Bone Formation and Joints - Virtual MicroscopyLab 7 –Bone Formation and Joints Slide 130: Membranous Bone, Fetal Skull A560 –Fall 2015 I. Introduction II. Learning Objectives III.

Lab7– BoneFormationandJointsA560– Fall2015

I. IntroductionII. LearningObjectivesIII. SlidesandMicrographs

A. Bone(cont.)1. Generalstructure2. Cellsa. Osteoblastsb. Osteoclasts

B. BoneFormation1. Intramembranousossification2. Endochondralossification

C. Joints1. Synovial2. Intervertebral

IV. Summary

BoneFormationandJoints

BoneFormationandJoints

1. Bone is a specialized type of connective tissue witha calcified (mineralized) extracellular matrix (ECM);it serves to support the body, protect internalorgans, and acts as the body’s calcium reservoir.

2. Major cells of bone include: osteoblasts (formosteoid which allows matrix mineralization tooccur), osteocytes (from osteoblasts; enclosed inlacunae and maintain the matrix), and osteoclasts(locally erode bone matrix during bone formationand remodeling).

3. Bone growth occurs via two basic mechanisms:intramembranous ossification (bone forms withinmesenchymal membrane) and endochondralossification (bone replaces hyaline cartilage)

4. Joints are places where bones meet (articulate),allowing at least the potential of bending ormovement; examples include, synovial joints(diarthrosis) and intervertebral joints






IV. Summary

Learning Objectives

1. Understand the differences and similarities between intramembranous andendochondral bone formation and the key function of the periosteum inbone growth.

2. Understand the organization of the epiphyseal growth plate and its role inendochondral bone formation and growth of long bones.

3. Understand the structure of a typical synovial joint, including the natureand functions of the synovium.






IV. Summary

104:Bone,H&EBone

Slide129:Tooth,H&E






IV. Summary

Slide129:Tooth,H&E

osteon

perforating(Volkmann’s)canals

centralcanal

osteocytein lacuna






IV. Summary

Slide104:Bone,H&E

osteon

centralcanal

resorptioncanals






IV. Summary

Osteon formation: resorption canals (with dimension of new osteon) are carved out of bone byosteoclasts; blood vessels and connective tissue invade and occupy the tunnel; osteoblasts beginto deposit new bone along the walls, forming lamellae; synthesis continues from periphery tocenter until only central canal with neurovascular bundle remains in center

Slide34:HealingBoneFracture,H&ELab7– BoneFormationandJointsA560– Fall2015





IV. Summary

1

2

Fracture

Callus

HyalineCartilage

ECOss

Corticalbone

Sections (1) and (2) are two rib segments from a fetal/newborn rabbit; (1) gives an example of a bonefracture and repair processes (the callus is a temporary formation of highly proliferative fibroblasts andchondroblasts extending from the periosteum down into the fracture to form new bone); (2) gives anexample of endochondral ossification (EC Oss) bone growth






IV. Summary

From the outside: (CT) is connective tissue proper surrounding the bone, note the abundant vasculaturepresent; (P) is periosteum with clearly visible collagen bundles and (Act P) is an area of more activeperiosteum with lots of cellular differentiation occurring; (M) is marrow with abundant blood cells; (HC)are areas of hyaline cartilage; (Wb) is woven or primary bone being deposited upon hyaline cartilage(basophilic); there is an irregular arrangement of collagen fibers (acidophilic), large cell number, andreduced mineral content; (Tb) is mature (mineralized), trabecular bone; close examination showsendosteum on the edge of the bone bordering the marrow

Tb

Wb

M

ActPP

CT

HC






IV. Summary

From the outside (top): (CT) is connective tissue proper surrounding the bone; (P) is periosteum withclearly visible collagen bundles; (Cb) is cortical bone with lamellar arrangement, note the adjacent centralcanal with surrounding rings of cartilage; (M) is marrow with abundant blood cells; (Tb) is trabecularbone, distinguished from cortical bone by the lack of osteons; close examination shows endosteum on theedge of the bone bordering the marrow; (Wb) is woven or primary bone with an irregular arrangement ofcollagen fibers, large cell number, and reduced mineral content; osteoid (unmineralized bone) is the pale,acellular layer just below osteoblasts which line the edges of the nearby marrow cavities

