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Connective tissueFunctions of CT:a. Structural supportb. A
medium for exchange (so it is vascular).c. Helps in defense &
protection of the body.d. A site for storage of fat.TYPES:1.CT.
proper. 2. Embryonic CT.3. Specialized CT (cartilage, bone and
blood).Components of CT:1-Cells.2-Extracellular matrix (ground
substances and fibers).
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Extracellular Matrix I-Ground substanceIt is a hydrated,
amorphous material that is composed of: Glycosaminoglycans,
proteoglycans and adhesive glycoproteins as laminin, chondronectin,
osteonectin and fibronectin. II-Fibers*Collagen fibers, are
inelastic and possess great tensile strength. Each fiber is
composed of fine subunits called tropocollagen molecule.Most of the
fibers show axial periodicity by EM.There are six major collagen
types: Type I: in CT proper, bone, dentin and cementum
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Type II: In hyaline and elastic cartilage.Type III: reticular
fibers.Type IV: Lamina densa of the basal lamina.It does not
possess a periodicity and not assembled into fibers.Type V:
Associated with type I collagen and in the placenta.Type VII:
Attaching the basal lamina to the lamina reticularis.The previous
types of collagen show fibrillar structure except type IV that form
meshwork.*Elastic fibers: Are composed of elastin and
microfibrils.They are formed by fibroblasts as well as smooth
muscle cells of blood vessels.
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CT.CellsI-Fixed cells: 1.Fibroblasts 2.Adipose cells.
3.Pericytes. 4.Mast cells. 5.Macrophages.II-Transient cells:
1.Plasma cells. 2.Lymphocytes. 3.neutrophils, eosinophils and
basophils. 4.Macrophages. 5.Monocytes.
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Fixed CT. Cells I-Fibroblasts They are the most abundant CT.
cells and derived from undifferentiated mesenchymal cells.Types: 1-
Active fibroblasts: are elongated, fusiform cells with
pale-staining cytoplasm rich in RER and dark-stained large ovoid
nucleus containing well-defined nucleolus. 2- Inactive fibroblasts
(fibrocytes): are smaller and are more ovoid with more acidophilic
cytoplasm.* Fibrobalsts are responsible for the synthesis of almost
of extracellular matrix.
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II-Adipose cells (fat cells or adipocytes):They are derived from
undifferentiated mesenchymal cells.They are fully differentiated
and do not undergo cell division.There are 2 types: A-Unilocular
fat cells, form white adipose tissue: they are large cells, they
store fat as one droplet, which enlarge pushing the cytoplasm and
the nucleus peripherally against cell membrane (signet-ring
appearance). They have few mitochondria. The fat droplet is not
bounded by a membrane. B-Multilocular adipocytes, form brown
adipose tissue:Are small cells with multiple fat droplets, central
spherical nucleus and many mitochondria. * They function in the
synthesis, storage and release of fat.
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IV.Mast cells:They are derived from bone marrow stem cells.Their
cytoplasm is rich in membrane-bound granules that stained
metachromatically with toluidine blue.The granules contain heparin,
histamine, eosinophil chemotactic factor and neutrophil chemotactic
factor.They are present in subepithelial CT.,CT. around small blood
vessels, and subepithelial CT. of digestive and respiratory tracts
(mucosal mast cells that secrete histamine).* Histamine causes
vasodilatation and increase permeability of blood vessels. It
causes bronchospasm and increase mucus production.
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V-MacrophagesThey are active mononuclear phagocytes.Some are
fixed and others are transient .They are irregular in shape and
have filopodia.Their cytoplasm Is basophilic with small eccentric
indented nucleus, prominent RER, well developed Golgi and an
abundance of lysosomes.They derived from monocytes that localized
in different regions, EX. in liver (Kupffer cells), in lung (dust
cells), in CT.(macrophage), in skin (Langerhans cells) and in blood
(monocytes). Several monocytes can fuse together forming
foreign-body giant cell.They phagocytose foreign substances and
debris.
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Transient CT. cellsI-Plasma cellsThey are derived from
B-lymphocytes after exposure to an antigen.They secrete
antibodies.They are large ovoid cells with intensely basophilic
cytoplasm that is rich in RER and Golgi (pale-staining region
adjacent to the nucleus that has chromatin radiating out from the
center (clock-face appearance).
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II-LeukocytesThey are white blood cells that circulate in blood
stream then migrate through capillaries to enter CT. during
inflammations, invasion by foreign elements and immune
response.
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Classification of CT.A-Embryonic CT:1.Mesenchymal CT: Is present
only in the embryo (in adult in pulp of teeth) and consists of
mesenchymal cells in a gel-like ground substance containing
scattered reticular fibers.Mesenchymal cells are small cells with
pale staining cytoplasm with small processes and a small pale
nucleus with prominent nucleolus.2.Mucous tissue:It is formed of
hyaluronic acid, collagen types I &III and fibroblasts.It is
known as Whartons jelly and is found in umblical cord and
sub-dermal CT. of the embryo
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B-Connective tissue proper1- Loose (areolar CT):It fills in the
spaces of the body just deep to the skin, below the mesothelium
lining of internal body cavity, adventitia of blood vessels and
surrounds the parenchyma of glands.The loose CT of mucous membranes
is called lamina propria.It is characterized by abundant ground
substance and tissue fluid housing the fixed CT. cells,
undifferentiated cells and collagen, reticular and elastic
fibers.
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2.Reticular CT: Type III collagen is its major component.
Collagen forms mesh-like networks interspersed with fibroblast and
macrophages. It forms the architectural framework of liver
sinusoids, adipose tissue, lymph nodes, spleen, smooth muscle and
islets of Langerhans
. 3.Adipose tissue: It is divided into white (unilocular)
adipose tissue and brown (multilocular) adipose tissue. It is rich
in blood vessels
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4. Dense CT:It has many more fibers and fewer cells than
ordinary CT.It is formed of three types:a.Dense irregular
collagenous CT: it is formed of randomly arranged collagen fibers,
few ground substances and fibroblasts scattering between collagen
fibers. It constitutes the dermis of skin, the sheath of nerves and
capsules of spleen, testes, ovary and lymph nodes.
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b. Dense regullar collagenous CT: it is composed of coarse
collagen bundles that oriented into parallel sheets. It has few
ground substances and fibroblasts between collagen bundles.Ex.
Tendons, ligaments and aponeuroses.
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c.Dense regular elastic CT: it has coarse branching elastic
fibers with only collagen fibers forming networks and scattered
fibroblasts. Elastic fibers are arranged parallel to one another
and form either thin sheets or fenestrated membranes as in large
blood vessels, ligamenta flava and suspensory ligament of
penis.
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Basement membrane It is a cellular region that interface between
epithelium and the underlying CT. It is visible by light microscope
and stained by PAS.External lamina, is similar to basement membrane
and surrounds smooth muscle, skeletal muscle, adipocytes and
Schwann cells.By EM. It is composed of: 1. Basal lamina which is
formed by the epithelium and it is consists of two layers : A.
Lamina lucida. It consists of extracellular glycoproteins laminin
and entactin.B. Lamina densa, comprises a meshwork of type IV
collagen.
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2. Lamina reticularis: which is composed of type I &III
collagen that manufacture by fibroblasts of underlying CT. It is
thick in skin and very thin around alveoli.
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