Morphological changes and protein secretion induced by progesterone in the endometrium during the luteal phase in preparation for nidation.

Progesterone induces morphological modifications and protein secretion during the luteal phase to permit implantation/nidation. It acts through specific receptors which are regulated by oestrogens. The proliferative phase is thus a prerequisite for progesterone activity. Changes in morphology are first located in the glandular component, with peak secretion occurring during the implantation window. The various components of the stroma are involved during the second part of the luteal phase, resulting in decidualization and the formation of spiral arteries. The subsequent fall-off in progesterone leads to menstruation. The next cycle begins with a regenerative process in which oestrogens induce the modifications typical of the proliferative phase. The luteal phase is usually assessed on morphological criteria but should also be accompanied by protein secretion profiles of glands and stroma.