Functional morphology of the pars intermedia of the rat hypophysis as revealed with the electron microscope. 3. Effects of dexamethasone on the pars intermedia of rats under various experimental conditions.

Abstract

The general structures of the pars intermedia of the rat hypophysis including two kinds of light cells, dark cells, marginal zone, cysts and nervous elements were described, and effect of dexamethasone on the pars intermedia, especially on the light cells of the first type of intact and experimental rats was studied by electron microscopy.Three hours after a single injection of dexamethasone secretory granules of the light cells of the first type were extremely increased in electron density, but after 3 weeks the electron density of the granules was decreased nearly to the normal level.Prolonged daily injection of dexamethasone caused a cumulative effect on the light cells as an increase of dilated endoplasmic reticulum at 7 days and considerable degeneration of whole cells at 10 days.One day after adrenalectomy the light cells which were also treated with dexamethasone 6 hours after the operation contained numerous clear vesicles, while in later adrenalectomized animals which received dexamethasone 24 hours prior to sacrifice the cells successively increased their dark secretory granules from 3 to 7 days and reached the maximal number at from 10 to 14 days after the operation. Four weeks after adrenalectomy the light cells appeared to have a large cell volume and to contain granules of various densities, extensive Golgi apparatus and cisternae of the endoplasmic reticulum filled with dense material.Two and a half minutes after laparotomy, the light cells which were pretreated with dexamethasone 24 hours before the operation contained a mixture of dark and light granules. The dark granules were decreased in number one day after the operation and were replaced by electron lucent vesicles 3 days after the operation.These results apparently indicated that the release of granule content of the light cells of the pars intermedia was inhibited by dexamethasone which had been known to prevent a release of ACTH from the adenohypophysis, and that the pars intermedia responded to stressful stimuli such as adrenalectomy and laparotomy, showing morphological alterations of the density of secretory granules. The morphological changes showed a good parallelism with changes of blood and pituitary ACTH concentrations. From these findings two possibilities may be suggested either the pars intermedia cells might produce an ACTH-like substance, or the intermedia hormone might be a precursor of ACTH in biosynthesis in the pars distalis.