Histology of Cartilage and Bone 12/4/24 PDF

Histology of Cartilage and Bone (12/4/24) **[Learning Objectives]** 1. **[Compare and contrast the histological make-up of the specialized connective tissues, cartilage and bone.]** [Cartilage:] - Extracellular matrix (ECM): 95% of tissue volume - Solid but flexible/resilient...

Histology of Cartilage and Bone (12/4/24) **[Learning Objectives]** 1. **[Compare and contrast the histological make-up of the specialized connective tissues, cartilage and bone.]** [Cartilage:] - Extracellular matrix (ECM): 95% of tissue volume - Solid but flexible/resilient - Avascular - High ratio of glycosaminoglycans (GAGs) to type II collagen fibers - Chondrocytes: sparsely located, fibroblastic cells for maintaining ECM - Large cells with distinct nuclei; sit within lacunae - 3 types: hyaline (articular), elastic, fibrocartilage [Bone: ] - Specialized CT that contributes to the skeleton, protects vital structures, provides mechanical basis for movement and harbors bone marrow - Immature (woven) bone: **bone tissue formed during development** - Mature (lamellar) bone differs in its collagen fiber arrangement - Classified as either compact (dense) or spongy (cancellous) - Compact bone: lies **outside and beneath periosteum** - Spongy/cancellous bone: formed by internal, **sponge-like meshwork of trabeculae** - ECM: mineralized matrix (hydroxyapatite crystals: calcium + phosphate) - Type I collagen + ground substance (non-collagen proteins) - Bone remodeling: involves interplay between osteoblast and osteoclast activity - Covered by periosteum - Outer fibrous layer: resembles dense connective tissue. High amount of collagen arranged parallel to the bone - Inner cellular layer: contains osteoprogenitor (periosteal cell) which can divide into active osteoblasts - Bone cavities are lined by endosteum - Single cell layer of osteoprogenitor (endosteal) cells, osteoblasts and osteoclasts 2. **[Detail the histological classifications of cartilage.]** [Hyaline cartilage:] - Homogenous and amorphous ECM; appears glassy - ECM contains 3 classes of molecules: collagen molecules (**mainly type II**), multi-adhesive glycoproteins, proteoglycan aggregates - 3 types of GAGs (hyaluronan, chondroitin sulfate, and keratan sulfate) - Aggrecan: **most abundant proteoglycan monomer** in hyaline cartilage - Large proteoglycan aggregates (hyaluronan + aggrecan molecules) have **negative charges** to **bind and** **hold large amount of water molecules** - Chondrocytes distributed singularly or in clusters (**isogenous groups** of 2 or 4) A diagram of a cell Description automatically generated - Perichondrium: dense irregular connective tissue surrounding hyaline cartilage; **provides new chondrocytes** - **Not present on** free surfaces of **articular cartilage in synovial joints** - Site of new chondrocyte growth - Hyaline cartilage is framework for **development of fetal skeleton** (endochondral ossification) and in most **growing bones** (epiphyseal growth plates)  - Articular cartilage: cartilage covering surfaces of movable joints; contains no perichondrium - **Remnant of** the original cartilage template of **developing bone** A diagram of a structure Description automatically generated  [Elastic Cartilage] - Similar to hyaline (**high type II collagen amount**) but with **elastic fibers and interconnecting sheets** of elastic material - Location: **external ear, middle ear (auditory tube) and larynx** - Does not calcify during aging process A close-up of a microscope Description automatically generated [Fibrocartilage] - Combination of dense regular connective tissue and hyaline cartilage - Chondrocytes distributed in rows or **isogenous groups** - Typically present in intervertebral discs, pubic symphysis, insertion of tendons and structures within certain joints - ECM: similar to hyaline but with abundant type I collagen fibrils - Ground substance: contains larger amounts of versican than aggrecan molecules - **No perichondrium present**  3. **[Detail the histological classifications of bone.]