucleolus, are also seen in the photograph as dark-stained corpuscles. Toluidine blue stain. High magnification.

Electron micrograph of a macrophage. Note the abundant cytoplasmic extensions (arrows). In the center is a centriole (C) surrounded by Golgi cisternae (G). Secondary

lysosomes (L) are abundant. x15,000.

What are lysosomes?Lysosomes . cells' garbage disposal system. . degrade the products of ingestion, such as the bacterium that has been taken in by phagocytosis. After the bacterium is enclosed in a vacuole, vesicles containing lysosomal enzymes (sometimes called primary lysosomes) fuse with it. The pH becomes more acidic and this activates the enzymes.

The vacuole thus becomes a secondary lysosome and degrades the bacterium. Lysosomes also degrade worn out organelles such as mitochondria. In this cartoon, a section of RER wraps itself around a mitochondrion and forms a vacuole. Then, vesicles carrying lysosomal enzymes fuse with the vesicle and the vacuole becomes an active secondary lysosome. A third function for lysosomes is to handle the products of receptor-mediated endocytosis such as the receptor, ligand and associated membrane. In this case, the early coalescence of vesicles bringing in the receptor and ligand produces an endosome. Then, the introduction of lysosomal enzymes and the lower pH causes release, and degradation of the contents. This can be used for recycling of the receptor and other membrane components.

Lysosomes carry hydrolases that degade nucleotides, proteins, lipids, phospholipids, and also remove carbohydrate, sulfate, or phosphate groups from molecules. The hydrolases are active at an acid pH which is fortunate because if they leak out of the lysosome, they are not likely to do damage (at pH 7.2) unless the cell has become acidic. A Hydrogen ion ATPase is found in the membrane of lysosomes to acidify the environment. Lysosomal morphology varies with the state of the cell and its degree of degradative activity. Lysosomes have pieces of membranes, vacuoles, granules and parts of mitochondia inside. Phagolysosomes may have parts of bacteria or the cell it has ingested. This electron micrograph shows typical secondary lysosomes. They have been detected by cytochemical labeling for acid phosphatase. This is a good marker for lysosomes.

The Golgi complex sorts the lysosomal enzyme in the Trans region. It is received from the RER in the cis region. There it has a phosphate radical attached to the mannose residue. This mannose-6 phosphate forms a sorting signal that moves through the cisternae to the trans region where it binds to a specific receptor. After it binds to the receptor, it begins to bud and a "cage" or "coat" made of clathrin forms around the bud (to strengthen it). It moves away to fuse with a developing lysosome (such as the vacuoles seen in the previous figure). This lysosome contains a hydrogen ion pump on its surface. The pump works to acidify the environment inside the lysosome. This removes the phosphate and dissociates the hydrolase from the receptor. The receptor is then recycled back to the Golgi complex.

Lysosomes can actually be detected by pH indicator dyes. This photograph shows dyes that indicate different pH's with different colors. The red lysosomes (pH 5.0) are probably typical lysosomes. The blue and green lysosomes are probably endosomes. This change can be detected if you link a ligand to fluorescein. Fluorescein will not fluoresce at pH's lower than 6.0. Therefore, one can follow entry of the receptor-ligand complex and then see the fluorescence disappear as the endosome containing the complex is acidified.

Electron micrograph showing 4 dark secondary lysosomes surrounded by numerous mitochondria.

Current concepts of the functions of lysosomes. ❋ Synthesis occurs in the (RER), and the enzymes are packaged in the G C. ❋ Note the heterophagosomes, in which bacteria are being

destroyed, and the autophagosomes, with RER and mitochondria in the process of digestion.

❋ Heterophagosomes and autophagosomes are secondary lysosomes. The result of their digestion can be excreted, but sometimes the secondary lysosome creates a residual body, containing remnants of undigested molecules. ✿ In some cells, such as osteoclasts, the lysosomal enzymes are secreted to the

extracellular environment.

TYPES OF LYSOSOMES1. Multivesicular bodies ☞ Formed by fusion of an early endosome containing endocytic vesicles with a

late endosome.2. Phagolysosome

☞ Formed by fusion of a phagocytic vacuole with a late endosome or lysosome.3. Autophagolysosome ☞ Formed by fusion of an autophagic vacuole with a late endosome or lysosome.4. Residual bodies

☞ Lysosomes that have expended their capacity to degrade material☞ Contain undigested material (lipofuscin, hemosiderin…).

