ABSTRACT
Hypothalamic-Pituitary Axis Mammalian tropic hormones from the anterior pituitary (Ch. 7) have molecular counterparts among nonmammalian (or submammalian) vertebrates, and several nonmammalian functions have been experimentally stimulated by these mammalian agents. Early in vertebrate evolution 3 cell types appear to have differentiated to elaborate glycoproteins (LH, FSH, and TSH), large polypeptides (i.e., proteins; GH and PRL), and the POMC-related peptides (ACTH, MSH, and endorphins). These cell types attained functional significance and gave rise via amino acid substitutions, modified cleavage of prohormones, or both, to the additional hormones that characterize each group. Nonmammals produce the same tropic hormones as mammals, with considerable homology among them. Nonetheless, there is evidence of certain functional alterations elicited by these molecules related to minor structural and receptor modifications.
