Cartilage and Bone

BIO 408

Histology

Dr. D. L. Daley

Cartilage

n Cells are called chrondrocytes found in small cavities called lacunae within the extracellular matrix they secrete

n Not vascularized, no nervous innervation, or lymph vessels

n Receives nourishment from blood vessels of nearby connective tissue

Cartilage

n Matrix composed of glycosaminoglycans and proteoglycans in association with collagen and elastic fibers

n Perichondrium - connective tissue sheath that covers most cartilage

n Has an outer fibrous layer and inner cellular layer (secrete the matrix of cartilage)

n Vascularized

n Cartilage of articular surfaces has no perichondrium & receives nourishment from synovial fluid

Three Types of Cartilage

n Hyaline cartilage

n Contains type II collagen - most abundant form

n Elastic Cartilage

n Contains type II collagen and abundant elastic fibers

n Fibrocartilage

n Contains type I collagen - can withstand strong tensile forces

Hyaline Cartilage

n Bluish-gray semitranslucent, pliable substance

n Found in the nose and larynx, on the ventral ends of the ribs where they articulate with the sternum, tracheal rings and bronchi and the articular surfaces of the movable joints

n Also forms the cartilage template of many bones during embryonic development and the epiphyseal plates of growing bones

Cartilage Cells

n Chrondrogenic cells - spindle shaped, narrow cells from mesenchyme

n Can differentiate into both chrondroblasts and osteoprogenitor cells

n Chrondroblasts - from either mesenchymal cells or from chrondrogenic cells from the inner cells layer of the perichrondrium

n Plump basophilic cells

n Chrondrocytes - chrondroblasts that are surrounded with matrix

Matrix of Hyaline Cartilage

n Semitranslucent & blue-gray in color

n Up to 40% dry weight can be collagen

n Primarily type II collagen but types IX, X and XI and other minor collagens are also present in small quantities

n Type II collagen does not form large bundles - fiber orientation is a function of stresses placed on the cartilage

Matrix of Hyaline Cartilage

n Matrix is subdivided into two regions

n Territorial matrix - around each lacuna - poor in collagen & rich in chondrotin sulfate

n Interterritorial matrix - bulk of the matrix - rich in type II collagen - poorer in proteoglycans than the territorial matrix

n Pericellular capsule - small region of matrix immediately around the lacuna - mesh work of collagen fibers imbedded in a basal lamina like substance

n Probably mostly collagen other than type II

Matrix of Hyaline Cartilage

n Rich in aggrecans - large proteoglycans

n Composed of a protein core with glycosaminoglycans molecules covalently linked

n As many as 100 to 200 aggrecan molecules are linked noncovalently to hyaluronic acid - forming huge aggrecan composites - 3 to 4 mm long

n These are negatively charged and attract Na+ which then attracts water

n The matrix can be so hydrated that up to 80% of the wet weight is water

n Also contains the adhesive molecule chondronectin - connects matrix molecules and the cells

Elastic Cartilage

n Located in the pinna of the ear, external and internal auditory tubes, epiglottis and cuneiform cartilage of the larynx

n Very similar to hyaline cartilage - however the outer layer of the perichondrium is rich in elastic fibers

n The matrix has abundant fine to coarse elastic fibers with type II collagen fiber bundles - more flexibility

n Chrondrocytes are more abundant and larger than in hyaline cartilage

Fibrocartilage

n No perichondrium & scant amount of matrix

n Present in intervertebral disks, in the pubic synthesis, in articular disks and attached to bone

n Fibrocytes often aligned in alternating parallel rows with thick and coarse bundles of collagen - which is parallel to the tensile forces attendant on the tissue

Bone

n Specialized connective tissue with a calcified matrix that incarcerates the cells that secrete it

n Bone typically contains a central cavity which houses bone marrow

n Bone is covered with a periosteum

n Outer layer of dense fibrous connective tissue

n Inner cell layer containing osteoprogenitor cells

n Inner central cavity lining - endosteum - thin connective tissue - monolayer of osteoprogenitor cells and osteoblasts

Bone Matrix - Inorganic

n 65% by dry weight, composed mainly of calcium and phosphorous plus some bicarbonate, citrate, magnesium, sodium and potassium

n Calcium and phosphorous exist as hydroxyapatite crystals

n [Ca₁₀(PO₄)₆(OH)₂]

n Also some calcium phosphate

n These are arranged along type I collagen fibers

n Surface ions of these crystals attract H₂O - form hydration shell - permits ion exchange with extracellular fluid

Bone Matrix - Organic

n Collagen - most type I - 80% to 90%

n Sulfated glycoaminoglycans - chondrotin sulfate and kerratin sulfate

n These form small proteoglycans - with short protein cores - to which glycosaminoglycans are covalently bound

n The proteoglycans link to hyaluronic acid - forming large aggregate composites

n Also several glycoproteins are present - osteocalcin and osteopontin - both bind to hydroxyapatite crystals

