Comparative Anatomy and Physiology of the Anterior Pituitary

Abstract

The most primitive living vertebrates already possess a fully differentiated pituitary body. The evolutionary antecedents of the pituitary body are unknown, and it seems improbable that they shall ever be found. Within the vertebrate group the structural relationships of the parts of the pituitary are fairly constant. The most remarkable variations are the presence of a "ventral" lobe in elasmobranchs, the presence of an "Ubergangsteil" in teleosts, and the absence of the pars intermedia in birds and some mammals. Exploration and comparison of the hormone content of pituitaries of lower vertebrates with mammals are still far from complete, but several interesting conditions have been shown to exist. The presence of the luteinizing hormone has been claimed for the hypophyses of fish and birds, neither of which has corpora lutea. The lactogenic hormone has been reported in all vertebrate classes, but its "target organ" is found only in one. Environmental factors influencing cyclical activity of the pituitary may be of a different nature even within rather closely related vertebrate groupings. The reduced efficiency of administered pituitary hormones from very divergent sp. may be explained by the concept of a "relative sp.-specificity.".