Skeletogenic Connective Tissues

Cartilages and Bones

Skeletogenic Connective Tissues

Cartilages

Cartilage is a kind of connective tissue

arises from the mesenchyme contains extensive matrix

more than 95% of cartilage volume

Types of cartilages

Hyaline cartilage Elastic cartilage Fibrocartilage

Hyaline cartilage is the most common cartilage in the body

Localization nose, larynx, trachea, and bronchi cartilaginous parts of the ribs articular surfaces of bones epiphyseal plates of the growing bones fetal skeleton

Hyaline cartilage matrix is firm but pliable

Composition: collagen fibrils amorphous ground substance

hyaluronic acidproteoglycansglycoproteins

water

Type II collagen is the major matrix protein

Collagen fibrils are not discernible in histologic sections

because the collagen fibrils are very fine have the same refractive index as the amorphous ground substance

Cartilage matrix organization

Proteoglycan monomers consist of core proteins and chondroitin or keratan sulfates

Monomers are bound to hyaluronic acid by link proteins to form large aggregates

Aggregates are bound to collagen fibrils

Multiadhesive glycoproteins (anchorin, fibronectin)anchor cells to the cartilage matrix

Proteoglycans have an affinity to water molecules

Much of this water is tightly bound to the proteoglycans

cartilage resilience

Some of this water is loosely bound to the proteoglycans

high cartilage matrix permeability

Hyaline cartilage matrix is highly hydrated

Cartilage is avascular tissue

Cartilage lacks blood vessels, lymphatics, and nerves Cartilage matrix is highly permeable Cartilage is supplied by diffusion Diffusion starts from the perichondrium

Substances freely pass cartilage matrix,except high molecular weight proteins

Perichondrium is a connective tissue cartilage envelope

Outer fibrous layerdense connective tissue

Inner cellular layer loose connective tissueblood vesselscondrogenic cells chondroblasts

Perichondrium functions

cartilage protection cartilage supply new cartilage cell formation cartilage growth

Articular cartilage lacks the perichondrium

it is supplied by diffusion from synovial fluid

Cartilage cells

Chondrogenic cells reside in the perichondrium give rise to chondroblasts

Chondroblasts reside in the perichondrium are capable of mitotic divisions secrete extracellular matrix give rise to chondrocytes

Chondrocytes are surrounded by matrix are housed in lacunae

are capable of mitotic divisions secrete extracellular matrix mature cells form isogenous groups

Isogenous groups

are located in the mature cartilage zone contain from 4 to 8 mature chondrocytes result from successive mitotic divisions are surrounded by cartilage matrix

Chondroblasts and chondrocytes secrete cartilage matrix and are similar in morphology

Chondrocyte ultrastructure

abundant rER extensive Golgi apparatus a few mitochondria (anaerobic cells)

many vacuoles with collagen precursors and glycoproteins large amounts of lipid and glycogen inclusions

Cartilage growth mechanisms

Appositional growthchondroblast mitotic divisionchondroblast matrix secretion

Interstitional growthchondrocyte mitotic divisionchondrocyte matrix secretion

Hormonal regulation of matrix secretion and cartilage growth

Stimulation - STH, thyroxine, testosterone Inhibition – glucocorticoids, estrogens

Age-related hyaline cartilage changes

matrix composition changes matrix permeability becomes worse chondrocytes swell and die matrix undergoes mineralization mineralized cartilage is replaced by bone

mineralized cartilage is basophilic

Elastic cartilage

is found in the ear, auditory tube, epiglottis, and the larynx contains matrix rich in elastic fibers possesses the perichondrium shows isogenous groups is never calcified with age

elastic fibrils are revealed by orcein

Fibrocartilage

exists in the annulus fibrosus of intervertebral disks the pubic symphysis menisci of the knee jointsin the large tendon insertions

its matrix contains fewer amorphous ground substance more collagen fibers

lacks the perichondrium is capable of calcification

Fibrocartilage matrix contains type I and type II collagen

collagen fibers are arranged in parallel bundles

chondrocytes lie longitudinally

between collagen bundles

Cartilage has limited ability for repair

owing to cartilage avascularity chondrocyte immobility limited mature chondrocyte proliferation

Some repair can occur if defect involves the perichondrium

Skeletogenic Connective Tissues

Bones

Bone as an organ

Consists of bone tissue periosteum endosteum blood vessels articular cartilages bone marrow (red and yellow)

Bone functions

serves as a support for the body provides for attachment of muscles protects the CNS and vital organs contains the red and yellow bone marrow is an important calcium reserve

Bone tissue matrix

Organic portiontype 1 collagen fibers amorphous ground substance (chondroitin sulphate, keratan sulphate, and osteomucoid)

Inorganic portion calcium, phosphorus, citrate, bicarbonate, magnesium,potassium, sodium, and hydroxyapatite crystals [Ca10(PO4)6(OH)2]