FUNCTIONS OF LYSOSOMES ➽ Intracellular digestion

➽ Degeneration of glycogen & its removal ➽ Initiate mitosis

➽ Release of thyroid hormones ➽ Destruction of bone matrix (osteoclasts)

✪ Intracellular digestion ➣ Nonlysosomal digestion

Turnover of short-lived proteins by different classes of nonlysosomal proteases

➣ Lysosomal digestion: Lysosomal enzymes ☞ Heterophagy: ingestion & degradation of foreign materials taken

into the cell by receptor-mediated endocytosis or phagocytosis. ☞ Autophagy: segregation of organelles & other cellular constituents

☞ Crinophagy: fusion of hormone secretory granules with lysosomes

✹ AUTOLYSIS occurs normally. - resorption of tadpole tail

- regression of mesonephros - regression of mammary tissue after cessation of lactation

Section of a pancreatic acinar cell showing autophagosomes. Upper right: Two portions of the RER segregated by a membrane. Center: An autophagosome containing mitochondria (arrow) plus RER. Left: A residual body, with indigestible material. Arrowhead shows a cluster of coated vesicles.

Electron micrograph of a pancreatic acinar cell from the rat. Numerous mature secretory granules (S) are seen in association with condensing vacuoles (C) and the Golgi complex (G). x18,900.

Why peroxisomes are not like lysosomesPeroxisomes are organelles that contain oxidative enzymes, such

as D-amino acid oxidase, ureate oxidase, and catalase.

They may resemble a lysosome, however, they are not formed in the Golgi complex. Peroxisomes are distinguished by a crystalline structure inside a sac which also

contains amorphous gray material.

They are self replicating, like the mitochondria. Components accumulate at a given site and they can be assembled into a

peroxisome. They may look like storage granules,

however, they are not formed in the same way as storage granules.  They also enlarge and bud to produce new peroxisomes.Peroxisomes function to rid the body of toxic substances like hydrogen peroxide, or other metabolites (ethanol). They are a major site of oxygen utilization and are numerous in the liver where toxic byproducts are going to accumulate.

FUNCTIONS OF PEROXISOMES . Regulates H2O2 within cells

. Gluconeogenesis (formation of glucose from non-carbohydrate precursors)

. Enzymes for lipid metabolism: β-oxidation of long chain FAs

. Detoxification (e.g., ethanol) . Formation of bile salts

Electron micrograph of fibroblast cytoplasm. Note the actin filaments (AF) and microtubules (MT). x60,000. (Courtesy of E Katchburian.)

The cytosolic actin filament. Actin dimers are added to the plus (+) end and removed at the minus (—) end, dynamically lengthening or shortening the filament, as required by the cell.

✹ structural framework within the cytosol. ✹ Functions: maintain cell shape, stabilize cell attachments, facilitate endocytosis and exocytosis, and promote cell motility.

Microtubules☞ are straight, hollow tubules 25 nm in diameter.

☞ have a rigid wall composed of 13 protofilaments, each of which consists of a linear arrangement of αβ- tubulin dimers.

☞ are polar, with polymerization (assembly) and depolymerization (disassembly) occurring preferentially at

one end (+ end). ☞ contain microtubule-associated proteins (MAPs), which stabilize them and bind them to other cytoskeletal

components and organelles. ☞ are associated with kinesin, a force-generating protein,

which serves as a “motor” for vesicle or organelle movement.

✹ maintain cell shape ✹ aid in the transport of macromolecules within the cytosol

(axoplasmic transport, melanin transport, vesicle movements b/n ER & GC and GC & membs)

✹ promote the movement of cilia, flagella, and chromosomes

Microfilaments (F actin or actin filaments) ✬ 6 nm in diameter.

✬ composed of globular actin monomers (G actin) linked into a double helix having a

36nm repeat. ✬ display polarity like microtubules

✬ more stable than microtubules. ✬ are abundant at the periphery of the cell

where they are anchored to the plasma membrane via one or more intermediary

proteins (e.g. α-actinin, vinculin, talin). ❁ are involved in the following cellular processes:

☞ Sol/gel transformation of the cytosol☞ Endocytosis and exocytosis

☞ Locomotion of nonmuscle cells

Electron micrograph of a skin epithelial cell showing

intermediate filaments of keratin associated with desmosomes.

INTERMEDIATE FILAMENTS

Protein Location Function

Keratin19 distinct forms (acidic, neutral & basic) * Tonofilaments

Epithelial cells(Keratinizing and Non-keratinizing)

Provides structural support or tension-bearing role; markers for tumors of epithelial origin.Desmosome-heidesmosome association

Desmin Skeletal muscleCardiac muscle Smooth muscle

Forms a framework linking myofibrils / myofilaments; marker for tumors of muscle origin

Vimentin FibroblastsEndothelial cellsVascular smooth muscleChondroblastsMacrophagesMesenchymal cells

Is associated with nuclear

envelope and pores,

Marker for connective tissue

tumors.

Glial fibrillary acidic protein (GFAP)

AstrocytesOligodendrocytesSchwann cells

Provides structural support; marker for glial tumors

Neurofilaments Neurons Provide support for axons and dendrites; facilitate gel state (cytosol)

Lamins A, B and C Nuclear lamina of all cells

Organize nuclear envelope and perinuclear chromatin

➲ provide mechanical strength to cells.

