Intra-ovarian events of ovulation. Vascular mechanisms and morphological alterations in the rat and the human

Abstract

Ovulation is the central event of the reproductive cycle and is generated by an LH-induced cascade of mediators with inflammatory-like effects on ovarian blood flow and tissue. Nitric oxide (NO) exhibits vasodilatory effects and the synthesis of NO is increased during inflammation. Recently, NO has gained recognition as a substance with possible ovulatory promoting effects. Biochemical changes in the ovary have been investigated thoroughly during the last decades, but there are few studies dealing with the relation between the biochemical- and vascular/morphological changes occuring during ovulation. The aims of the present thesis were to (i) investigate the presence and localization of nitric oxide synthase (NOS) in the rat ovary, (ii) to study effects of NO in the nonpregnant human female genital tract, (iii) to examine blood flow in the human follicle prior to ovulation, (iv) to develop a method for intravital microscopy of the rat ovary and oviduct, (v) to adapt laser Doppler for ovarian blood flow measurements and (vi) to investigate the possible causal relationship between blood flow and ovulation. By histochemical and immunohistochemical methods, localization of NOS were characterized in the rat ovary during follicular development, ovulation and luteal formation. The results demonstrated high levels of the endothelial NOS (eNOS) in endothelial cells, stroma, and the theca cell compartment. Immunoblotting revealed generally lower levels of the inducible isoform (iNOS), with a small increase in stratum granulosum during the ovulatory phase. In women, examined with transvaginal ultrasonography (TVS)/color Doppler, NO caused a reversible redistribution of blood flow to the subendometrial area and most likely a reversible inhibition of the circadian blood flow changes in the uterine arteries. Examinations of women during the late follicular and the ovulatory phase by TVS/color Doppler, demonstrated a gradual redistribution of blood flow with increased flow in the basal-, unchanged flow in the lateral-, and undetectable blood flow in the apical region of the follicle.A method for in vivo microscopy of the rat ovary and the oviduct revealed short term variations of follicular blood flow. A typical pattern of oviductal motility, trapped the oocyte/cumulus masses in the ampullary segment, where by oscillatory movements a gradual denudation ocurred. Experiments, combining intravital microscopy with computer analysis of blood flow and laser Doppler flowmetry, demonstrated high blood flow with regular and high amplitude short term variations of blood flow (vasomotion). The follicular morphology showed heterogeneity during the process of ovulation, with a variable amount of bleeding and change in shape of the follicle apex. Ligation of either of the two major ovarian vessels caused reduced ovulation rate in the rat in vivo. The decrease of ovulation was more pronounced after ligation of the ovarian artery (OL) compared to the ovarian branch of the uterine artery (UL), where reductions only were observed when the procedure was performed during the early stage of the ovulatory phase. Progesterone levels were reduced after OL.