© 1972 by The Society for Integrative and Comparative Biology
Ovarian Control in Gyclostomes and Elasmobranchs
Department of Zoology, University College of North Wales Bangor, Caerns , U. K.
All vertebrates show some degree of periodicity in their reproductive physiology, and there is evidence that light and temperature are the external stimuli most commonly involved in the regulation of reproductive cycles. Furthermore, except in cyclostomes and elasmobranchs it is already evident that these cues are integrated by the hypothalamus which in turn controls the gonadotrophic activity of the pituitary gland. The existence of such mechanisms has not as yet been demonstrated in these two groups though more work is essential before it can be said that they do not exist.
In lampreys the gonadotrophic function of the pituitary appears to be restricted to controlling rate of growth of the oocytes and steroidogenesis. In hagfishes the situation is less well understood though potentially more interesting, since both corpora lutea and corpora atretica are encountered in these animals.
The main, if not the only, gonadotrophic region of the elasmobranch pituitary is its ventral lobe. Nothing is known of the mechanisms which control the cyclical activity of this lobe, though such activity must be postulated in at least some elasmobranchs. Vitellogenesis appears to be under pituitary control since removal of the ventral lobe prevents it and causes the granulosa to lose its vitellogenic function and become phagocytic. It also seems likely that steroidogenesis by the elasmobranch ovary is under pituitary control though this has not been demonstrated directly. Only further work will reveal whether parts of the pituitary other than the ventral lobe are also implicated in ovarian control in elasmobranchs.