Calcium in the turkey yolk sac during late development and postembryonic involution.

Yolk sacs (YS) were examined by gross, histochemical, and electron microscopic techniques from incubation day 12 to 10 days of age. Weights were maximal at 22 days of incubation and declined markedly thereafter; by 7 days of age, the yolk was completely absorbed. Endodermal cells of YS epithelium were highly polar with nuclei, mitochondria, and glycogen in basal areas and lipid globules and intralysosomal yolk granules near luminal surfaces. Yolk laminar spherules were first seen at day 16 of incubation. They increased progressively with incubation and disappeared rapidly after hatching. Spherules stained for calcium, glycosaminoglycans, and alkaline phosphatase in late incubation and were considered calcium storage structures. Histochemical evidence of intracellular calcium loading of yolk matrix (from 18 days of incubation until 5 days of age) suggested a specific calcium transport function of YS epithelium.