CT P Cb

M

Tb

PCT

Wb

Slide34:HealingBoneFracture,H&E

endosteumliningtrabecula

marrow






IV. Summary

Endosteum lines all internal surfaces of bone (both cortical and trabecular); itis generally only a single cell‐layer thick, and consists of inactive and activeosteoblasts


osteoidlighterstainedareabetweenendosteum andmaturebone

endosteum

maturebone






IV. Summary

Osteoid is collagen‐rich, non‐mineralized precursor to true bone matrix; itis secreted by osteoblasts during bone growth, repair, and remodeling;osteoblast subsequently calcify the osteoid into bony hard bony matrix; inthe process, they become trapped in the matrix (in lacunae) and becomeosteocytes

inactiveperiosteum:w/flattenedosteoprogenitorcells

maturingbonewithosteocytes

Slide34:HealingBoneFracture,H&ELab7– BoneFormationandJoints

A560– Fall2015





IV. Summary


osteocyte

activeperiosteum:w/roundedorcuboidalosteoblasts






IV. Summary

Osteoblasts develop from osteoprogenitor cells (from mesenchymal cells) and are found lining theexternal and internal bone surfaces; they make osteoid (collagen‐rich matrix) which they thencalcify into true hard, bony matrix; they become trapped in the matrix and become osteocytes

Slide130:MembranousBone,FetalSkull

periosteum

osteoblasts






IV. Summary


Osteoclast

Howship’s lacuna(space)

Osteoclasts are large, multinucleated cells; they resorb bone by secreting organic acids, whichdissolve hydroxyapatite, and lysosomal enzymes, which break down the osteoid matrix; at the bonesurface, osteoclasts lie in Howship's lacunae, surface depressions caused by the resorption of bone

Howmanynucleidoesthisosteoclasthave?

upto200ispossible,5‐20isusual






IV. Summary


Osteoclastwithruffledborder(visibleinEM)inHowship’s lacuna






IV. Summary


osteoclasts






IV. Summary

IntramembranousOssification






IV. Summary

1. Primary center of ossification:mesenchyme cells osteoblasts

2. Osteoblast begin depositing bony matrix to form trabeculae,extending radially from the ossification center

3. Marrow develops in spaces between trabeculae

4. Periosteum and endosteum develop from mesenchymemembrane

5. Surfaces are remodeled to form compact bone

Slide130:MembranousBone,FetalSkullLab7– BoneFormationandJoints

A560– Fall2015





IV. Summary

SideView FrontalView developingteethwithsurroundingintramembranousbonedevelopmentofthejaw

tongueintheoralcavity

nasalcavitywithcartilaginousnasalseptuminthemiddle;surroundedbyintramembranousbonedevelopmentoftheskull

Slide130– FetalSkull


wovenbone






IV. Summary


hyalinecartilage

periosteum

wovenbone






IV. Summary

EndochondralOssificationLab7– BoneFormationandJoints

A560– Fall2015





IV. Summary

1. ZoneofReserveCartilage:hyalinecartilageactsassourceofcartilagetoundergoossification

2. ZoneofProliferation:normalchondrocytesmultiply

3. ZoneofHypertrophy:chondrocytesenlargeandalign

4. ZoneofCalcification:cartilagematrixcalcifies;scaffoldfornewbone

5. ZoneofOssification andResorption:chondrocytesdeteriorateanddie;osteoblastinvadeandboneisdepositedonthecalcifiedmatrix;osteoclastsbeginremodelingprocess

Because the rates of proliferation and destruction are approximatelyequal, the epiphyseal plate does not change thickness; instead, it isdisplaced away from the middle of the diaphysis, resulting ingrowth in length of the bone


GrowthplateEpiphysis(End)

Diaphysis(Middle)

endochondralossification






IV. Summary


zoneofreservecartilage

zoneofchondroblastproliferation

zoneofchondrocytehypertrophy

zoneofcalcification,

ossification,andresorption






IV. Summary


zoneofreservecartilagetypicalhyalinecartilagewithchondrocytesarrangedinsmall

clusters,usuallyonlyonecellperlacuna(notgrouped)






IV. Summary


zoneofchondroblast proliferationchondrocytesaredividingandincreasinginnumber;chondrocytesareslightlylargerinsize,areclosertoneighboringcells,andarebeginningtoformrowsorstacks






