Bone development and growth

Question 1

Bone vs cartilage

Answer 1

• Matrix of bone: osteoid calcified by deposition of hydroxyapatite crystals, with type I collagen (covered by periosteum, vascular)
• Matrix of cartilage: proteoglycan (hyaluronic acid) aggregates covered by perichrodrium (avascular)
• Cells in bone: osteobasts, osteocytes, and osteoclasts
• Cells in cartilage: chrondroblasts, chrondrocytes
• Growth mode of bone: appositional
• Growth mode of cartilage: interstitial and appositional

Question 2

Cartilage degeneration

Answer 2

• Normal part of endochondral bone formation
• Chrondrocytes die and matrix calcifies
• Normal process of aging as well

Question 3

Intramembranous bone formation 1

Answer 3

• Bone is formed from vascularized condensed mesenchymal tissue
• Growth of flat bone
• Mesenchymal cells differentiate then clusters together to form ossification center
• Osteoblasts are generated secrete osteoid, which becomes crystalized by hydroyapatite on the surface of bone

Question 4

Intramembranous bone formation 2

Answer 4

• The ostoblasts are trapped in the osteoid matrix and become osteocytes
• Trabeculae from osteocytes join together to create spongy bone (primary bone)
• Undifferentiated mesenchymal cells surrounding the trabeculae differentiate into BM

Question 5

Endochondral bone formation 1

Answer 5

• Bone is formed from cartilage, mostly long bones
• Vascularization of perichondrium of the cartilage model at the midshaft (diaphysis) causes chondrogenic cells to differentiate into osteogenic cells
• These give rise to osteoblasts which lay down osteoid
• Osteoid at the diaphysis is calcified to form a (subperiosteal) bone collar around the diaphysis
• Chondrocytes in the center of the diaphysis undergo apoptosis and the area is vascularized and invaded by periosteum (periosteal bud) then lined w/ osteoblasts (ossification center)

Question 6

Endochondral bone formation 2

Answer 6

• The osteoblasts divide and osteoid replaces the cartilage except for at the ends of the bone (epiphyses) which remain cartilaginous (calcified cartilage-bone complex)
• The epiphyses form secondary ossification centers after birth
• Proliferation and hypertrophy of chondrocytes w/in the epiphyseal plate followed by calcification of cartilage matrix leads to increase in bone length
• Bone deposition on the outer surface of bone collar and resorption of bone on the inner surface results in increase in diameter of diaphysis (width)

Question 7

Epiphyseal (growth) plate

Answer 7

• Made of hyaline cartilage and only found in children/adolescents, allows for interstitial growth of cartilage-> bone via calcification
• Is replaced by epiphyseal line in adults
• Epiphyseal plate composed of 5 distant zones: zone of resting (reserve cartilage), proliferation (stacks of chondrocytes), maturation/hypertrophy (large chondrocytes in columns), calcification (thin layer of calcified dead chondrocytes, ossification (bone)
• Together these areas look like columns of enlarged chondrocytes

Question 8

Formation of fracture hematoma

Answer 8

• Blood vessels are broken, clots form around the site of fracture (hematoma)
• Lack of circulation results in inflammation and cell death
• Dead bone and fragments will be resorbed by osteoclasts

Question 9

Cartilaginous callus formation

Answer 9

• Capillaries, fibroblasts, and osteogenic cells from broken periosteum and BM resolve the fracture hematoma into granulation tissue
• Fibrous connective tissue btwn ends of fractured bone are formed from fibroblasts and osteogenic cells
• Some osteogenic cells in the CT become chondrogenic cells and produce a hyaline cartilage callus around the ends of the bones

Question 10

Bony callus formation

Answer 10

• Osteogenic cells in fibrous lining of the callus produce bony trabeculae by intramembranous bone formation
• Blood supply to callus improves and endochondral ossification occurs in the cartilaginous callus
• This forms bony callus consisting of primary (woven) bone (weaker than secondary bone, found in fetal bones, eventually replaced by secondary)

Question 11

Bone remodeling

Answer 11

• The primary bone in bony callus is remodeled into secondary bone
• Original bone architecture is restored

Question 12

Synovial membrane

Answer 12

• Lines the synovial cavity, is a layer of squamous to cuboidal epithelial cells
• 2 types of cells: type A (phagocytic) and type B (fibroblast-like)
• The membrane secretes viscous fluid (synovial fluid) rich in hyaluronic acid and proteins
• Bone surfaces within synovial cavity are lined by persistent layer of hyaline cartilage (articular cartilage)