➲ 8-11 nm in diameter

Molecular organization of a

microtubule. In this polarized structure there is an alternation of the two subunits (alpha and beta) of the tubulin molecule. Tubulin molecules are arranged to form 13 protofilaments, as seen in the cross section in the upper part of the drawing.

Electron micrograph of a section of a photosensitive retinal cell. Note the accumulation of transversely sectioned microtubules (arrows). Reduced slightly from x80,000.

Schematic representation of microtubules, cilia, and centrioles.

A: Microtubules as seen in the electron microscope after fixation with tannic acid in glutaraldehyde. The unstained tubulin subunits are delineated by the dense tannic acid. Cross sections of tubules reveal a ring of 13 subunits of dimers arranged in a spiral. Changes in microtubule length are due to the addition or loss of individual tubulin subunits.

B: A cross section through a cilium reveals a core of microtubules called an axoneme. The axoneme consists of 2 central microtubules surrounded by 9 microtubule doublets. In the doublets, microtubule A is complete and consists of 13 subunits, whereas microtubule B shares 2 or 3 heterodimers with A. When activated by ATP, the dynein arms link adjacent tubules and provide for the sliding of doublets against each other.

C: Centrioles consist of 9 microtubule triplets linked together in a pinwheel-like arrangement. In the triplets, microtubule A is complete and consists of 13 subunits, whereas tubules B and C share tubulin subunits. Under normal circumstances, these organelles are found in pairs with the centrioles disposed at right angles to one another.

Drawing of a centrosome with its granular protein material surrounding a pair of centrioles, one shown at a right angle to the other. Each centriole is made of 9 bundles of microtubles, with 3 microtubules per bundle.

9+0 axoneme pattern

✪ Exist as a pair of cylidrical rods oriented at right angles to one another✪ Self-duplicate in the S phase of the cell cycle.✪ Associated with microtubule-organizing centers (MTOCs).

Photomicrograph of the epithelium covering the inner surface of the respiratory airways. Most cells in this epithelium contain numerous cilia in their apices (free upper extremities). N, cell nuclei; M, cytoplasmic mucus secretion, which appears dark in this preparation. H&E stain. High magnification.

• Microtrabecular lattice• is a three-dimensional meshwork of slender strands

in the cytosol, which is observed only by high-voltage electron miscroscopy.

• is not universally accepted as an authentic structure.• may compartmentalize metabolic activities in the

cytosol, influence the movement of organelles, and affect the viscosity of the cytosol.

CELL INCLUSIONS• I. Stored substances

– Lipid droplets (non-memb) – triglycerides, cholesterol (adipose tissue, adrenal cortex, liver cells)– Glycogen (non-memb) - PAS+

• II. Secretory granules– Diameter = 0.2-2 um, (memb-bounded)

• III. Pigments– 1. Endogenous

• Hematogenous –Iron-containing (hemosiderin: spleen, BM, phagocytic cells of liver); without iron.

• Melanin – Skin, retina, iris• Lipofuscin – “wear & tear” pigment (Cardiac m., neurons, hepatocytes)

– 2. Exogenous – Dust particles, Heavy metals…)

• IV. Crystals– Leydig cells

Section of adrenal gland showing lipid droplets (L) and abundant anomalous mitochondria (M). x19,000.

MELANIN Skin scalp

Melanin-containing cells are found in the basal layer of the epidermis (stratum germinativum) in some sites of the body. The pigment is practically absent from the palms and soles, while the areola of the mammary gland, the circumanal region, the labia majora, and the scrotum are more richly pigmented. The melanin pigment is found as granules primarily within the cells of the basal layer. Some granules appear to be scattered among the cells but are actually located in cellular processes. Cells that synthesize melanin (melanocytes) can be distinguished from those to which melanin is subsequently transferred (cytocrine secretion) by means of histochemical methods. The collagenous connective tissue shown is in the dermis of the skin.

Human, 10% formalin, H. & E., 1416 x.

PIGMENT-CONTAINING CELLS Choroid of eye

Human, glutaraidehyde, H. & E., 452 x.The choroid of the eye lying superficial to the neural retina contains many vessels ramifying within a loose connective tissue. Within this connective tissue are abundant melanocytes. The cytoplasmic inclusion melanin is complexed with the structural protein of specialized granules, the melanosomes.

Section of amphibian liver shows cells with pigment deposit (PD) in the cytoplasm, a macrophage (M), hepatocytes (H), and a neutrophil leukocyte (N). In this resin-embedded material it is possible to see mitochondria (pale red) and lysosomes (blue) in the cytoplasm of the hepatocytes. Only with resin embedding is it possible to obtain such information. Giemsa stain. Medium magnification.

GLYCOGENLiver

Rabbit, absolute alcohol, Best's*carmine and hematoxylin stains, 612 x.

