Intraovarian mechanisms influencing the human corpus luteum

Abstract: Introduction: The human corpus luteum (CL) is a transient endocrine gland, only functionally active for about 14 days. Its principal function is to produce and secrete progesterone and thereby support the endometrium for implantation of a blastocyst and prevent rejection of the developing embryo. In the event of a non-conceptive cycle, functional and structural demise of the CL follows and a new ovulatory cycle begins. The aims of the thesis were to study different mechanisms involved in the extrinsic and intrinsic regulation of the CL and correlate these findings to available clinical investigations tools.Materials and methods: Sixty women volunteered to donate their CL prior to scheduled surgery due to benign conditions. They were grouped according to CL-age, based on the occurrence of a preovulatory luteinizing hormone (LH) surge where days 2-5 post LH surge were designated as early luteal phase, days 6-9 as mid luteal phase and days 11-14 as late luteal phase. The CL bearing ovary was investigated using transvaginal ultrasonographical B-mode and color Doppler imaging prior to surgery. Following excision, the CL was further characterized using anatomical and morphological measurements, in vivo and in vitro hormone synthesis, isolation and cultures of steroidogenic luteal cells. Moreover, active transcription of putative regulatory genes of interest was targeted using semi-quantitative reverse-transcription polymerase chain reaction, in situ hybridization and Northern blots, and these genes' respective translation products were characterized by immunocytochemistry.Results: The bulk of progesterone is stimulated by human chorionic gonadotropin (hCG) in the peripheral (steroidogenic) layer of the CL, where the LH/hCG receptor, as well as progesterone receptor (PR) isoform A/B and estrogen receptor type β (ER-β), but not ER-α, was localized. The sesitivity towards hCG was highest during the mid luteal phase in concordance with the value of LH/hCG receptor coding mRNAs. During this phase, despite low levels of PR-B mRNA, hCG treatment initiated a significant rise in progesterone output which could be blunted by the PR antagonist mifepristone. Increased amounts of prostaglandin (PG) F2α and its respective receptor (FP) mRNA were detected during the later developmental stages of the CL. However, steroidogenic luteal cells were unresponsive to added PGF2α until late luteal phase, indicative of an acquisition of sensitivity to the proposed luteolytic signal during this stage. Intraluteal vascular density was highest in early luteal phase and dramatically decreased during the course of CL development, a finding which was inversely correlated to resistance to blood flow in intraovarian blood vessels supplying the CL. Furthermore, a high degree of agreement between ultrasonographical and anatomical measurements of the CL was found.Conclusions: Based on the novel findings herein, the hypothesis of steroid influence, acting within or near the steroidogenic luteal cells is confirmed. Alongside the classical extrinsic signals (e.g. hCG) and locally produced factors (e.g. PGF2α) these findings may further explain their modulatory roles during luteolysis or very early pregnancy. Furthermore, transvaginal ultrasonography in combination with color Doppler measurements may serve as a clinical tool to evaluate CL function.