The pars distalis (anterior pituitary) in the fetal sheep: an ultrastructural study

The pars distalis from 32 fetal sheep (gestational ges ranging from 60 to 143 days), was examined by light and electron microscopy. The pars distalis was principally composed of clusters of parenchymal cells, which were a mixture of secretory and non-secretory folliculo-stellate cells. As gestation progressed the clusters grew larger and more numerous and the cytoplasm of the secretory cells became increasingly granular. From as early as the 60th day of pregnancy it was possible to recognise several secretory cell types. Mammotrophs and somatotrophs were the most abundant. These cells generally showed signs of a high level of activity throughout gestation for they usually contained large secretory granules, well developed Golgi apparatus and much rough endoplasmic reticulum. The gonadotrophs, initially angular in profile, became larger, rounder and more granular as gestation progressed. Thyrotrophs and corticotrophs were sparsely distributed. The study suggests that the secretory cells of the fetal sheep pars distalis may be active in the production and secretion of hormones from at least the 60th day of pregnancy.