The pars intermedia (intermediate lobe of the pituitary) in the fetal sheep: an ultrastructural study.

The pars intermedia from 13 fetal sheep (gestational ages ranging from 64 to 143 days) was examined by light and electron microscopy. The pars intermedia consists of a thin zone of cells sandwiched between the pars distalis and the neurohypophysis. During removal of the pituitary gland the neurointermediate lobe usually separated from the pars distalis; the line of division was along Rathke's cleft. The pars intermedia and neurohypophysis were intimately associated with large blood vessels and connective tissue marking the junction. As gestation progressed the overall width and vascularity of the pars intermedia increased. The tissue of the pars intermedia contained two distinct cell types; secretory and stellate. The secretory cells were the larger and more abundant, and showed signs of a high level of activity. The stellate cells were small, contained few organelles and possessed long cytoplasmic processes which extended between secretory cells. These cells also lined Rathke's cleft.