Week 1: Connective Tissues III-Bone development and growth

Question 1

Describe Intramembranous bone formation.

Answer 1

Bone formed directly in membranous mesenchymal cells

• formation of flat bones, e.g. calveria, via appositional growth
• in richly vascularized mesenchymal tissue
• mesenchymal cells differentiate and cluster, forms primary ossification center–>osteoblasts
• osteoblasts differentiate and secret osteoid
• osteoblasts bc trapped in matrix, become osteocytes
• bone spicules–>bony trabecular join together to form spongy bone
• blood vessels invade and bone marrow forms ossification center

Question 2

Describe endochondral bone formation.

Answer 2

Hyaline Cartilage replaced by bone

• produces most bones in the body, e.g. long/short bones, vertebrae, pelvis, base of skull
  1. formation of cartilage models, surrounded by perichondrium
  2. vascularization of perichondrium
  3. mesenchymal cells–>osteoblasts–>osteoid–>subperiosteal bone collar formed, appears around diaphysis.
  2. cartilage cells hypertrophy and matrix calcified inside diaphysis
  3. from periosteum, vascular periosteal bud invades interior of diaphysis, lined with osteoblasts, forms primary ossification center and primitive marrow cavity (spongy bone replaces cartilage)
  4. secondary ossification centers formed in epiphysis
  5. osssification zone progresses form diaphysis toward epiphyses as 2 fronts, 5 zones seen in epiphyseal plates

Question 3

What are the 5 zones of the epiphyseal plates?

Answer 3

Found at end of long bone in children/teens
A. Zone of resting (reserve cartilage): layer nearest epiphysis, small, scattered chondrocytes
B. Zone of cell proliferation: larger chondrocytes, stacks of cones
C. Zone of cell maturation and hypertrophy: layer of large chondrocytes in columns
D. Zone of calcification: few cells thick, mostly dead chondrocytes and calcified matrix
E. Zone of ossification: layer that bone is deposited

Question 4

How do bones grow in width?

Answer 4

-Occurs through net bone deposition underneath periosteum on outer circumferential lamellae by appositional growth and net bone resorption within medullary cavity by osteoclasts

Question 5

Describe formation of fracture hematoma (1st step in bone fracture repair)

Answer 5

• vessels crossing fracture are ruptured
• blood clots around site of fracture
• cell death and inflammation–>migration of PMNs, macrophages, osteoclasts
• fragments of bone resorbed by osteoclasts

Question 6

Describe cartilaginous callus formation (2nd step in bone fracture repair)

Answer 6

• within 1 week
• capillaries, fibroblasts, osteogenic cells from broken periosteum and bone marrow resolve fracture hematoma into granulation tissue
• produce fibrous connective tissue between ends of bone
• some osteogenic cells become chondrogenic cells and produce hyaline cartilage callus

Question 7

Describe bony callus formation (3rd step in bone fracture repair).

Answer 7

-osteogenic cells in fibrous inner and outer lining of cartilaginous callus produce bony trabeculae
by intramembraneous bone formation
-as blood supply improves, endochondral ossification occurs within cartilage callus
-cartilage replaced with primary (woven) bone

Question 8

Describe bone remodeling (4th step in bone fracture repair)

Answer 8

• primary bone in bony callus remodeled into secondary bone.
• takes 4weeks-several months

Question 9

Describe characteristics of synovial joints.

Answer 9

• capsule is DCT
• synovial cavity lined by synovial membrane. layer oc cells that secret synovial fluid
• synovial fluid: colorless viscous fluid rich in hyaluronic acid and proteins
• bones within synovial cavity lined by hyaline cartilage called articular cartilage
• cell types: Type A phagocytic cells and Type B fibroblast like cells