THE FINE STRUCTURE OF THE PARS INTERMEDIA OF THE PITUITARY BODY IN THE MONKEY, MACACUS EUSCATUS

The pars intermedia of the pituitary body in the adult male monkey (Macacus f uscatus) was investigated by means of electron microscope. The pars intermedia contains glandular cells, agranular cells, and a great number of unmyelinated nerve fibers. The glandular cells are further composed of three types of cells : Type I contains numerous large secretory granules (about 2500 - 4000 Å in diameter) with variable electron density, and are most dominant in number. Type II possesses smaller dense secretory granules (about 2000 - 2200 Å) and the well-developed rough-surfaced endoplasmic reticulum. Type III contains the smallest dense secretory granules (about 1300- 1500 Å). Both types, II and III, are quite small in number. Agranular cells have no granules suggestive of secretion, and may serve as the supporting cell. In the monkey pars intermedia, three types of nerve endings can be distinguished ; (I) nerve endings only containing synaptic vesicles of about 500 Å, (II) nerve endings with synaptic vesicles and large dense-cored vesicles (large granular vesicles), about 900 Å, and (III) nerve endings containing synaptic vesicles and elementary granules of neurosecretory substance, about 1500 Å. The results may indicate that the pays intermedia of the adult male monkey is still active in secretory function.