Lecture 10. Skeletal System, Chondrogenesis and Osteogenesis (review lecture 3)

Cartilage: semi-rigid matrix; virtually avascular (nutrients supplied by diffusion from vessels in perichondrium and synovial fluid); no lymphatic drainage or nerves.

cartilage canals: convey small vessels to other tissues but not cartilage; brought into center of cartilage mass where particularly thick (e.g., costal cartilage)
chondroblasts: formed from stellate mesenchyme cells or embryonic fibroblasts; proliferate during growth; synthesize ground substance and fibrous ECM;
chondrocytes: mature chondroblasts; maintain integrity of cartilage matrix; small nuclei with dispersed chromatin and basophilic, granular cytoplasm reflecting a well developed rER; space (shrinkage artifact) in ECM = lacuna; lg lipid droplets esp. in larger chondrocytes; separated by matrix;
isogenous clusters: 2 or 4 with thin layer matrix; permits interstitial growth in embryonic cartilage
ground substance: merges with perichondrium inner (territorial or capsular) zone surrounds chondrocytes: rich in GAGs, poor in collagen; basophilic; chondrocytes in clusters appear to secrete fresh ECM; continuously turned over, dependent on viabiity of chondrocyte
outer  (interterritorial) zone : pale-staining; contains numerous collagen fibrils

fibers: collagen type II, with collagen type I and small amounts of other types, especially IX, X and XI; elastin in special cartilages

glycosaminoglycans (GAGs): chains of hyaluronic acid;  chondroitin 4-sulfate, chondroitin 6-sulfate, keratan sulfate
proteoglycans: bottlebrushes with stems of core proteins and with bristles of sulfated glycosaminoglycans covalently linked to core proteins
proteoglycan aggregates (up to 4 µm in length): proetoglycans noncovalently associated with hyaluronic acid via link protein and interacting with collagen

glycoprotein: chondronectin (binds collagen type II and GAGs): adhesion chondrocytes to matrix

perichondrium: chondrogenic layer: contains chondroblasts; provides appositional growth
fibrous layer: dense regular ct; attachment sites; peripheral chondrocytes resemble fibroblasts; collagen type I
hyaline cartilage: small aggregates of chondrocytes (isogenous groups) embedded in matrix; perichondrium; growth both interstitial and appositional; type II collagen ; type I in articulate surfaces; perichondrium except at articulate surfaces

elastic (fibroelastic) cartilage: perichondrium; growth both interstitial and appositional growth; type II collagen; perichondrium

Bone: ECM mineralized (hydroxyapatite) ground substance; fibrous matrix; collagen type I (banded), dynamic state of growth & resorption. Endosteum and periosteum line internal cavities and cover external surface; osteoclasts resorb mineralized bone.

two main developmental varieties (and stages of development):

primary, immature or woven: immature; random weave of coarse collagen fibers; first bone to develop; remodelled into lamellar bone; first bone laid down at site of fracture

secondary, mature or lamellar: most mature skeleton; successive layers with organized infrastructure [note: lamellar bone is either circumferential at edge of bone or concentric in osteons]

osteoprogenitor cells: derived from periosteum and endosteum
osteoblasts: synthesize and secrete osteoid = organic (matrix) components of ECM before mineralization; mineralized to form bone;
osteocytes: bone-entrapped osteoblast; maintains matrix; connected via gap junctions; secretion Ca²⁺ controlled by local concentrations, parathormone and calcitonin; also respond to piezo-electric currents induced by deformation;
monocyte-macrophage derived osteoclasts: multinucleated; bone resorption associated with continuous remodeling; in resorption lacuna (Howships lacuna); fine microvilli form ruffled border that secretes organic acids capable of dissolving mineral component of bone; lysosomal proteolytic enzymes destroy organic matrix.

Endochondral ossification: long bones, vertebrae, pelvis and bones of base of skull; preceded by cartilage model diaphysis (two growing points): bony collar formed appositionally around shaft of cartilagenous model

epiphysis (upon the growing point): originally hyaline cartilage at end of shaft

growth of epiphysial plate: hyaline cartilage plate at junction of diaphysis and epiphysis (5 zones)

zone of reserve cartilage: typical hyaline cartilage

zone of proliferation: columns of chondrocytes formed by successive mitotic divisions; matrix deeply stained (rich in proteoglycan)

zone of hypertrophy ([enlarging or swelling] hypertrophying cells and lacunae) = (zone of maturation: end division; increase in size of lacunae)

