The Ossification of the Long Bones
The ossification of the long bones occurs from two ossification centers: the epiphyseal ossification centers in the proximal and distal parts of the bone and the ossification from the periost of the diaphysis. Between the epiphysal centers and the diaphysis are ring- shaped discs of cartilage (the epiphyseal plates) that continue to produce cartilage for a long period of time. The bone spicules that originate from enchondral ossification are parallel and are situatedlengthwise in the direction of the bone. The bone increases in thickness because it is built up peripherally and is dissolved in the center of the shaft such that the bone marrow cavity is created. This type of skeletal forming has a direct connection with the impact of gravity on the organism. The morphology and the degree of calcification of the long bones are, to a great degree, determined by the effect of gravity. Research on the structure of the bone spicules in relation to the effect of gravity is unequivocal in this. Bone spicules are formed in accordance with the lines of gravity that are operative in the skeleton (fig. 3.9.). In a weightless situation, the skeleton of the torso and the extremities is particularly threatened by decalcification because of the effect of insufficient gravity.
Enchondral ossification of the skeletal bones in the head also occurs in that portion of the cranium that develops from the branchial arches and the paraxial sclerotomes (fig. 3.3.). The architecture of the bone spicules in this portion of the skeleton displays the same characteristics as the architecture of the extremitiesIn this area, we also find the forces that are exerted on the skeleton by the chewing musculature. Most of the bone tissue disappears from those areas where these forces can no longer influence the skeleton. This is particularly the case after the loss of teeth, which normally transfer these mechanical forces onto the skeleton.