The stain used here is specific for glycogen, although the rationale for its selectivity is uncertain.

Sinusoids: Vascular channels larger in diameter than ordinary capillaries but composed of a single layer of fenestrated endothelial cells separating sheets of hepatic cells.

Hepatic cells: Arranged in plates. Each cell has a distinct central nucleus. These cells may contain more than one nucleus.

Glycogen: Stored throughout the cytoplasm. Glycogen is normally stored in hepatic cells whose content varies with the functional state of the liver and the dietary intake. Glycogen is a polysaccharide, a polymer composed of many molecules of glucose. It

stains red by this method. Central vein: Center of the hepatic lobule. Smallest radicle of the hepatic vein, which

receives the contents of all the sinusoids comprising the hepatic lobule.

✪Larger than robosomes.

✪Not memb-bounded.

✪ Serves as a stored energy source that can be degraded to glucose.

Electron micrograph of a section of a liver cell showing glycogen deposits as accumulations of electron-dense particles (arrows). The dark structures with a dense core are peroxisomes. x30,000.

LIPID Striated muscle of diaphragmlongitudinal and cross section 

Rat, frozen section, osmiumtetroxide fixation and stain, 612 x.Cells contain lipid droplets, neutral fats that are liquid at body temperature. Lipids are metabolized for energy or incorporated in cell membranes and other structures. In routine preparations, lipid is extracted and is not seen. In well-fixed tissues, clear round holes may sometimes be seen; these mark the site of lipid droplets within the cytoplasm of cells. The lipid in this preparation was fixed with osmium tetroxide, which renders it insoluble, and appears black. Lipid droplets: Irregularly distributed within smaller muscle fibers. in contrast, larger fibers are devoid of lipid droplets.

✪Not memb-bounded.

✪ Are storage forms of triglycerides (an energy source) & cholesterol (used in the synthesis of steroids).

LIPOCHROME PIGMENT Spinal cordlower motor neuron

Rhesus monkey, 10% formalin, Glees' method, 612 x.This plate shows a large multipolar motor neuron in the anterior horn of the spinal cord. Note the characteristic large, rounded central nucleus, the prominent nucleolus, and processes that extend from the cell body (perikaryon). Note also the stout dendrite. A collection of lipochrome pigment (lipofuscin) is seen in one corner of the cytoplasm. Lipochrome pigment is believed to be a product of normal metabolic activity and accumulates with age. It can also be seen in cardiac muscle.

PARTS FUNCTIONS

A. Centrioles found within the centrosome; aid in reproduction

B. Centrosome contain the centrioles; aid in reproduction

C. Golgi body (complex) manufactures carbohydrates and packages it with protein

D. Nucleus control center

E. Nucleolus produces ribosomes and RNA

F. Nuclear membrane allows material in and out of the nucleus

G. Nucleoplasm gives the nucleus shape

H. Plasma (cell) membrane allows material in and out of the cell

I. Microvilli projections that increase cell's surface area

J. Mitochondria produce energy

K. Cytoplasm gives the cell shape and holds the organelles

L. Lysosome contain digestive enzymes

M. smooth ER synthesize lipids (without ribosomes)

N. rough ER transports material thru cell ( with ribosomes)

O. Ribosomes produces protein

P. Pinocytic vessicleindentation in the cell membrane that allows for entrance of

large molecul

Q. Vacuole storage areas for food and/or water

S U M M A R Y

1. SECRETION (Exocytosis) a) Nucleus

b) RER c) Golgi Apparatus

d) Secretory vesicle 2. ABSORPTION (Endocytosis)

a) Endosome b) Lysosome

3. METABOLISM a) Mitochondria

b) SER c) Peroxisome

Female sex chromatinA. peripheral blood smear

B. buccal epithelium, and C. corpus luteum

A. Human, air-dried smear, Wright's stain, 1416 x.

B. Human, buccal epithelium scraping, aceto-orcein stain,

500 x. C. Human, 10 % formalin, H. &

E., 1416 x.

SHAPES, SIZES & NUMBER. rounded, loosely packed…………………... hepatocytes

. relatively small with more tightly packed content…sinusoids . distinctive shape, deeply indented (segmented)…..neutrophils

. Binucleated……parietal cells (stomach), some hepatocytes, some cardiac m. cells

. Multinucleated……………osteoclasts, skeletal m. cells . exceptionally large (multiple amounts of DNA)…megakaryocytes

. no nuclei at all…………….erythrocytes, blood platelets

. Double memb. – nuclear pores . Genetic material within – chromosomes

(DNA + protein). Own cytoskeleton

. No cytoplasmic organelles . No protein synthesis

. STORES – DNA . IMPORTS – PROTEINS

. EXPORTS - mRNA

A. Motor neuronB1. Basophilic & B2. Orthochromatic erythroblasts

A. Rhesus monkey, van Gehuchten's fluid,methylene blue and erythrosin, 612 x.

B. Human, air-dried smear, Wright's* stain, 1416 x.

Liver cells (hepatocytes). Several dark-stained nuclei are shown. Note the apparent nuclear membrane consisting mainly of a superficial condensation of chromatin. Several nucleoli are seen inside the nuclei, suggesting intense protein synthesis. One hepatocyte contains 2 nuclei. Pararosaniline—toluidine blue (PT) stain. Medium magnification.

