Localization of the pituitary lactotropes and thyrotropes within Ambystoma gracile by histochemical and immunochemical methods. A developmental study of two populations.

The prolactin-producing cells are the first hormone-producing cells of the pars distalis to be differentiated within Ambystoma gracile. They first appear when the larvae attain a length of approximately 3.0 cm snout to vent length (SVL). Thyrotropes are observed as the next chromophilic cells to appear, and they occur when the larvae are approximately 4.5 cm SVL. Both thyrotropes and lactotropes increase in numbers until metamorphosis. Gonadotropes begin to appear when larvae attain a size of 5.0 cm SVL and become extremely abundant when larvae are in excess of 7.0 cm SVL. Animals, generally, exhibit the greatest number of thyrotropes just prior to the mean size for metamorphosis, and metamorphosing animals exhibit a dramatic reduction in the number of thyrotropes. Neotenous larvae have an abundant number of thyrotropes which are mainly located along the caudal periphery of the pituitary.