Cartilage/Bone

Basic structural features of hyaline cartilage:

1. Perichondrium:

• outer fibrous layer of dense IR CT (usual components of CT including capillaries)

• inner cellular (protochondral/chondrogenic) layer populated by mesenchymal cells that can produce fibro/chondroblasts

2. types of cells and arrangement:

• chondrocytes—fill lacunae and maintain matrix; are not replaced

• chondroblasts—actively produce cartilage (GAGS and type 2 collagen)

• chondrocytes are found embedded in lacunae in isogenic cell nests (2-4 cells) at regular intervals

3. components of matrix:

• type 2 and small type 11 collagen

• 75% tissue fluid: hyaluronic acid backbone with proteoglycan bottle brushes; chondroitin sulfate binds to type 2 collagen via chondronectin

• territorial matrix—high concentration of GAGS and low collagen

4. nourishment:

• cartilage is not vascularized; cells obtain nutrients via diffusion through the matrix

• if diffusion is poor or is blocked,the cells hypertrophy, die and the matrix calcifies

5. Types of cartilage:

• Hyaline—most common, glassy

• Articular—hyaline cartilage at joints where synovial fluid (provides nutrients) is present; has no perichondrium

• Fibrocartilage—contains type 1 collagen as well as type 2; collagen oriented in one direction; does not have perichondrium

• Elastic—similar to hyaline but has elastic fibers replace some of type 2 collagen

2. Development and growth of cartilage

1. chondroblasts come from mesenchymal stem cells; also produce fibroblasts that produce fibrous perichondrium; some stem cells remain to form chondrogenic layer

2. an avascular mesenchymal membrane forms and then cartilage forms within it

3. interstitial—cartilage grows from within so it rises like bread dough: chondroblasts divide and secrete more matrix

4. appositional—adds on to outer surface: chondroblasts are generated by perichondrium and secrete matrix near the outside

5. these two mechanisms are not separate and a cell can participate in both. Ex. A chondroblast generated by perichondrium participates in appositional, but if it divides again it can contribute to interstitial

6. isogenic cells nests in adult cartilage is evidence of interstitial growth; small chondroblasts in lacunae near edge of cartilage is evidence of appositional

3. Development of flat bone

   1. develop via intramembranous ossification

   2. initiated in mesenchymal membrane of pluripotential mesenchyme cells surrounded by GAGS and ground substance similar to cartilage except it is vascularized.

   3. development starts when mesenchyme cells near capillary convert to osteoblasts and produce bone matrix—forms spicules

   4. cells lay bone down on surface of spicules by appositional method—makes trabeculae

   5. trabeculae are enlarge in close relationship to BV and grow to encircle them—makes 1 (woven) bone

   6. mesenchyme also generates fibroblasts to make periosteum and some mesenchymal cells remain to form the osteogenic layer or periosteum

   7. diploe—spongy bone, contains marrow

   8. BONE MUST GROW AROUND CAPPILARIES!

4. development of long bone

   1. grow by endochondral ossification

   2. production of cartilage model would be the same as how hyaline cartilage develops since the model for a long bone is hyaline (see B)

   3. mesenchyme converts to chondroblasts and fibroblasts; fibroblasts and mesenchymal cells within perichondrium convert to osteoblasts

   4. vessel penetrates outside of cartilage—converts perichondrium to periosteum—produces osteoblasts

   5. osteoblasts form bony collar underneath periosteum on outside of cartilage (via intramembranous oss)

   6. bony collar blocks diffusion, chondroblasts die, matrix calcifies

   7. calcified cartilage causes more degeneration

   8. dead cells cause osteoclasts to come clean up and BV follow

   9. pluripotential cells make osteoblasts which make bone on surfaces of calcified cartilage near BV

   10. 1⁰ center of ossification—diaphysis

   11. 2^o center of ossification--epyhysis

   12. BONE MUST GROW AROUND CAPILLARIES

Category: Orthopedics Notes