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Normal fetal bone

There are important differences between fetal bone (woven or immature bone) and adult bone. Bone production begins by secretion of collagen molecules and ground substance by osteoblast cells. Collagen molecules polymerize to form collagen fibres, which make up 90±95% of osteoid tissue. In fetal bone, collagen fibres lie in a random arrangement forming a complex interwoven meshwork. The bone is therefore laid down in an irregular arrangement, with primary calcification and no secondary calcification. Fetal bone does not have the organized lamellar appearance of adult bone. The bone salts, principally those of calcium and phosphate which make up the crystalline salt hydroxyapatite Ca10(PO4)6(OH)2, precipitate on the surface of the collagen fibres. The osteoblasts become trapped in the osteoid as they secrete collagen around themselves and, once trapped, they become osteocytes. The figure is a photomicrograph taken from a human fetal lumbar vertebra at autopsy at 24 weeks gestation, which clearly illustrates this process (B, bone; M, marrow; OO, osteoblasts; O, osteoblast becoming trapped in the osteoid). Trabecular bone is being laid down by the osteoblast cells and one has almost become surrounded by osteoid.