Supporting tissues (cartilage and bone)

Bone lamella

-3-5um -Mature bone Components: -Mineralized extracellular substance with paralelly oriented collagen fibers -Lacunae and canaliculi containg ostecytes and their processes Two types: -Spongy(cancellous) bone -Compact bone

Osteon

-A cylinder a few mm long, up to 200 um in diameter -Built of 5-15 concentric bone lamellae with vascular canal in center -Containing a capillary, nerve fibres, a few osteogenic cells, inacive osteblasts and -clasts -Surrounded by mineralized cementing line -No communication between osteons -oxygen and nutrient supply from blood vessels located in Haversian canal (centrifugal diffusion)

Joint (diarthrosis)

-Articular cartilage har no perichondrium. Joint capsule: -Fibrous layer(dense connective tissue) -Synovial layer(membrane): Loose connective tissue, macrophages(A cells), fibroblasts(B cells)-secrete synovial fluid -Blood vessels

Fibrocartilage

-Chondral territories(few) -Parallel bundles of collagen fibres type 1 -Ground substance(small amounts) -No perichondrium Location: -Intervertebral disks -Pubic symphysis -Some attachments to bone (continous with tendons) Mechanical properties: Resistant to stretch

Hyaline cartilage

-Chondral territories. -Meshwork of thin fibres built of collagen type 2. -Ground substance rich in giant proteoglycan (aggrecan) aggregates. Location: -Articular surfaces -Nose, larynx, trachea, bronchi -In fetus: models of long bones Very resistant to compression(result from the properties of highly hydrated proteoglycans)

Elastic cartilage

-Chondral territories. -Meshwork of thin fibres built of collagen type 2. -Network of elastic fibres. -Ground substance. Location: -Auricle -Auditory tubes -Larynx(epiglottis) Mechanical properties: Elastic

Compact bone

-Concentric lamellae form osteons(Haversian systems) around blood vessels located in the central vascular canals (Haversian canals) Location: Shafts of long bones, superficial(cortical layer) of the bones

Osteoblasts

-Cuboidal, basophilic cytoplasm -RER, Golgi -Produce components of extracellular substances and control its mineralization. -Active in fetal bone development, mature bone repair and remodeling -Orginate from osteoprogenitor cells -In mature bone found in areas of repair and remodeling -After cessation of activity, transform into ostecytes Control differentiation and recruitment of osteclasts: -Secrete M-CSF which induces expression of RANK receptors in mononuclear osteoclasts precursors. -Express ligand for that receptor(RANKL)- binding of ligand to receptor induces differentiation and fusion of precursors into multinucleated osteoclasts (Stimulation of osteoclast formation) -Produce osteoprotegerin (OPG) which blocks RANK(inhibition of osteoclasts formation)

Osteocytes

-Large nucleus -Poorly developed organelles -Long processes interconnected by gap junctions Ocupy the nonmineralized spaces in extracellular substance: Lacunae and bone canaliculi Participates in bone remodeling and in keeping the calcium balance in blood. Respond to changes in mechanical load by changing their functional states and sending signals to other ostecytes, osteoblasts and osteoclasts

Characteristics common for all cartilage types

-No blood vessels -Ground substance rich in chondroitin sulphates -Chondrocytes residing in lacunae: singly or in small groups(isogenous groups) -Lacunae are surrounded by condensed ground substance (matrix) and are called chondral territories

Spongy bone

-Parallel lamellae build bone trabeculae. -Interconnected trabeculae form a network, with bone marrow(containg blood vessels) in the meshes Location: Flat and irregular bones, epiphyses of long bones

Osteoprogenitor cells

-Spindle-shaped, pale cytoplasm -Active in fetal bone developement, mature bone repair and remodeling Location in mature bone: in the periostenum, Haversian canals and endosteum(inner bone surface)

Types of lamellae in long bone shafts

-Systemic -Intersystemic(interstitial) -Circumferential (outer/inner) Periosteum: A layer of dense connective tissue containg blood vessels, osteogenitor cells and inactive osteoclasts. Endosteum: A single layer of flat cells(bone lining cells-inactive osteblasts) lining the inner surface of bone

Inframembranous ossification

1.Mesenchymal cells produce a small area of mineralized bone matrix(osteoid) 2. Mesenchymal cell differentiate into osteoprogenitor cells and osteoblasts which produce and mineralize the ECF 3.Osteoblasts become interconnected by cytoplasmic processes, surround themselves by mineralized ECS and are transformed into osteocytes Primary bone trabeculae are formed and undergo further remodeling. Spongy bone is formed this way.

