Ultrastructural study of the development of the pars distalis (anterior pituitary) in the foal.

The pituitary glands of 4 horse embryos (41-55 days of gestation) were examined by light microscopy, and the pars distalis from 10 fetal foals (75-300 days) was examined by electron microscopy. At Day 41 the development of Rathke's pouch and the saccus infundibuli was advanced; the former had almost lost its connection with the stomodaeum and the latter had started to differentiate into infundibular process and infundibular stalk. By Day 43 Rathke's pouch was completely dissociated from the stomodaeum and its walls were beginning to show uneven growth. The ventral wall of the pouch, the future pars distalis, exhibited the most growth. The growth of this wall was extensive by Day 55: the region consisted of cell cords surrounded by blood vessels. From Day 75 the epithelial cells of the pars distalis were arranged into clusters. The cells were a mixture of secretory and non-secretory folliculo-stellate cells. As gestation progressed the clusters grew larger and the granulation increased within individual secretory cells. Mammotrophs were the most abundant and readily identifiable of the secretory cell types; they showed signs of intense activity throughout pregnancy. Corticotrophs were also observed in the earliest fetus examined. Somatotrophs and gonadotrophs could not be differentiated before Day 250 of gestation. The study suggests that secretory cells of the fetal foal pars distalis may be active in the production and secretion of hormones throughout the major part of pregnancy.