The nuclear envelope Schematic diagram of the nuclear envelope. The inner nuclear membrane is lined by the nuclear lamina, which serves as an attachment site for chromatin. (A, David M. Phillips/Photo Researchers, Inc.; B, courtesy of Dr. Werner W. Franke, German Cancer Research Center, Heidelberg.)

Schematic representation of a cell nucleus. The nuclear envelope is composed of two membranes of the endoplasmic reticulum, enclosing a perinuclear cisterna. Where the two membranes fuse, they form nuclear pores. Ribosomes are attached to the outer nuclear membrane. Heterochromatin clumps are associated with the nuclear lamina, whereas the euchromatin (EC) appears dispersed in the interior of the nucleus. In the nucleolus, note the associated chromatin, heterochromatin (Hc), the pars granulosa (G), and the pars fibrosa (F).

Three-dimensional representation of a cell nucleus showing the distribution of the nuclear pores, the heterochromatin (dark regions), the euchromatin (light regions), and a nucleolus. Note that there is no chromatin closing the pores. The number of nuclear pores varies greatly from cell to cell.

Electron micrograph of a nucleus, showing the heterochromatin (HC) and euchromatin (EC). Unlabeled arrows indicate the nucleolus-associated chromatin around the nucleolus (NU). Arrowheads indicate the perinuclear cisterna. Underneath the cisterna is a layer of heterochromatin, the main component of the so-called nuclear membrane seen under the light microscope. x26,000.

Nuclear characteristics of dead cells (Necrosis)

1. Pyknosis (Gk. Condensation)…. Intensely basophilic

uniform mass 2. Karyorrhexis

(Gk rhexis=breaking)….total fragmentation 3. Karyolysis

(Gk.Lysis=dissolution)….faded & ghost-like

Heterochromatin ✪ intensively basophilic (electron-dense)

✪ collective name for karyosomes (individual masses of chromatin) ✪ condensed chromatin (coiled portions of chromosomes)

✪ visible as - chromatin granules associated with the nuclear envelope (peripheral chromatin)

- nucleolus- associated chromatin (nucleolus) - some aggregates distributed in karyolymph

✪ transcriptionally inactive (gene transcription suppressed) ✪ heterochromatin mass in females (one of the X chromosomes)

- Barr body….. in epithelial cell - Drumstick …. In neutrophilic leukocyte

Euchromatin (Interchromatin) ✪dispersed (diffuse) region of chromosomes

✪ transcriptionally active (active DNA, control metabolic activities)☞ contains - chromatin

- nucleoproteins…. attachment sites for DNA - RNA….ribosomal, messenger & transfer

➣Distinction b/n Heterochromatin & Euchromatin disappears during cell division.

A small lymphocyte. The nucleus contains predominantly heterochromatin clumps. Its cytoplasm shows scattered free ribosomes (circled areas marked by arrows).

A neuron. The round nucleus contains mainly euchromatin (dispersed chromatin). What does this indicate? A prominent nucleolus is located in the centre.

Illustration to show the structure, the localization, and the relationship of the nuclear lamina with chromosomes. The drawing also shows that the nuclear pore complex is made of 2 protein rings in an octagonal organization. From the cytoplasmic ring, long filaments penetrate the cytosol, and from the intranuclear ring arise filaments that constitute a basketlike structure. The presence of the central cylindrical granule in the nuclear pore is not universally accepted.

Simplified representation of 2 nuclear pore complexes. In this model, the final nuclear portion is seen to be a more continuous structure, in the shape of a ring.

Electron micrographs of nuclei showing their envelopes composed of two membranes and the nuclear pores (arrows). A, B: Transverse sections; C: a tangential section. Chromatin, frequently condensed below the nuclear envelope, is not usually seen in the pore regions. x80,000.

Electron micrograph obtained by cryofracture of a rat intestine cell, showing the two components of the nuclear envelope and the nuclear pores. (Courtesy of P Pinto da Silva.)

Schematic representation of a nucleosome. This structure consists of

a core of 4 types of histones (2 copies of each)–H2A, H2B, H3, and H4–and one molecule of H1 or H5 located outside the DNA filament.

The orders of chromatin packing believed to exist in the metaphase chromosome. Starting at the top, the 2-nm DNA double helix is shown; next is the association of DNA with histones to form filaments of nucleosomes of 11 nm and 30 nm. Through further condensation, filaments with diameters of 300 nm and 700 nm are formed. Finally, the bottom drawing shows a metaphase chromosome, which exhibits the maximum packing of DNA.

