PATTERNS OF MOBILIZATION AND DEPOSITION OF CALCIUM IN EMBRYOS OF OVIPAROUS, AMNIOTIC VERTEBRATES

ABSTRACT

Embryos of oviparous, amniotic vertebrates obtain calcium for skeletogenesis from both the yolk and the eggshell, but the importance of these sources varies among major groups, as does the pattern of deposition of calcium during development. Snake eggs have a rich supply of calcium in the yolk, and snake embryos rely more on the yolk than on the lightly calcified eggshell for the calcium needed for skeletogenesis. Embryonic lizards obtain a larger proportion of their calcium from the eggshell, but even so, more than half of the calcium required for development is provided by the yolk. Chelonian and archosaurian embryos, on the other hand, rely on the eggshell as the principal source of calcium. Calcium obtained from the eggshell by embryonic cheloni—ans is deposited primarily, if not exclusively, in the carcass of the developing embryo. Some archosaurian embryos deposit a small proportion of shell calcium in the yolk, whereas others deposit shell calcium almost exclusively in the yolk. Control of shell calcium mobilization and control of calcium transport by the yolk sac and chorioallantoic membranes have the potential to contribute importantly to calcium homeostasis during embryogenesis. Unfortunately, our knowledge of the mechanism and control of these processes is rudimentary even for embryos of well-studied species like the domestic fowl, and the subject has not been addressed at all in other species of oviparous, amniotic vertebrates.