The Fine Structure of the Pars Intermedia of the Pituitary in the Human Fetus

The pars intermedia of the pituitary of the human fetuses aged from 5 to 9 months was studied with the electron microscope. The pars intermedia consists of epithelial cells bounding the Rathke's hypophysial cleft and an underlying interstitial space containing blood capillaries, collagen fibrils and some fibrocytes. Two kinds of epithelial cells are distinguishable: the marginal cells which line the cleft and the glandular cells which are located under the former and contain secretory granules of various sizes. The marginal cells are columnar and characterized by the presence of a large number of glycogen granules which tend to decrease in number with advancing age. The cells of this kind show no dense granules suggesting secretion. The glandular cells are composed of three types: Type I cells contain granules about 2, 000Å in diameter, and type II cells larger ones about 3, 500Å. Both types of cells are commonly observed and have a well-developed Golgi apparatus and rough endoplasmic reticulum besides numerons secretory granules. The cells of type III possess small granules about 1, 200Å and are few in number. Many of the capillary endothelial cells found in the interstitial space show fenestrations of the cytoplasm as early as the fifth month of fetal life. Possible activity of the pars intermedia in hormonal secretion in this early stage of development was discussed.