Morphologic features of sex chromatin in human female oral (buccal)

epithelium and in a polymorphonuclear leukocyte. In the epithelium, sex chromatin appears as a small, dense granule adhering to the nuclear envelope. In the leukocyte, it has a drumstick shape.

Human karyotype preparation made by means of a banding technique. Each chromosome has a particular pattern of banding that facilitates its identification and also the relationship of the banding pattern to genetic anomalies. The chromosomes are grouped in numbered pairs according to their morphologic characteristics.

CHROMOSOMESA. Metaphase chromosomes

B. Karyotype

Human, peripheral

lymphocyte culture,air-dried trypsin-banded, Giemsa*

stain, 2142 x.

Photomicrograph of 2 primary oocytes, each one with its pale cytoplasm and round, dark-stained nucleus. In each nucleus the nucleolus, very darkly stained, is clearly seen. The sectioned chromosomes are also seen, because they are condensed. These cells stopped at the first meiotic division. Meiosis will proceed just before ovulation (extrusion of the oocyte from the ovary).

❋ is a well-defined, but not membrane-bounded, nuclear inclusion (sometimes more than one) present in cells

that are actively synthesizing proteins. ❋ generally is detectable only when the cell is in

interphase is involved in the synthesis of rRNA and its assembly into precursors of ribosomes

❋ contains nucleolar organizers – regions of some chromosomes (in humans, chromosomes 13, 14, 15,

21, and 22) where r RNA genes are located; these regions are involved in reconstituting the nucleolus

during the G1 phase of the cell cycle

. Well-defined, not memb-bounded nuclear inclusion.

. Actively synthesize proteins (contains histones, enzymes & RNA.. Generally detectable only during interphase.

➽ contains the following distinct regions:1. Fibrillar centers ✪ spherical areas of inactive rDNA surrounded by a dense fibrillar region2. Fibrillar regions ✪ composed of fibrils (5nm in diameter) around and b/n the fibrillar centers ✪ contain transcriptionally active rDNA ✪ represent early stages in the formation of rRNA precursors

3. Granular regions ✪ are composed of 15nm particles representing maturing ribosomal precursors

Electron micrograph of a nucleolus. The nucleolar organizer DNA (NO), pars fibrosa (PF), pars granulosa (PG), nucleolus-associated chromatin (NAC), nuclear envelope (NE), and cytoplasm (C) are shown.

• ✹ Sequense of events that is executed repetetively: repeated divisions at regular intervals • ✹ Dynamic & continuous process.• ✹ Induced by phytohemmagglutinin (NGF, EGF, FGF, precursors of RBC GF=erythropoietin)• ✹ Arrested in metaphase stage by colchicine (chalones)• ✹ Consists of two major periods

– Interphase: interval between cell divisions.– Mitosis: periods of cell division (M phase).➲ Interphase:

• Considerably longer than the M phase• Cell doubles in size and DNA content✹ Phases

– G1 Phase . Cell growth & protein synthesis (restore daughter cells to normal vol. & size). . Restriction point - “trigger proteins” synthesized to reach a

threshold. . Cells that fail to reach the restriction point = resting cells, enter G0 phase. . Lasts from few hours to several days.

– S Phase . DNA replication & protein synthesis = duplication of chromosomes . Centrioles self-duplicate . Lasts 8-12 hours in most cells.

– G2 Phase . Cell prepares to divide . Centrioles grow to maturity . Energy required for the completion of mitosis is restored . RNA & proteins necessary for mitosis are synthesized . Lasts 2-4 hours.

✹ Control factors . S-phase activator triggers the initiation of DNA synthesis . M-phase delaying factor

. M-phase promoting factor

Cell Lifespan · Neutrophil: 6-7 hours circulating, 4 days in tissue · Red blood cell: 120 days

· Brain neuron, heart: 50 - 100 years Lifespan Processes · Birth: Mitosis, except germ cells meiosis · Death: Apoptosis (programmed cell death), NecrosisCell Cycle · Mitosis (M phase): Cell birth(division), small time of cell cycle · Interphase: Most cell life, cell growth, function, DNA synthesis, organelle

development Cell Cycle · Time cell comes into existence until that cell divides again · Rapidly growing human cells 20-24 hr · Liver cells 1-2 year · Neurons 1 only: Quiescent G0 Cell Cycle- Stages · Rapidly dividing cell every 20-24hr · Mitosis: M 1 hr · Interphase: G1 Phase, cellular growth 9hr, Most variable time, Can exit to G0,

S Phase, DNA duplication 9hr, G2 Phase, prepare for mitosis 4 hr

Phases of mitosis

    Nuclear events in mitosis - described in the sequence in which they occur.

