Structure and fine structure of the hypophyseal pars distalis in endigenous African species of the genus Tilapia

Abstract

The structure and fine structure of the pars distalis hypophyseos was examined in five species of Tilapia fishes (T. alcalica, T. grahami, T. leucosticta, T. zillii, T. nigra) which were collected from lakes of a wide range of salinities. The pars distalis in all the species is composed of 5 granulated (“secretory”) and 1 chromophobic cell types. The rostral pars distalis prolactin cells appear most numerous and active in the fresh water species and smaller and least active in the “soda” lake fish. The evidence from nuclear measurements suggests that the species adapted to hyposmotic media have compensated for the freshwater environment (and the subsequent need for greater prolactin secretion) by increasing the number of prolactin cells rather than by increasing the synthetic activity of individual cells.

In “soda” lake species which were acclimated to fresh water the prolactin cells are markedly hyperactive and degranulated when compared with any other group.

The ACTH cells appear more active in the “soda” lake species than in the fresh water groups, however, these cells are maximally active in “soda” lake fish acclimated to fresh water.

The rostral pars distalis stellate cells are described and discussed in relation to their possible involvement in the release of hormone from the pars distalis “secretory” cells.

The proximal pars distalis somatotrophs appear active in all the species investigated although they were maximally active in fresh water acclimated “soda” lake species. The structure of the proximal pars distalis gonadotrophs and thyrotrophs is variable both within the same animal and between the species but the variation is not consistent with environmental salinity parameters.

The means by which granules are released from the different cell types is discussed.