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2. What are the functions of different types of cells present in bone?

Osteocytes are the most abundant cells present in bone. They are mature osteo-

blasts and are embedded in the bone matrix. They act as mechanosensor and initi-

ate bone remodeling. These cells also secrete various phosphatonins including

FGF-23 which play a crucial role in the phosphate homeostasis. In addition,

osteocytes also secrete dickkopf-1 and sclerostin which inhibit bone formation.

Osteoblasts lay down the matrix (osteoid) and promote mineralization of osteoid

by secreting osteocalcin, osteopontin, and alkaline phosphatase. Once the process

of mineralization is complete, mature osteoblasts may undergo apoptosis or can

differentiate into osteocyte or bone-lining cells. Osteoblasts also promote osteo-

clastogenesis through RANK-RANKL pathway. Osteoclasts secrete various pro-

teolytic enzymes like cathepsin K which results in bone resorption, a key step in

bone remodeling. Bone-lining cells serve as a blood–bone barrier and regulate the

influx and efflux of calcium and phosphorus from extracellular fluid. Further,

these cells can redifferentiate into osteoblasts, when they are exposed to PTH.

3. What are the types of bone?

Human skeleton can be classified on the basis of anatomy or structure.

Anatomically, bone can be classified into long bone (e.g., femur), short bone

(e.g., carpal bones), and flat bone (e.g., skull bones). Structurally, bone can be

classified either as cortical and trabecular or as woven and lamellar bone.

Constituents of Bone

Cells (2%)

-Osteoblasts

-Osteoclasts

-Osteocytes (95%)

-Bone lining cells

Matrix (98%)

Inorganic (60-70%)

-Calcium

-Phosphate

-Magnesium

Organic (30-40%)

-Type 1 collagen(90%)

-Non-collagenous proteins(5%)

• Osteocalcin


• Osteopontin

Fig. 5.2 Constituents of bone

5 Rickets–Osteomalacia