❁ Interphase (A): Non-dividing or resting stage. The chromatin appears as an irregular reticular meshwork. The nuclear membrane(envelope) and the nucleolus are distinctly seen. Chromosomes are not visible.

❁Early prophase (B): Nuclear membrane and nucleolus disappear. Granularity of the nucleus is markedly increased, and filamentous structures are seen. These granules and filaments represent the chromosomes, which become shorter and thicker in this stage.

❁ Late prophase (C): The thread- or rod-like character of the chromosomes is more apparent. Each chromosome consists of two coiled chromatids, which are not visible in this preparation. The disappearance of the nuclear membrane allows mixing of nuclear and cytoplasmic material.

❁ Metaphase (D): Chromosomes appear condensed and line up in the equatorial plane (metaphase plate) of the cell. Each chromosome is still composed of two paired chromatids.

❁ Anaphase (E): The daughter chromosomes (chromatids) separate and are drawn to opposite poles of the cell. They remain separate and tightly coiled, and appear at this magnification to be fused. Cytoplasmic division begins.

❁ Telophase (F): The two distinct groups of daughter chromosomes (chromatids) appear fused and tightly packed. Cytoplasmic division is completed. Nuclear membranes re-form and nucleoli reappear.

CELL DIVISION Lymph node

➽ Why mitosis?✹ Cells differ in their average life spans. ExamplesEpith. cells of the urinary bladder = 66.5 days Epidermal cells = 19.2 daysEpith. cells of the small intestine = 1.4 days Tracheal cells = 47.6 days

Photomicrograph of cultured cells to show cell division. Picrosirius-hematoxylin stain. Medium magnification. A: Interphase nuclei. Note the chromatin and nucleoli inside each nucleus. B: Prophase. No distinct nuclear envelope, no nucleoli. Condensed chromosomes. C: Metaphase. The chromosomes are located in a plate at the cell equator. D: Late anaphase. The chromosomes are located in both cell poles, to distribute the DNA equally between the daughter cells.

Onion Root Tip Find cells in various stages:Metaphase (green) Telophase (red)

Notice the new cell wall developing between the two daughter nuclei. Others are in interphase. Bar = 30 Microns

Onion Root Tip Find cells in various stages:Prophase (black) Metaphase (green) Anaphase (blue)

Telophase (red) Others are in interphase. Bar = 50 Microns

Onion Root Tip Find cells in various stages:Prophase (black) Metaphase (green) Anaphase (blue)Others are in interphase. Bar = 30 Microns

Onion Root Tip Find cells in various stages:Prophase (black) Metaphase (green) Anaphase (blue)

Most other cells with diffuse nuclei are in interphase.Bar = 50 Microns

Images obtained with a confocal laser scanning microscope from cultured cells

DNA appears red, and microtubules in the cytoplasm are blue

Images obtained with a confocal laser scanning microscope from cultured cells. An interphase nucleus and several nuclei are in several phases of mitosis. DNA appears red, and microtubules in the cytoplasm are blue. Medium magnification.

A: Interphase. A nondividing cell.

B: Prophase. The blue structure over the nucleus is the centrosome. Note that the chromosomes are becoming visible because of their condensation. The cytoplasm is acquiring a round shape typical of cells in mitosis.

C: Metaphase. The chromosomes are organized in an equatorial plane.

D: Anaphase. The chromosomes are pulled to the cell poles through the activity of microtubules.

E: Early telophase. The two sets of chromosomes have arrived at the cell poles to originate the two daughter cells, which will contain sets of chromosomes similar to those in the mother cell.

F: Telophase. The cytoplasm is being divided by a constriction in the cell equator. Note that the daughter cells are round and smaller than the mother cell. Soon they will increase in size and become elongated. (Courtesy of R Manelli-Oliveira, R Cabado, and G Machado-Santelli.)

FUNCTION STAGE

✹ Chromosomes appear Prophase

✹ Cell pinches in two Telophase

✹ Chromosomes line up across the middle Metaphase

✹ Cemtrioles move to opposite sides Prophase

✹ Nucleolus and nuclear membrane reappear Telophase

✹ DNA duplicates Interphase

✹ Nucleolus and nuclear membrane disappears

Prophase

✹ Separated chromosomes move to opposite sides

Anaphase

✹ Chromosomes become chromatin Telophase

Electron micrograph of a section of a rooster spermatocyte in metaphase. The figure shows the two centrioles in each pole, the mitotic spindle formed by microtubules, and the chromosomes in the equatorial plane. The arrows show the insertion of microtubules in the centromeres. Reduced from x19,000. (Courtesy of R McIntosh.)

Electron micrograph of the metaphase of a human lung cell in tissue culture. Note the insertion of microtubules in the centromeres (arrows) of the densely stained chromosomes. Reduced from x50,000. (Courtesy of R McIntosh.)