IV. Summary


zoneofchondrocytehypertrophychondrocytesarealignedinrows(alongaxisofgrowth)andaresignificantly

largerthaninprecedingzones






IV. Summary


zoneofcalcification,ossification,andresorptionchondrocytesundergoapoptosis(sonucleiaremorecondensed);smallbloodvesselandosteoprogenitor cellsbegintoinvade,givingrisetoosteoblasts

whichbegintolaydownboneandbecomeosteocytes






IV. Summary

Fig8‐19SynovialJointLab7– BoneFormationandJointsA560– Fall2015





IV. Summary

See:Fig8‐19,8‐20,8‐21

Fig8‐22IntervertebralDiscLab7– BoneFormationandJointsA560– Fall2015





IV. Summary

Vertebrawithbonemarrowcavity(BM)

Vertebrawithbonemarrowcavity

Concentriclayersoffibrocartilageformtheannulusfibrosus (AF)(Lt.“fibrousring”)

Nucleuspulposus (NP)istheinnercoreofthevertebraldisc;itiscomposedofagel‐likematrixconsistingofwaterandaloosenetworkofcollagenfibers






IV. Summary

Common Confusion:Cartilage vs. Bone

Hyaline cartilage

Cartilage: a specialized type of solid connective tissuewhich, along with bone, is distinguished by its relativerigidity of the extracellular matrix (ECM); it isdistinguishable from bone by its avascularity in addition toits lack of lymphatics and nerves; hyaline, the mostcommon type, is pictured here

Look for: (1) lack of vasculature, lymphatics, and nerves;(2) large proportion of cells to ECM; (3) chondrocytes mayoccur as pairs within a lacuna and tend to be clusteredtogether (isogenous groups); (4) chondrocytes are usuallylarger and rounder than osteocytes; (5) matrix is often notuniform and tends to heterogeneously stain

Bone: a specialized type of solid connective tissuecharacterized by a mineralized ECM that stores calciumand phosphate

Look for: (1) vasculature; note the central canal (osteon)containing visible erythrocytes; (2) more ECM and fewercells; (3) osteocytes are singular and spaced apart; (4)osteocytes are smaller and more condensed thanchondrocytes; (5) canals and lamellar organization may bevisible; (6) canaliculi may be visible linking osteocytes

Decalcified bone






IV. Summary

Summary

1. During bone formation, woven bone (haphazard arrangement of collagenfibers) gets remodeled into lamellar bone (parallel bundles of collagen inlayers known as lamellae).

2. Periosteum is a layer of dense connective tissue on the outer surface ofbone; endosteum is a thin layer (generally only one cell layer) which linesall the internal surfaces of bone.

3. Major cells of bone include: osteoblasts (from osteoprogenitor cells; formosteoid which allows matrix mineralization to occur), osteocytes (fromosteoblasts; enclosed in lacunae and maintain the matrix), and osteoclasts(from hematopoietic lineage; locally erode bone matrix during boneformation and remodeling).

4. Bone growth occurs via two basic mechanisms:• intramembranous ossification occurs when bone forms within

mesenchymal membrane; forms bones of skull and jaw; primarilyoccurs only during development or fracture repair

• endochondral ossification occurs when bone replaces hyalinecartilage; forms and grows all other bones except as noted for IM;occurs during development and throughout life






IV. Summary

Summary(cont.)

5. During epiphyseal growth (elongation of bone), the growth plate, with itszonal organization of endochondral ossification, allows bone to lengthenwithout the epiphyseal growth plate enlarging; zones include:

• Zone of reserve cartilage• Zone of proliferation• Zone of hypertrophy• Zone of calcification• Zone of ossification and resorption

6. Joints are places where bones meet (articulate), allowing at least thepotential of bending or movement; examples include, synovial joints(diarthrosis) and intervertebral joints (with tough outer layer offibrocartilage known as annulus fibrosus, and gel‐like core known asnucleus pulposus).

Osteoblasts Osteocytes Osteoclasts

Precursor cell

Location

Percentage of all cells in bone

Function

Appearance

Sketch

Lab 7: Summary Features of Major Cells of Bone Tissue

Intramembranous Ossification Endochondral Ossification

Bones produced

Cartilage present

Type of bone produced

When occurs

Steps Involved

Lab 7: Comparison of Mechanisms of Bone Formation

Bone Formation and Joints - Virtual MicroscopyLab 7 –Bone Formation and Joints Slide 130: Membranous Bone, Fetal Skull A560 –Fall 2015 I. Introduction II. Learning Objectives III.

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