Development and aging of the teleost pituitary: qualitative and quantitative observations in the annual cyprinodont Cynolebias whitei.

The rapidly aging annual cyprinodont Cynolebias whitei (maximum age under natural conditions: approximately six months) was used to study the histology and growth of the pituitary gland throughout the entire lifespan of a teleost fish. Immunoreactive prolactin and corticotropic cells were present in the prehatching larvae. Somatotropic cells and pars intermedia could be distinguished by histological staining in five days old fish. After three weeks basophilic, GTH-immunoreactive, cells could be observed in the proximal pars distalis while in five weeks old fish PAS-positive and melanotropic cells could be distinguished in the pars intermedia. A weakly chromophilic and GTH-immunoreactive cell type appeared in the proximal pars distalis of 6 weeks old fish. These cells were identified as thyrotropic cells. In fish older than four months cysts and haemorrhagia appeared in all parts of the pituitary gland. Allometric (log-log) plots of fish length and pituitary volume revealed two inflections in the slope of the regression line. Such inflections indicate stanzas (stages or periods) in fish growth. Both inflections found in this study reflect an increased rate of pituitary growth in relation to the growth rate of body length in the subsequent stanzas. The first inflection occurs at about three weeks, coincides with the appearance of GTH-immunoreactive cells and probably represents the onset of maturation. The second inflection occurs at four months and is followed by pituitary hyperplasia and histological deteriorations. This inflection is considered to reflect the onset of aging.