Musculoskeletal

Question 1

Endochondrial ossification

Membranous ossification

Answer 1

Endochondral ossification
-Bones of axial skeleton, appendicular skeleton, and base of skull. Cartilaginous model of bone is
first made by chondrocytes. Osteoclasts and osteoblasts later replace with woven bone and then
remodel to lamellar bone. In adults, woven bone occurs after fractures and in Paget disease.
Defective in achondroplasia.

Membranous ossification
Bones of calvarium, facial bones, and clavicle. Woven bone formed directly without cartilage. Later
remodeled to lamellar bone.

Question 2

What do each of these do in bone 
Osteoblast 
Osteoclast
PTH
Estrogen

Answer 2

Osteoblast: Builds bone by secreting collagen and catalyzing mineralization in alkaline environment via ALP. Differentiates from mesenchymal stem cells in periosteum. Osteoblastic activity measured by
bone ALP, osteocalcin, propeptides of type I procollagen.

Osteoclast: Dissolves bone by secreting H+ and collagenases. Differentiates from a fusion of monocyte/
macrophage lineage precursors.

Parathyroid hormone: At low, intermittent levels, exerts anabolic effects (building bone) on osteoblasts and osteoclasts
(indirect). Chronically elevated PTH levels (1° hyperparathyroidism) cause catabolic effects (osteitis
fibrosa cystica).

Estrogen Inhibits apoptosis in bone-forming osteoblasts and induces apoptosis in bone-resorbing

osteoclasts. Causes closure of epiphyseal plate during puberty. Estrogen deficiency (surgical or
postmenopausal) leads to increased cycles of remodeling and bone resorption increase risk of osteoporosis.