A light and electron microscopic study of Rathke's pouch in fetal rabbits

Abstract

Rathke's pouch, the anlage of the adenohypophysis, has been studied in fetal rabbits with the light and electron microscope from its first appearance on the tenth day of gestation until the sixteenth day when secretory granules are observed in some of the cells of the pars distalis portion of the pouch. The pouch is considered undifferentiated from the tenth through fifteenth day since secretory granules were not observed within any of its cells during this period. The secretory granules observed for the first time on the sixteenth day of gestation within cells of the pars distalis closely resemble those found in basophils in the adult rabbit pars distalis. The cytologic development of the partes intermedia and tuberalis is essentially identical to that of the pars distalis from the tenth to fifteenth day of gestation. Rathke's pouch at ten days is a single-layered epithelium composed of a single cell type characterized by a high nuclear-cytoplasmic ratio, irregular contours, and very few organelles except for an abundance of ribosomes. Lipoprotein inclusions are common near the luminal surface from ten to twelve days but diminish thereafter. In specific regions of the pouch large, polymorphic structures are observed located intra- and extra-cellularly. These are tentatively designated “odd bodies.” A striking feature of the pouch at ten and eleven days are large, extracellular spaces across which processes of the pouch cells contact one another. Cell processes also extend between the pouch and adjacent neural tube and notochord. The extracellular spaces gradually diminish from the twelfth to fourteenth day as the number of cells increases. Organelles increase gradually during this period though the cells are still not richly endowed. At fifteen days (the day preceding the appearance of secretory granules and mesenchymal infiltration) the entire cell population of Rathke's pouch undergoes a radical change. The cells appear swollen, electron lucent, and organelles are diminished in quantity. The extracellular spaces are almost completely obliterated. These changes in the cytology at fifteen days are considered a normal prelude to further cyto-differentiation which is evident in the pars distalis at sixteen days and later.