Comparative aspects of calcium metabolism in embryonic reptiles and birds

Abstract

Eggs of oviparous, amniotic vertebrates must be endowed at oviposition with all of the organic and most of the inorganic components required for embryonic growth. A major inorganic constituent of these eggs is calcium used for ossification of the skeleton. The two main sources of this element are the yolk and the eggshell, but the proportion of calcium supplied by these compartments varies among species. Some of the calcium absorbed from the eggshell by embryos of domestic fowl is stored in the yolk. causing the calcium content of this compartment to increase appreciably during incubation. In contrast, the calcium content of yolk declines throughout incubation in eggs of reptiles, and the yolk that is withdrawn into the abdominal cavity just prior to hatching contains only small quantities of this element. Thus, major differences in the pattern of calcium metabolism characterize avian and reptilian embryos, and studies of embryos of domestic fowl may not provide a broadly-based model with which to characterize calcium metabolism in embryos of other species. Control of calcium transport across the cellular epithelia (the yolk sac and chorioallantois) that separate embryos from their sources of calcium (yolk and eggshell) represents one aspect of control of calcium metabolism during embryogenesis, but this process has been examined only in eggs of domestic fowl and only in the chorioallantois. Calcium transport across the chorioallantois of embryonic chicks is influenced by a vitamin K-dependent calcium-binding protein, carbonic anhydrase, the level of calcium to which the chorioallantois is exposed, and vitamin D. However, a complete story concerning control of calcium transport across the chorioallantois and its relationship to calcium regulation in embryos of domestic fowl is not yet possible.