An electron microscopic study of the histogenesis of the parathyroid gland in the sheep.

Definitive chief cells, characterized by the presence of specific secretory granules and typical membrane configurations, were present in the developing parathyroid glands of sheep embryos from 26 days of gestation (term = 150 days). During earlier stages of gestation developing chief cells contained lysosomal systems which appeared to be concerned with the autophagy of retained secretion product. The EM evidence suggests that physiologic release of the hormone does not occur until at least 30 days of gestation, which correlates well with the experimentally established time of onset of functional capacity in the parathyroid gland in the sheep. 'Bark cells' were found in the parathyroid primordia of embryos between 20 and 27 days of gestation. The significance of these "dark cells" is discussed with respect to glutaraldehyde fixation and to their possible relationship to chief cells.