Role of prolactin in regulation of reproductive cycles

Abstract

Prerequisities for viviparity are internal fertilization, megalecithal eggs (although yolk may be reduced secondarily), and a “uterus.” The evolution of refinements in mammals have led to control of gestation, at least initially, by progestins produced by the corpora lutea under the influence of pituitary prolactin. This control may be replaced by placental prolactin or progestin. Corpora lutea are ubiquitous in the vertebrates; a possible role of progestins or prolactin in viviparity in nonmammalian forms is examined. Viviparity does not occur in cyclostomes; there is no evidence of progestins or prolactin in this group. Chondrichthians are commonly viviparous; a yolk-sac placenta may be present. Progestins occur, but estrogens may be more important in stimulating “uterine” gestational changes. Osteichthians are infrequently viviparous; yolk-sac placentae are not formed, and evidence for progestins is lacking. Neither of these two groups of fish in general show a prolactin with mammalian type of activity. Viviparity is found in very few amphibians, and placental attachments are lacking in these. Progestins occur in oviparous species, as does pituitary prolactin with mammalian-type activity (mammotropic and pigeon-crop stimulatory effects). However, there seems to be no basis for considering that a prolactin-luteal-progestin axis is concerned in gestation in any of the preceding forms. Viviparity is common in the ophidian and saurian reptiles; both yolk-sac and allantoic placentae are found. Evidence for a function of corpora lutea or pituitary in gestation is conflicting, but progestins occur and are elevated in pregnancy. Pituitary prolactin with mammalian-type activity is present. It is possible that a role of prolactin and progestin in viviparity evolved in this group. Finally, viviparity is absent in the Aves, despite the presence of progestins and a fully-active prolactin. However, the neuroendocrine mechanisms controlling prolactin release appear to have been modified for parental care in such a way that the hormone is unavailable for gestational purposes.