Immunocytochemical studies on the pituitary gland of Anguilla anguilla L., in relation to early growth stages and diet-induced sex differentiation.

Mammalian and teleost antisera against pituitary hormones were used to identify and localize pituitary cell types in the European eel (Anguilla anguilla L.). The investigation was conducted on unpigmented glass eels of 5.6-6.2 of total body length (L(T)) caught in river mouths, then on yellow eels reared from the pigmented glass eel (or elver) stage up to 12-14 cm of L(T), in an eel farm in warm freshwater. Treated elvers were fed with commercial paste food supplemented with mature carp ovaries, containing oestradiol, that induced an early ovarian differentiation and a higher growth rate. The antisera detected seven types of immunoreactive (ir) cells, six of which were already found in glass eel adenohypophysis, suggesting differentiation of these cell types during the leptocephalus stage. In 12-14 cm treated yellow eels with small ovaries, a seventh type (ir-GtH) was detected in the proximal pars distalis; in the same animals the ir-TSH cells increased in number and size. From unpigmented glass eels to 12-14 cm yellow eels, the whole pituitary volume of controls increased nearly four times, while that of treated ones increased nearly six times. The larger volume of pituitary in treated eels was mainly due to volume increase of proximal pars distalis and rostral pars distalis. The %GH, that is the potential index of GH production, was significantly higher in treated yellow eels with gonads differentiating into ovaries than in controls; no difference was detected in %PRL between treated and control eels. The above results strongly suggest that in eels the feminizing effects of oestrogen is first exerted on the pituitary, probably through the hypothalamus, and later on the gonads.