Endometrial responses to corpus luteum products in cycles with induced ovulation: theoretical and practical considerations.

Products of the corpus luteum have targeted actions on the endometrium. Besides steroid hormones, the corpus luteum produces biologically active substances which may be either unique or shared by the endometrium and other tissues. Here we review selected markers of the corpus luteum and the endometrium as candidates for functional markers of the interplay between the two sites and relative to various treatment modalities. In clinical routine, the assessment of luteal phase is performed by morphological criteria. The timing of endometrial biopsy is important because specimens taken at different stages of the luteal phase give different results. After human menopausal gonadotrophin (HMG) superovulation, there is dyssynchrony in the morphological maturation of endometrial glands and stroma, and a marked difference has been found in endometrial development between progesterone-supplemented and non-supplemented cycles. The expression of steroid hormone receptors in endometrium is affected by ovarian stimulation regimens. After gonadotrophin-releasing hormone analogue/HMG superovulation, the progesterone receptor (PR) has been found less frequently in progesterone-supplemented cycles than in non-supplemented cycles. The relative distributions of oestrogen receptor and PR between epithelium and stroma have been reported to vary according to the number of days of progesterone exposure. Thus, the detection of PR in endometrial glands in the late luteal phase indicates that exposure of the endometrium to the action of progesterone is short. Certain biochemical changes in the uterus are not reflected in endometrial morphology. Under the influence of progesterone, secretory glandular epithelium synthesizes placental protein 14, more recently named glycodelin. Glycodelin inhibits the innate immune system and also has contraceptive actions. Endometrial glands secrete glycodelin into glandular lumen, uterine fluid and blood, where the concentrations rise during the last week of the secretory phase. The effects of various ovarian stimulation protocols on serum glycodelin concentrations are reviewed, along with recent studies on relaxin, prolactin and insulin-like growth factor binding protein 1, all products of the secretory endometrium.