Chapter 6 Cartilage and Bone Functions of cartilage

Chapter 6 Cartilage and Bone

Functions of cartilage • Support soft tissue • keep trachea open • cushions vertebrae • Provide soft, gliding surface at articulations (joints) • Provides model formation of new bone • Comes in 3 types • hyaline cartilage • fibrocartilage • elastic cartilage

Supporting connective tissue • Cartilage and bone are considered connective tissue • protect and support soft body tissues • Cartilage has firm, gel-like extracellular matrix • protein fibers and ground substance • chondrocytes are mature cartilage cells • sit in spaces called lacunae (sing. lacuna) • collagen fibers give strength; stronger than other connective tissue • elastic fibers give resilience • avascular (nutrients absorbed by diffusion)

Hyaline cartilage Table 4. 11 a-1 • Chondrocytes (cartilage cells) sit in lacunae (sing. lacuna) • Think of the lacuna as a pocket. The chondrocyte sits in the pocket. • Dividing chondrocytes often seen sharing lacuna • Extracellular matrix surrounds cells • Collagen fibers are very fine; can’t be seen in microscope Lacuna Chondrocyte Extracellular matrix LM 250 x

Growth of cartilage • Interstitial growth • happens within cartilage • chondrocyte in lacuna undergoes mitosis • now two cells in one lacuna • cells called chondroblasts • chondroblasts secrete cartilage matrix, get pushed apart • lacuna forms around each cell, now called a chondrocyte • Happens widely during embryonic development, less as cartilage matures

Fig. 6. 2 a Interstitial Growth Chondrocyte in lacuna Hyaline cartilage LM 320 x 1. Chondrocyte starts to undergo mitosis Lacuna Chondrocyte Cartilage matrix 2. Two chondroblasts occupy one lacuna. Chondroblast Lacuna © The Mc. Graw-Hill Companies, Inc. /Photos by Dr. Alvin Telser

Fig. 6. 2 a Interstitial Growth Hyaline cartilage 1. Chondrocyte starts to undergo mitosis 3. Chondroblasts produce new matrix, begin to separate. Separate cells called chondrocytes Lacuna Chondrocyte Cartilage matrix 4. Cartilage continues to grow internally; chondrocytes produce matrix. New matrix Chondrocyte 2. Two chondroblasts occupy one lacuna. Chondroblast New matrix Lacuna Chondrocyte Chondroblast Lacuna © The Mc. Graw-Hill Companies, Inc. /Photos by Dr. Alvin Telser

Growth of cartilage • Appositional growth • • happens at edges of cartilage stem cells in perichondrium divide, create chondroblasts new chondroblasts start to secrete cartilage matrix new cells push apart, each forms own lacuna

Perichondrium Fig. 6. 2 b Appositional Growth 1 Hyaline cartilage Matrix Chondrocyte in lacuna LM 320 x Stem cells within perichondrium do mitosis Perichondrium Mesenchymal cells New cartilage matrix Dividing undifferentiated stem cell Older cartilage matrix

Fig. 6. 2 b Perichondrium Appositional Growth Matrix Chondrocyte in lacuna Hyaline cartilage LM 320 x 2 New undifferentiated stem cells and committed cells that differentiate into chondroblasts are formed. New matrix formed at periphery Undifferentiated stem cells Committed cells differentiating into chondroblasts New cartilage matrix Older cartilage matrix Chondroblast secreting new matrix

Perichondrium Fig. 6. 2 b Appositional Growth Matrix Chondrocyte in lacuna Hyaline cartilage 3 Chondroblasts push apart and become chondrocytes, produce more matrix at periphery Perichondrium Undifferentiated stem cells New cartilage matrix Chondrocyte secreting new matrix Older cartilage matrix Mature chondrocyte

Fig. 6. 5 Endosteum • Osteoprogenitor cell is stem cell (b) Endosteum Osteoprogenitor cell Periosteum Compact bone Endosteum • mitosis creates another stem cell and “committed cell” • Osteoblasts secrete osteoid • bone matrix • later calcium deposits harden osteoid • osteoblasts become trapped in bone matrix, become osteocytes Osteoblasts Endosteum Osteoclast Nuclei Bone matrix Canaliculi Osteocyte in lacuna Osteoid Copyright © Mc. Graw-Hill Education. Permission required for reproduction or display.

