Once the appendicular skeleton starts to develop, the progress is rapid. Early in the sixth week, only vague concentrations of mesenchyme represent the primordia of future bones. By the end of the sixth week, these cellular concentrations are sufficiently molded so that some of the larger future bones can be detected. During the seventh week, the primordia of many of the smaller bones of the hand and foot are present.

By the eighth week, well-molded cartilage rudiments represent all the major future bones of the appendicular skeleton.

BONE FORMATION

Bone forms in areas occupied by either connective tissue or cartilage. Bone formed in connective tissue is of intramembranous origin and is called membrane bone. Most of the bones of the calvaria, the facial bones, and, in part, the clavicle and mandible, are membrane bones. All the other bones of the body form in areas occupied by cartilage, which they gradually replace. These bones are of endochondral origin and are called cartilage bones. The terms membrane bone and cartilage bone merely describe the environment in which a bone forms, not the microscopic structure once the bone is completely developed.

Membrane Bone. The cells of the mesenchymal rudiment of a membrane bone begin to produce a mucoprotein matrix in which collagen fibers are embedded (see Plate 1-10). Within this organic matrix, which is known as osteoid, inorganic crystals of calcium phosphate are deposited between, on, and within the collagen fibers. This mineralization of the osteoid is known as ossification. The calcium-to-phosphate ratio increases in the bone matrix as ossification proceeds before birth, chiefly in the form of a series of minerals known as apatites. As development proceeds to the time of birth, hydroxyapatite emerges as the dominant component of bone mineral. Hydroxyapatite is the basic inorganic constituent of mature bone, and its hydroxyl groups are partially substituted by other chemical elements and radicals, such as fluoride or carbonate.

The mesenchymal cells involved in bone formation become known as osteoblasts. As bone formation proceeds, the osteoblasts divide and some become completely surrounded by osteoid. The trapped osteoblasts, then known as osteocytes, send out long, thin extensions of their cell bodies in all directions, which make contact with the cellular extensions of adjacent osteocytes also laying down osteoid (see Plate 1-10). When bone mineral is deposited in the osteoid, the space in the matrix housing the portion of the osteocyte containing its nucleus is known as a lacuna, and the tiny, tubular spaces radiating out from the lacuna containing the extensions of the osteocyte are known as canaliculi (see Plate 1-11).