Mineralized bone matrix prevents free diffusion

Bone contains blood vessels enter from the periosteum and endosteum penetrate the matrix via Volkmann’ canals run perpendicularly to the bone surface branch to form the haversian canal vessels

Periosteum covers bones, endosteum lines the marrow cavities

Outer fibrous layerdense connective tissue

Inner cellular layerloose connective tissueblood vesselsosteogenic cells and osteoblasts

Functions: bone growth bone repair

blood supplySharpey’s fibers attach the periosteum to bone

Bone cells

Osteogenic (stem) cells- reside in the periosteum, endosteum, and around bone blood vessels- give rise to osteoblasts

Osteoblastsgive rise to osteocytes

Osteocytes Osteoclasts

arise from blood monocytes

Osteoblasts

reside in the periosteum, endosteum, and around the bone blood vessels

contain well-developed rERGolgi apparatusgranules with matrix precursors

Functions: secrete the bone matrix organic portion take part in matrix mineralization

Osteogenic cell and osteoblasts (electron microphotograph)

Osteocytes

reside in their own lacunae in the bone matrix possess narrow cytoplasmic processes extending through the canaliculi communicate with each other via gap junctions contain heterochromatic nuclei, sparse rER,

and small Golgi apparatus

Function: maintain the bone homeostasis

Osteocyte morphology

Osteocyte in its lacuna (scanning electron micrograph)

Osteoclasts

reside in the periosteum, endosteum, and around the bone blood vessels

are located in the bone surface depressions are multinucleated giant cells display the plasmalemma ruffled border contain numerous lysosomes and phagosomes

Functions: are bone macrophages, responsible for bone resorption and bone remodeling

Osteoclast morphology

Bone matrix resorption

is activated byparathyroid hormone

is inhibited bycalciotonin

Bone tissue types

Coarsely bundled bone tissue (primary, immature)its collagen fibers are arranged in thick bundlesexists mostly in embryogenesisis replaced by secondary bone tissueremains in the tooth sockets, skull sutures, and tendon insertions

Lamellar bone tissue (secondary, mature)its collagen fibers are arranged in lamellaereplaces primary bone tissueexists in postnatal life

Lamellar bone substance types

Spongy bone substance is found at the epiphyses and within the long bone diaphyses, within the flat and short bones lamellae are arranged in irregular trabeculae contains bone marrow spaces

Lamellar bone substance types

Compact bone substance is found at periphery of the long bone diaphyses, the flat and shot bones its lamellae are arranged in osteons

Osteon or haversian system

is cylindrical in shape is composed of 4 to 20 concentric lamellae osteocytes are in lacunae between lamellae

Central haversian canal

Contains blood vessels nerve fibers loose connective tissue osteoblasts and osteoclasts

Compact substance of the long bone diaphysis

Lamella arrangement outer circumferential lamellae osteon layer

osteonsinterstitial lamellae

inner circumferential lamellae

Bone remodeling continues throughout life

it contributes the bone adaptation

Interstitial lamellae are remnants of former osteons

Bone tissues arise from mesenchyme

Intramembranous bone formation direct osteogenesis is typical of flat bone development

Endochondral bone formation indirect osteogenesis occurs in the place of a hyaline cartilage model is typical of long bone development

Intramembranous bone development

mesenchymal cells differentiate intoosteogenic cells

osteogenic cells give rise to osteoblasts osteoblasts elaborate the bone matrix osteoblasts entrapped in matrix become

osteocytes bone matrix, osteoblasts, and osteocytes

constitute a bony trabecula

Intramembranous bone development

bony trabeculae calcify and join together primary bone is replaced by secondary bone

Endochondral bone development

Primary center of ossification occurs at the cartilage model diaphysis midriff vascularization of the perichondrium transformation of chondrogenic cells to osteogenic cells development of osteoblasts perichondrium transformation into the periosteum

Endochondral bone development (continuation)

formation of the perichondral bone (collar or cuff) calcification of cartilage under the perichondral cuff penetration of the cartilage model by the periosteal buds

Endochondral bone development (continuation)

resorption of the mineralized cartilage formation of the endochondral bone

endochondral bone (red) includes mineralized cartilage (blue)

Endochondral bone development (continuation)

appearance of the secondary center of ossification in the epiphyses formation of the epiphyseal plate of growth

Epiphyseal plate of growth

is between the diaphysis and epiphysis includes 5 zones:(1) Zone of resorption(2) Zone of calcified cartilage(3) Zone of hypertrophy(4) Zone of proliferation(5) Zone of reserve cartilage

(1)

(2)

(3)

(4)

(5)

Epiphyseal plates provide bone growth in length

Epiphyseal closure occurs about 20 years of age STH stimulates bone growth (chondrocyte proliferation) Sex hormones stop bone growth (chondrocyte proliferation)

The End

Thank you for attention!