Fig. 6. 5 Endosteum • Osteoclasts reabsorb bone matrix (b) Endosteum Osteoprogenitor cell Periosteum Compact bone Endosteum • derived from marrow cells • usually located in pit or depression called resorption lacuna • free calcium and potassium for use elsewhere in the body (osteolysis) • clean up damaged bone Osteoblasts Endosteum Osteoclast Nuclei Bone matrix Canaliculi Osteocyte in lacuna Osteoid • Osteoclasts remove bone, osteoblasts replace it Copyright © Mc. Graw-Hill Education. Permission required for reproduction or display.

Copyright © Mc. Graw-Hill Education. Permission required for reproduction or display. Page 154 Paget’s disease of bone • Excessive osteoclast activity • osteoclasts are too large and too active, remove too much bone • Osteoblasts replace lost bone • New bone structurally unstable • more susceptible to deformation and fractures X-ray of a skull with osteitis deformans. White arrows indicate areas of excessive bone deposition. © Science Source

Osteocytes • Osteocytes in lacunae in matrix • maintain bone matrix • send information about stress on bone • communicate with each other through canaliculi • in spongy bone, osteocytes not in osteons

Fig. 6. 4 Parts of long bones Proximal epiphysis Metaphysis • outside of bone lined with periosteum • dense irregular connective tissue • same tissue found where else? • outer layer is fibrous • inner layer is cellular • perforating fibers anchor periosteum to bone Periosteum Perforating fibers Diaphysis Metaphysis Distal epiphysis

(a) Periosteum Circumferential lamellae Perforating fibers Periosteum • Protects bone • provides stem cells for bone width growth and fracture repair Copyright © Mc. Graw-Hill Education. Permission required for reproduction or display. Fig. 6. 5 Fibrous layer Cellular layer Canaliculi Osteocyte in lacuna Periosteum Compact bone Endosteum

Case Study: Achondroplasia • Matt is a toddler who does not appear to be developing normally • Mentally he is normal, but he is very small. His head size is normal, but his arms and legs are very short and don’t appear to be growing normally

Ossification • AKA osteogenesis • formation and development of bone • Intramembranous ossification • produces flat bones of skull, some facial bones, part of clavicle • osteoid from osteoblasts becomes calcified • bone slowly becomes more organized, osteoblasts captured in matrix

Fig. 6. 10 Intramembrous Ossification Flat bone of skull 1. Ossification centers form within thickened regions of mesenchyme. Collagen fiber Mesenchymal cell Ossification center Osteoblast Osteoid

Fig. 6. 10 Intramembrane Ossification Osteoid 2. Osteoid undergoes calcification. Osteoblast Osteocyte Newly calcified bone matrix

Fig. 6. 10 Intramembrous Ossification 3. Woven bone and surrounding periosteum form. Mesenchyme condensing to form the periosteum Blood vessel Trabecula of woven bone

Fig. 6. 10 Intramembrous Ossification 4. Lamellar bone replaces woven bone, as compact and spongy bone form. Periosteum Compact bone Spongy bone Lamellar bone

Fig. 6. 7 Periosteum Flat bone of skull Spongy bone Periosteum Compact bone

Ossification • Endochondral ossification • • • produces most bones of skeleton chondroblasts secrete cartilage matrix chondrocytes trapped within matrix die, leaves holes in matrix cartilage matrix is calcified osteoblasts lay down calcium phosphate matrix, get trapped in matrix, etc. process continues until growth is complete (teenage years)

