Carbonic anhydrase activity in the chick embryo chorioallantois: regional distribution and vitamin D regulation.

An electron microscopical and biochemical study of the chorionic epithelium from 16and 17-day-old chick embryos was conducted. Using a combination of sections made either parallel or perpendicular to the chorionic surface, it was confirmed that the cell populations of the polar and equatorial regions of the membrane are different; the former contains a very reduced number of VC cells which are, contrariwise, widely distributed in the latter. This evidence was required for the interpretation of data on carbonic anhydrase activity. This enzymatic activity is known to increase in the chorioallantoic membrane at the time at which shell resorption begins but the enzyme is believed to be localized in other cell types in addition to VC cells. The results of the present experiments show that carbonic anhydrase activity is significantly higher in the region where VC cells are more frequent (equatorial) than in the region where they are rarely found (polar) and thus suggests that a substantial proportion of the activity measured in the whole membrane corresponds to VC cells. Carbonic anhydrase activity in the equatorial region of the chorioallantoic membrane increased after 4 h and peaked 24 h after the administration of 1,25-DHCC. This time course is similar to the one on previous experiments in which serum calcium concentrations were determined after similar treatment. It is suggested that one of the effects of 1,25-DHCC on its target cells in the membrane (VC cells according to previous evidence) is to increase synthesis and/or activity of carbonic anhydrase.