The development of the calcifying mechanism in avian cartilage and osteoid tissue

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Abstract
The process of calcification in normal fowl osteoid tissue and cartilage at different stages of development was studied. The extent to which the normally uncalcified osteoid and cartilage are calcifiable was tested by immersion in various calcifying solns. in vitro. Osteoid tissue in its earliest stages is neither calcified in normal development nor artificially calcifiable under the conditions of these exps. At a slightly later stage the osteoid tissue, though still uncalcified in vivo, can be calcified experimentally; normal calcification in vivo quickly succeeds this stage. Fowl cartilage in the long bones begins to hypertrophy early in embryonic life, but calcification begins only ca. the 15th day of incubation, proceeds very slowly and is never complete. The early hypertrophic cartilage, though containing phosphatase, cannot be artificially calcified; but at later stages calcification can be induced in limited regions which expand as the age of the embryo advances. It is concluded that developing osteoid tissue and hyper-trophic cartilage do not at once acquire the calcifying mechanism, but that this mechanism is gradually developed during the course of differentiation. Cultures of fibrous tissue from the periosteum were uncalcifiable under exp. conditions which produce heavy deposits in control cultures of osteoid tissue. The results agree with the views that a specialized mechanism is developed in hypertrophic cartilage and osteoid tissue before these tissues can be calcified in the body and that phosphatase is an important component of this calcifying mechanism but not the sole factor concerned. The elaboration of this complex biochemical mechanism during the later stages of development of hypertrophic cartilage is a fact difficult to reconcile with the view expressed by Harris (1932) that the hypertrophy of the cells represents a process of senescence.