Phylogenetic distribution, regulation of secretion, and prolactin-like effects of placental lactogens.

The occurrence of placental lactogens has been reported for members of 4 orders, 7 families, and 15 genera, while its absence has been documented in representatives from 4 orders, 5 families, and 5 genera. Placental lactogens from several genera have been isolated, purified, and characterized biochemically. The cellular sites of production and patterns of secretion of placental lactogens have been described. The regulation of placental lactogen secretion is not understood, although its synthesis and release can be modified experimentally by altering metabolic, ionic, and hormonal factors in the placental environment. Placental lactogens stimulate lactogenesis and steroidogenesis. The mechanism underlying placental lactogen's role in these processes has not been clarified.