Cells of Bone

n Osteoprogenitor cells - found in the inner layer of the periosteum, lining haversian canals and in the endosteum

n From embryonic mesenchyme - spindle shaped with pale staining oval nucleus

n Retain ability to undergo mitosis

n Can differentiate into osteoblasts

Cells of Bone

n Osteoblasts - derived from osteoprogenitor cells - responsible for the synthesis of the organic components of the extracellular matrix

n Cells become surrounded by their own matrix - called osteocytes

n They are called bone-lining cells when inactive

n Have parathyroid hormone (PTH) receptors

n When bound with PTH they secrete - osteoprotegerin ligand (OPGL) - this induces preosteoclasts to become osteoclasts

n Also osteoclast stimulating factor is produced

Cells of Bone

n Osteocytes - mature bone cells - housed in lacunae within the bony matrix

n 20,000 to 30,000 per mm³ of bone

n Radiating from the lacunae - tunnel-like spaces called canaliculi

n These hold the cytoplasmic processes of the osteocyte

n These processes make contact with neighboring osteocytes - forming gap junctions

n Small molecules, ions and metabolites are carried in the extracellular fluid of these tiny canals

Cells of Bone

n Osteoclasts - large , motile, multinucleate cells - precursors originate in bone marrow

n These cells are responsible for resorbing bone

n Have receptors for osteoclast-stimulating factor, colony-stimulating factor-1, OPGL, osteoprotegrin and calcitonin

n Occupy shallow depressions, called Howship’s lacunae (region of bone resorption)

Bone Structure

n Long bones - display a shaft between two heads

n Short bones - more or less the same length and width (e.g. carpals)

n Flat bones - flat, plate-like (e.g. bone of the skull)

n Irregular bones - irregular shape - (e.g. sphenoid & ethmoid bones of the skull)

n Sesamoid bone - develop within tendons - they increase mechanical advantage for the muscle across a joint (e.g. patella)

Gross Structure of a Long Bone

n Cut longitudinal section - two types of bone

n Very dense bone on the outside - compact bone

n Porous portion lining the marrow cavity is cancellous or spongy bone

n Has branching bony trabeculae and spicules jutting out from the internal surface of the compact bone into the marrow cavity

n No haversian systems

Gross Structure of a Long Bone

n Bone marrow

n Red bone marrow - forms blood cells

n Yellow bone marrow - stores fat

n Shaft - diaphysis & articular ends - epiphyses

n Each epiphysis is separated from the diaphysis by an ephyseal plate (cartilage) in growing individuals

n Diaphysis covered by a periosteum except where tendons and ligaments insert into the bone

n Sharpey’s fibers connect the periosteum to the compact bone

Lamellar Systems of Compact Bone

n Compact bone composed of very thin layers of bone called lamellae

n Lamellae are arranged in four systems

n Outer circumferential lamellae

n Inner circumferential lamellae

n Osteons

n Interstitial lamellae

Lamellar Systems of Compact Bone

n Outer Circumferential Lamellae

n Outer is just deep to the periosteum - form outer most regions of the diaphysis - contain Sharpey’s fibers anchoring the periosteum to the bone

n Inner Circumferential Lamellae

n Found to completely encircle the marrow cavity

n Trabeculae of spongy bone extend from the inner circumferential lamellae into the marrow cavity

Lamellar Systems of Compact Bone

n Haversian Canal System (Osteon)

n The bulk of compact is haversian canal systems

n Each is composed of cylinders of lamellae, concentrically arranged around a vascular space called the haversian canal

n The osteons commonly bifurcate and each is bounded by a cementing line - mostly calcified ground substance & some collagen

n Collagen fibers are parallel to each other within a lamella but are oriented almost perpendicular to those of adjacent lamellae

Lamellar Systems of Compact Bone

n Haversian Canal System (Osteon)

n Each haversian canal is lined by a layers of osteoblasts and osteoprogenitor cells, houses a neurovascular bundle

n Volkmann’s canals join adjacent osteons - oriented obliquely or perpendicularly to haversian canals

n Lamella are added from the outside(nearest the cementing line) toward the inside of each osteon

n Most osteons are only made of 4 to 20 lamellae

Lamellar Systems of Compact Bone

n Interstitial Lamellae

n As bone is remodelled, osteoclasts resorb the osteons and osteoblasts replace them

n Remnants of of osteons remain as irregular arcs of lamellar fragments, known as interstitial lamellae - these are surrounded by osteons

n Like osteons, interstitial lamellae are surrounded by cementing lines