** - Osteocytes: cells surrounded by matrix (lacunae); connect via canaliculi to other osteocytes (gap junctions) - Osteoprogenitor cells: mesenchymal stem cell origin; produce osteoblasts - **Appear as flattened/squamous cells** with lightly staining, elongated or ovoid nuclei and inconspicuous acidophilic/slightly basophilic cytoplasm - Osteoblasts: secrete bone matrix; **become osteocytes when trapped** - Have cuboidal or polygonal shaped cells and aggregated into a single layer of cells lying in apposition to forming bone - Appear to be separated from the bone by a light band A blue and pink cells with a black arrow Description automatically generated - Bone-lining cells: osteoblasts on surfaces where no remodeling is occurring - Osteoclasts: reabsorb bone tissue - **Derived from** fusion of **hemopoietic progenitor cells** in bone marrow that give rise to neutrophilic granulocyte and monocyte lineages - Large, multinucleated cells found at sites where bone is being removed - Strongly acidophilic - Tartrate-resistant acid phosphatase (TRAP): used clinically as **a marker of osteoclast activity** and differentiation  A close-up of a cell Description automatically generated 4. **[Compare and contrast appositional and interstitial growth in cartilage and bone.]** - Interstitial growth of cartilage: occurs with chondrocytes dividing in their lacunae and then producing matrix to slowly push away from each other - **Occurs from inside out** - Appositional growth: **adds to outer circumference** 5. **[Detail the histological characteristics of intramembranous ossification and endochondral ossification.]** - Endochondral ossification: **utilizes a cartilage model** to be replaced and built upon; accounts for **majority of bones formed**  A white text with black text Description automatically generated - Intramembranous ossification: simpler model with ossification beginning within sheets of mesenchymal tissues - Responsible for formation of flat bones  - Osteoprogenitor cells differentiate into osteoblasts - Osteoblasts secrete collagen and bone matrix (osteoid) into surrounding spaces - [Epiphyseal growth plate/endochondral ossification:] -  **Zone of reserve cartilage**: exhibits **no cellular proliferation** or active matrix production. -  **Zone of proliferation:** has cartilage cells undergoing divisions that organize into distinct columns. These cells are larger than the reserve zone and **actively produce collagen matrix proteins**. -  **Zone of hypertrophy:** contains greatly enlarged (hypertrophic) cartilage cells that have a clear cytoplasm and are metabolically active. Cartilage matrix is compressed to form linear bands between the columns of hypertrophied cartilage cells. -  **Zone of calcified cartilage**: the hypertrophied cells degenerate, and the cartilage matrix becomes calcified. - Calcified cartilage serves as a scaffold for deposition of new bone - Chondrocytes positioned in the more proximal part of this zone undergo apoptosis. -  **Zone of resorption**: calcified cartilage is in direct contact with marrow cavity CT. Small blood vessels and osteoprogenitor cells invade the region and leave the calcified cartilage as longitudinal spicules. A diagram of a section of a human body Description automatically generated 6. **[Describe the histological basis of bone fracture repair.]** - [Cartilage:] - **Limited ability to repair** - Most optimal outcome would be damage to perichondrium - Far more collagen produced at injury site than new chondrocytes - [Bone:] - Direct (primary): involves stabilization of the fracture to promote internal remodeling - Indirect (secondary): involves responses from periosteum along with intramembranous and endochondral ossification processes 1. **Fracture hematoma forms** and **bone necrosis occurs** at the end of fractured bone fragments 2. **Osteogenic buds** from new bone **invade the callus** and begin to deposit bone within callus gradually **replacing the original** fibrous and cartilaginous **callus with a hard callus** 3. **Granulation tissue replaces the hematoma** and **transforms into a fibrocartilaginous soft callus** giving the fracture site a stable, semirigid structure 7. **[Compare and contrast the distinctive structures of cartilage and bone and relate these to functions of these specialized connective tissues. ]** 8. **[Discuss clinical scenarios of altered intramembranous and endochondral ossification.]** **[Additional Notes]** [Haversian System:] - Lamellar bone: mostly composed of osteons (Haversian systems) - Concentric lamellae: organized around an osteon (Haversian) canal; contains vascular and nerve supply of osteon - Interstitial lamellae: occupy spaces between osteons; often represent remodeled areas - Perforating (Volkmann) canals: perpendicularly arranged and connect osteonal canals to one another - Circumferential lamellae: form a collar around all osteons; surround entire bones - Lacunae between concentric lamellae contain osteocytes interconnected with other osteocytes via canaliculi - Endosteum lines the osteon canals and perforating canals - Cleidocranial dysplasia: affects intramembranous ossification (skull, teeth and clavicle are affected) - Canaliculi: provide nutrients to osteocytes within lacunae