An electron microscopic study of the pars distalis of the dog adenohypophysis

The parenchymal cells of the pars distalis of the dog adenohypophysis were studied with the electron microscope. Criteria for the differentiation of four cell types included the size and shape of the cells and the size, shape and distribution of organelles and secretory granules. Three secretory granule-containing cells designated as I, II and III were observed. Cell type I was further divided into subtypes (Ia, Ib, Ic) on the basis of the granule size of maximal frequency. A fourth cell type (IV) contained no or only a few secretory granules. It was unique in having a highly irregular shape with cytoplasmic processes extending between the other cell types and also occasionally enclosing follicular cavities.