Endochondral Ossification Fig. 6. 11 Perichondrium Deteriorating cartilage matrix Developing periosteum Epiphyseal capillaries Periosteal bone collar Hyaline cartilage Blood vessel of periosteal bud Epiphyseal plate Articular cartilage Epiphyseal blood vessel Spongy bone Developing compact bone Primary ossification center Calcified cartilage Compact bone Epiphyseal line (remnant of epiphyseal plate) Medullary cavity Periosteum Secondary ossification centers Epiphyseal plate Spongy bone Epiphyseal line Articular cartilage

Fig. 6. 12 Zone 1: Zone of resting cartilage Zone 2: Zone of proliferating cartilage Epiphyseal plates Epiphyses Diaphysis Zone 3: Zone of hypertrophic cartilage Zone 4: Zone of calcified cartilage LM 70 x (a) Epiphyseal plate Zone 5: Zone of ossification Epiphyseal plates Diaphyses (b) X-ray of a hand

Page 160 Epiphyseal plates • When a person is growing, epiphysis and diaphysis are separate pieces • When bones stop growing longer, cartilage section calcifies, unites epiphysis and diaphysis (Left) the epiphyses are partially fused; likely age 15 to 23 (Right) No fusion; likely younger than 15 years of age. Copyright © Mc. Graw-Hill Education. Permission required for reproduction or display. © David Hunt/Smithsonian Institution

Long bone development Fig. 6. 13 Bone deposited by osteoblasts Bone resorbed by osteoclasts Infant Child • Growth in length is interstitial growth • Growth in width is appositional growth • does not require cartilage matrix • osteoblasts created by cells in periosteum Young adult Adult

Case Study: Achondroplasia • What if the body doesn’t produce cartilage correctly? • Flat bones of head develop normally • osteoprogenitor cells continue to produce osteoblasts, which make osteoid • Long bones don’t develop normally • endochondral ossification can’t happen • osteoblasts get made, but don’t have cartilage matrix to lay calcium phosphate onto • bones don’t grow

Roloff family (Little People, Big World) Parents Matt and Amy and one son are achondroplasic dwarfs Other 3 children are average height

Bone remodeling • Happens throughout life • • maintains levels of calcium and phosphate in blood heals fractures responds to stresses (including running, weight training) increases size of bone attachment sites on bone • Osteoclasts remove bone, osteoblasts replace it • usually results in loss of bone density

Fig. 6. 14 Epiphyseal artery Metaphyseal artery Articular cartilage Epiphyseal line Periosteal arteries Periosteum Cellular layer Fibrous layer Periosteum Nutrient artery (in nutrient foramen) Medullary cavity (contains yellow bone marrow) Branch of nutrient artery Compact bone

Oblique Fractures Fig. 6. 15 Greenstick Pott Transverse Comminuted Spiral Colles Compound (open)

Fig. 6. 16 Medullary cavity Hematoma Periosteum Compact bone 1 A fracture hematoma forms.

Fibrocartilaginous (soft) callus Fig. 6. 16 Medullary cavity Hematoma Periosteum Compact bone 1 A fracture hematoma forms. 2 Regenerating blood vessels A fibrocartilaginous (soft) callus forms.

Fig. 6. 16 Medullary cavity Hematoma Periosteum Compact bone 1 A fracture hematoma forms. Hard callus Fibrocartilaginous (soft) callus Regenerating blood vessels 2 A fibrocartilaginous (soft) callus forms. Primary bone 3 A hard (bony) callus forms.

Fig. 6. 16 Compact bone at break site Medullary cavity Hematoma Periosteum Compact bone 1 A fracture hematoma forms. Hard callus Fibrocartilaginous (soft) callus Regenerating blood vessels 2 A fibrocartilaginous (soft) callus forms. Primary bone 3 A hard (bony) callus forms. 4 The bone is remodeled.