zone of calcification (calcifying cartilage): matrix calcified around enlarged spaces = (zone of cartilage degeneration: degeneration chondrocytes; invasion calcified cartilage by osteogenic cells in wake of vasculature)

zone of erosion and ossification (osteogenic zone: osteogenic cells) differentiate into ostebolasts; commence formation woven bone on surface of spicules of calcified cartilage; followed by extensive remodelling to produce mature compact and spongy bone

metaphysis (change in growth): junction of shaft with growth plate
  primary ossification: between ends of original cartilage model
secondary ossification: conversion of central epiphysial cartilage to bone
  at maturity: endochondral ossification ceases; fusion of diaphysis with epiphysis resulting in obliteration of growth plates

fracture repair: blood clot initially; replaced by highly vascular collagenous tissue = granulation tissue; becomes more fibrous; mesenchymal cells differentiate into chondroblasts; replace fibrous granulation tissue with hyaline cartilage = provisional callus; strengthened by deposition of calcium salts within cartilage matrix; osteoprogenitor cells lay down meshwork of woven bone within and around provisional callus which becomes bony callus; fracture site is bridged by woven bone in bony union; remodelled to mature lamellar bone

Intramembranous ossification: bones of vault of skull, maxilla and most of mandible; deposition bone within primitive mesenchymal tissue; direct replacement mesenchyme by bone; membranous bone mesenchyme begins synthesis of osteoid at several centers of ossification; osteoprogenitor cells at surface undergo mitosis; produce further osteoblasts; lay down more bone; adjacent centers fuse; bone spongy in gross appearance; progressive remodelling by osteoclastic resorption and osteoblastic deposition; forms mature compact or spongy bone; Joints: two main functional groups synovial  (NL;  between oval surfaces)= diarthroses (Gk. to fasten by a joint): extensive movement at articular surfaces; maintained in apposition by fibrous capsule and ligaments; surfaces lubricated by synovial fluid synovium: specialized layer of collagenous tissue lining inner aspect of capsule; inner surface discontinuous layer cells up to four synovial cells deep; no junctional complexes or basement membrane; mesenchymal origin; type A synovocytes: plump, majority; extensive Golgi complex, numerous lysosomes suggestive of macrophages; type B synovocytes: profuse rER; represent fibroblasts; areolar synovium: synovium of loose CT

fibrous synovium: denser collagenous type; rich network of capillaries and thick strands of collagen

adipose synovium: compoed of fat; intra-articular fat pads
 

synovial fluid: fluid from synovial extracellular matrix (not a secretion); hyaluronic acid and associated glycoproteins secreted by type B synovocytes; transudate from synovial capillaries; also small number of leucocytes, predominantly monocytes.

articular cartilages: no perichondrium; collagen of type I (in contrast to type II collagen of hyaline cartilage)

bony end plate; lacks osteons and canaliculi; osteocytes occupy large lacunae; thick layer of glycoprotein-rich substance (resembles cement lines between osteons) demarcates bony plate from articulate surface.

temporomandibular and knee joints: fibrocartilage completely or partially interposed btwn articular surfaces but unattached

intervertebral joints: thick ligaments extending down anterior aspect of spinal column merges with anulus fibrosus; thinner ligament posterior;

zygoapophyseal joints: (zyg = yoked) synovial joints btwn vertebral arches; stabilized by strong elastic ligaments connecting bony processes

intervertebral disk = symphysial joints (i.e., fibrocartilage type); fibrocartilage arranged in concentric rings - annulus fibrosus reinforced peripherally by circumferential ligaments; central cavity containing viscous fluid = nucleus pulposus containing physaliphorous cells (nucleus pulposus remnant of notochord)

slipped disk: fibrocartilage of annulus fibrosus thins and weakens; nucleus pulposus extruded, particularly postero-lateral aspect. tendon: densest form of collagenous supporting tissue

nonsynovial: limited movement; no free articular surfaces; joined by dense collagenous tissue dense fibrous tissue: sutures btwn bones of skull; syndesmoses = fibrous tissue joints; synostoses = after replacement by bone

hyaline cartilage: synchondrosis or primary cartilaginous joint; unites first rib with sternum

fibrocartilage: hyaline cartilage at ends of apposed bones connected to each other by plate of fibrocartilage; = symphyses or secondary cartilaginous joints; public symphysis (develops central cavity) and intervertebral disks (have fluid-filled central cavity)
 

last revised: 02-01-05