The ACTH cells in the pituitary gland of the nine-spined stickleback, Pungitius pungitius L.

The ACTH cells form a layer 1-8 cells thick, dorsal to the prolactin cells in the rostral pars distalis. They react only mildly with a variety of stains including PAS-lead haematoxylin. Their nuclear diameters vary seasonally in a manner that closely parallels that of the prolactin cells. The relative volumes of the ACTH and prolactin cell zones are remarkably constant in animals of different sizes. It is suggested that the two hormones may act synergistically at various body sites and that this accounts for the related morphological features of the ACTH and prolactin cells. There are no changes in the surface density of the ACTH zone with increasing animal size. Consequently, the ACTH/neurohypophysial border is highly convoluted in large animals. The proximity of the neurohypophysis also influences cell ultrastructure, as small processes, packed with secretory granules are more numerous near the basal lamina separating the adeno- and neurohypophyses. A morphometric analysis of ACTH cells provides base-line ultrastructural data for experimental studies and for comparisons with other teleosts. The cells have small, secretory granules, 100-300 nm in diameter, and of variable electron density. There is little rough endoplasmic reticulum and a small Golgi apparatus. There is no evidence of granule release by exocytosis and various explanations for this are suggested.