How osteoclasts digest bone

1: Sealing off the space beneath the ruffles border by integrin ring 2: Release of H+ ions (proton pump)-acidification-dissolution of hydroxyapatites(local demineralization) 3: Release of lysosomal enzymes(exocytosis of hydrolase vesicles)-extracellular digestion 4:Endocytosis of predigested bone fragments and further intracellular digestion.

Endochondral ossification

A complex of series of events leading to transformation of a fetal long bone model built of hyaline cartilage into long bone with bone marrow cavity and epiphyseal (growth) plates 1: Hypertrophy of chondrocytes in the center of the shaft (primary ossification center) and formation of bone collar around the shaft. 2:Degeneration and mineralization of cartilage inside the shaft. 3: Invasion of periosteal bud with blood vessels and osteoprogenitor cells. 4: Formation/destruction of new bone-formation of bone marrow cavity 5: Formation of secondary ossification centers in the epiphyses, ossification of epiphyses, formation of cartilaginrous growth plates. 6: Ossification of growth plates, termination of bone growth.

Perichondrium

A layer of dense connective tissue covering the cartilage surface. Contain blood vessels supplying oxygen and nutrients.

Chondroblasts

Actively produce components of the extracellular substance during cartilage formation, afterwards transform into chondrocytes

Ossification

Bone formation -Inframembranous -endochondral

Osteoclasts

Bone marrow-derived(macrophage-releated) cells able to destroy the bone tissue by combined extra and intracellular digestion: they are crucial for bone formation, repair and remodeling. -Large multinucleated -Ruffled border -Numerous endocytotic vesicles, hydrolase vesicles and lysosomes -Orgin: bone marrow(by fusion of mononuclear precursor cells.

Chondrocytes

Cells of mature cartilage, continue to produce components of the extracellular substances (in lesser amounts)

Bone (cells+extracellular substance)

Cells: Osteoprogenitor cells-osteoblasts-osteocytes, osteclasts Extracellular substance: -Calcium phosphate(70%)- hydroxyapatite crystals(40x25x3 nm, mounted between and within collagen fibrils, also present in the ground substance) -Collagen type 1 fibres -Ground substance(osteold): proteoglycans, glycoproteins

Cartilage

Cells: chondrogenic cells-chondroblasts-chondrocytes Extracellular substances: -Collagen fibres -Aggregates of proteoglycans rich in chondroitin sulphates -Glycoproteins (anchorin, tenascin, fibronectin, chondronectin)

Formative osteocytes

More RER and golgi, produce small amounts of ECS

Bone glycoproteins

Osteonectin: Binds HA crystals to collagen Osteocalcin: Binds Ca, participates in mineralization Osteopontin: Binds cells with ground substance Bone sialoprotein 2: As above, also promotes mineralization Enzymes(Proteases, alkaline phosphatase): Participate in mineralization and bone remodeling Bone morphogenetic proteins: Promote differentiation of osteoblasts, induce formation of new bone.

Quiescent(resting) osteocytes

Poorly developed organelles

Haversian canals

Recieve blood vessels from larger vascular(Volksmanns) canals, which contain vessels entering the bone from periosteum or from bone marrow cavity

Resorptive osteocytes

Some RER, Golgi, lysosomes- break down the surrounding ECS by releasing proteinases(MMPs), liberate Ca ions

Chondrogenic cells

Stem cells for cartilage, can transform into chondroblasts.