The differentiation of the chick chorionic epithelium: an experimental study

An electron microscopical study of the chorionic epithelium under two different experimental situations was conducted in order to gain knowledge on the mechanisms controlling the differentiation of ‘calcium-absorbing’ cells. In chorioallantoic membranes from 10-day-old embryos cultured on a semi-synthetic medium, the chorion became avascular but ‘calcium-absorbing’ cells differentiated, showing that the differentiation of these cells is not dependent on the vascular changes which in vivo occur simultaneously. The number of ‘calcium-absorbing’ cells was much larger in the explants cultured on parathyroid hormone-enriched medium than in controls, indicating that the hormone is capable of stimulating their differentiation. Since it is known that parathyroid glands are active at the time in which ‘calcium-absorbing’ cells normally appear, our results suggest that the hormone may be the normal inductor of their differentiation. Finally, a small number of ‘calcium-absorbing’ cells differentiated in vivo in a portion of chorion which had been artificially separated from the shell membranes, showing that their differentiation does not depend on the proximity of the external source of calcium.