Utero-ovarian relationships in placental mammals: role of uterus and embryo in the regulation of progesterone secretion by the corpus luteum. A review.

The role of the uterus and that of the embryo in regulating luteal progesterone secretion appear to differ greatly, depending on the mammal. However, the situation is analogous in some mammals. The main role of luteal progesterone is to permit the establishment and maintenance of pregnancy, and there appears to be a relationship between the type of placenta and the uterine and embryonic modes of regulating luteal activity. When placentation is endotheliochorial (carnivores), the corpus luteum is active over a remarkably long period; the uterus does not have any limiting action in any of the species studied and the presence of an embryo does not profoundly modify luteal progesterone secretion. When placentation is epitheliochorial (artiodactyls), hysterectomy, like the presence of an embryo in the uterus, considerably prolongs luteal activity in all the species studied. The uterus limits the duration of progesterone secretion by producing luteolytic PGF2 alpha. The embryo maintains progesterone secretion by acting directly on the uterus, causing a change in the synthesis of prostaglandins; this change eventually favors the synthesis of luteotropic PGE's. Moreover, the embryo secretes LH-like and possibly prolactin-like luteotropic factors. When placentation is hemochorial (rodents, lagomorphs, higher primates), the uterus plays a slight role or none at all in regulating luteal activity in all the species studied. The presence of an embryo prolongs and often increases progesterone secretion. The embryo acts in two ways: (a) by causing decidual tissue to form in the endometrium; this tissue secretes the luteotropic factors, uterine "prolactin" and sometimes PGE, and (b) by secreting an LH-like chorionic hormone that is produced in large quantities in primates.