Phases of the cell cycle in bone tissue. The G1 phase (presynthesis) varies in duration, which depends

on many factors, including the rate of cell division in the tissue. In bone tissue, G1 lasts 25 h.

The S phase (DNA synthesis) lasts about 8 h. The G2-plus-mitosis phase lasts 2.5—3 h.

Phases of the cell cycle The division cycle of most eukaryotic cells is divided into four discrete phases: M, G1, S, and G2. M phase

(mitosis) is usually followed by cytokinesis. S phase is the period during which DNA replication occurs. The cell grows throughout interphase, which includes G1, S, and G2. The relative lengths of the cell cycle phases shown here are typical of rapidly replicating mammalian cells.

Embryonic cell cycles. Early embryonic cell cycles rapidly divide the cytoplasm of the egg into smaller cells. The cells do not grow during these cycles, which lack G1 and G2 and consist simply of short S phases alternating with M phases.

Determination of cellular DNA content. A population of cells is labeled with a fluorescent dye that binds DNA. The cells are then passed through a flow cytometer, which measures the fluorescence intensity of individual cells. The data are plotted as cell number versus fluorescence intensity, which is proportional to DNA content. The distribution shows two peaks, corresponding to cells with DNA contents of 2n and 4n; these cells are in the G1 and G2/M phases of the cycle, respectively. Cells in S phase have DNA contents between 2n and 4n and are distributed between these two peaks.

Regulation of animal cell cycles by growth factors The availability of growth factors controls the animal cell cycle at a point in late G1 called the restriction point. If growth factors are not available during G1,

the cells enter a quiescent stage of the cycle called G0.

Cell cycle checkpoints. Several checkpoints function to ensure that complete genomes are transmitted to daughter cells. One major checkpoint arrests cells in G2 in response to damaged or unreplicated DNA. The presence of damaged DNA also leads to cell cycle arrest at a checkpoint in G1. Another checkpoint, in M phase, arrests mitosis if the daughter chromosomes are not properly aligned on the mitotic spindle.

Role of p53 in G1 arrest induced by DNA damage DNA damage, such as that resulting from irradiation, leads to rapid increases in p53 levels. The protein p53 then signals cell cycle arrest at the G1 checkpoint.

The four phases of the cell cycle. In G1 the cell either

continues the cycle or enters a quiescent phase called G0.

From this phase, most cells can return to the cycle, but some stay in G0 for a long time or even for their entire lifetime. The checking or restriction point (R) in G1 stops the cycle under conditions unfavorable to the cell. When the cell passes this restriction point, it continues the cycle through the synthetic phase (S) and the G2 phase, originating two daughter cells in mitosis (M) except when interrupted by another restriction point (not shown) in G2.

Comparison of meiosis and mitosis. Both meiosis and mitosis initiate after DNA replication, so each chromosome consists of two sister chromatids. In meiosis I, homologous chromosomes pair and then segregate to different cells. Sister chromatids then separate during meiosis II, which resembles a normal mitosis. Meiosis thus gives rise to four haploid daughter cells.

Section of a malignant epithelial skin tumor (squamous cell carcinoma). An increase in the number of cells in mitosis and diversity of nuclear morphology are signs of malignancy. PT stain. Medium magnification.

Section of a fast-growing malignant epithelial skin tumor showing an increased number of cells in mitosis and great diversity of nuclear morphology. PT stain. Medium magnification.

Section of a mammary gland from an animal whose lactation was interrupted for 5 days. Note atrophy of the epithelial cells and dilation of the alveolar lumen, which contains several detached cells in the process of apoptosis, as seen from the nuclear alterations. PT stain. Medium magnification.

Electron micrograph of a cell in apoptosis showing that its cytoplasm is undergoing a process of fragmentation in blebs that preserve their plasma membranes. These blebs are phagocytized by macrophages without eliciting an inflammatory reaction. No cytoplasmic substances are released into the extracellular space.

I). Plasma (cell) Membrane        A). Structure            1). Lipid Bilayer Consists of a double layer of phospholipids :

hydrophilic= polar phosphorous head attract water and face the inner and outer environment

  hydrophobic= nonpolar fatty acid tail avoid water and line up in the center of the membrane     Permeable to molecules that are soluble in lipids: Not permeable to molecules that are soluble in H2O: 2). Proteins     Provide specialized functions for the cell  i).  Receptor Proteins                         Bind with external molecules (hormones) and                         then carry messages into the inside of the cell ii).  Integral proteins    Act as channels for ions.       iii). Peripheral proteins: Associated with part of the membrane and act as enzymes and in signal transduction.       iv).  Glycoproteins contain a branching sugar                         Function in cell adhesion and cell recognition. 

            3). Cholesterol 

        B). Function of cell membrane:  

        C). Membrane Junctions 1). Tight junctions

2). Desmosomes 3). Gap junctions

         D). Cell Adhesion Molecules

                Sticky glycoprotein 1). Selectin 2). Integrin