Endometrial prolactin and implantation.

The role of prolactin in human implantation and human endometrial function is still unclear. Synthesis of prolactin from human endometrium and decidua was first demonstrated in 1977. Prolactin mRNA isolation from term decidua subsequently confirmed expression of the prolactin gene in human endometrium. More recently, a glycosylated form of prolactin has been isolated from both the human pituitary and human endometrium. It appears that predominately glycosylated prolactin is secreted in the late luteal phase of the menstrual cycle, with increasingly greater amounts of non-glycosylated prolactin secreted as pregnancy advances. The biological significance and regulation of prolactin glycosylation is uncertain, although glycosylated ovine prolactin has only 20-33% of the lactogenic activity of ovine prolactin. Pituitary lactotropes, of ectodermal origin, produce pituitary prolactin and are regulated by dopamine, oestradiol and thyrotropin releasing hormone. Decidual cells, of mesodermal origin, are not influenced by these pituitary secretogogues. Progesterone and calcium both appear to stimulate pituitary and decidual prolactin secretion, while arachidonic acid seems to inhibit decidual prolactin release. More recently, insulin like growth factor-1, a polypeptide growth factor implicated in the oestrogen promoted growth reproductive tissues, has been found in high concentration in the pig uterus and has been shown to stimulate the synthesis and secretion of prolactin from human decidua. Prolactin receptors have not been reported on endothelium of blood vessels and no clear evidence exists that endometrial prolactin may modulate the secretion of endothelin, endothelium derived relaxing factor or other potentially important substances controlling menstruation. Prolactin does appear necessary for the normal production of macrophage activating factors, including interferon, and may have a local immunomodulatory role upon human implantation for this reason. Human alpha 2-interferon has sequence homology to ovine trophoblast protein-1 and is found in high concentration in human pregnancy and fetal tissues. There is some evidence that interferon may reduce human endometrial prolactin secretion, but whether endometrial or decidual prolactin is critical to human implantation awaits further study.