Hormonal control of the corpus luteum and embryonic diapause in macropodid marsupials

Embryonic diapause in the macropodid marsupials is associated with an undeveloped corpus luteum (CL) and low levels of progesterone in plasma. The fertilized egg will enter diapause after ovariectomy and can survive for several months but for reactivation to occur the CL must be present and grow, and there must be a rise in plasma progesterone concentrations. Therefore the central question in the endocrine control of this phenomenon is how the activity of the CL is controlled. The pituitary exerts a tonic inhibition on the CL, probably through prolactin, and when this is released, even temporarily, the CL is reactivated. Once reactivated, no pituitary support is needed for either CL growth or steroidogenesis. The membranes of the luteal cells contain abundant prolactin receptors and prolactin probably exerts its inhibitory effect directly on the CL. Prolactin does not appear to affect steroidogenesis directly, since progesterone production in vitro does not differ between quiescent and reactivated CL and is not altered by addition of prolactin or LH. The increase in plasma progesterone after reactivation can be accounted for by the increase in CL mass. We conclude that circulating prolactin inhibits hyperplasia and hypertrophy of luteal cells and that as the prolactin level declines the CL increases in size, causing plasma progesterone levels to rise and blastocyst growth and development to resume.