Immunoreactive prolactin in the pituitary gland of cyprinodont fish at the time of hatching.

In the developing pituitary gland of embryos of the annual fish Cynolebias whitei and the medaka, Oryzias latipes, prolactin cells have been identified before hatching by means of a light-microscopic immunocytochemical method with antiserum against ovine prolactin. At the time of hatching, changes in the intensity of the immunoperoxidase staining occur. Histological staining by Cleveland and Wolfe's trichrome shows differentiation of cell types in the adenohypophysis only later in ontogeny. Our results indicate that, in teleosts, differentiated prolactin cells are present before hatching and that prolactin may be involved in the endocrine control of the hatching process.