A simple method was devised for the maintenance of the chorioallantoic membrane (CAM) of chick embryos in organ culture. Explants of CAM survived for up to 5 days in this system and retained the characteristic three-layered morphology (ectoderm, mesoderm, and endoderm). Induction of the CAM calcium-binding protein (CaBP) by effectors of calcium metabolism was studied in these organ cultures. Vitamin K was found to elicit a seven- to eightfold increase in CaBP, whereas no increase in CaBP activity occurred on supplementation with vitamin A, parathyroid hormone, an analogue of vitamin D, vitamin D and its hydroxylated metabolites, or with elevated calcium levels. The vitamin K-mediated induction of CaBP was dose-dependent, inhibited by the vitamin K antagonists warfarin and dicoumarol, selective for vitamin K5, and maximal at the developmental stage (13-15 days of incubation) corresponding to the onset of calcium transport by the CAM in vivo. CaBP levels increased after 60-70 h in cultures of 13-15 day CAM supplemented with vitamin K and reached maximal levels around 80-90 h of culture. The CAM ectoderm underwent extensive proliferation and often assumed a villuslike morphology in the vitamin K cultures.

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