Autoradiographic Studies of the Utilization of Ca45 by the Chick Embryo

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Abstract
Calcium-45 was injected into the dense albumen of fertile hen''s eggs, to the extent of 25 [mu]c/egg. The eggs were incubated under standard conditions and 3 or more embryos removed daily and fixed in 10% neutral formalin. Stripping-film autoradiograms were prepared from paraffin sections of the tibio-fibulae. Exposure varied with the isotope concentration. The tissue sections with their autoradiograms in place were stained with dilute Giemsa, while other sections were stained with hematoxylin-azure-eosin and by von Kossa to demonstrate bone salt. At about 9 days, Ca45 iS found in the cartilage template both intra- and extracellularly. Between 9 and 11 days, a primary diaphyseal lamella is deposited which is largely acellular. The lamella is eroded by capillaries from the periosteum and a resorption center is established in the cartilage. New lamellae of bone are deposited centrifugally in an imbricated pattern. Bone matrix formation precedes calcification by about 1 to 1/2 days, and calcification in a particular lamella is not uniform. Endochondral bone formation is described, as well as calcification of the epiphyseal/diaphyseal cartilage. Calcium-45 occurs intracellularly in the osteocyte during bone formation.