The microvasculature of the ovary. A review by SEM of vascular corrosion casts

The vascular changes of the ovary were studied by scanning electron microscopy of vascular corrosion casts in estrous, pseudopregnant (stimulated with human chorionic gonadotropin - hCG) and pregnant rabbits. The results demonstrated that ovarian cycle and pregnancy may induce both structural and functional changes in the ovarian vessels. In fact the ovarian blood vessels adapted their structure to the temporary functional needs of the recruited follicles or corpora lutea. These changes involved both larger ovarian blood vessels (supplying the hilus and the medulla) and cortical blood vessels (supplying the luteo-follicular complexes). Hilar and medullary blood vessels also displayed morphological devices for the continuous control of the blood flow (spiral arteries) and for the local recirculation of endocrine products (artero-venous contacts). Such contacts, may likely sustain a "countercurrent mechanism" which was also shown in ovaries of other species. Scanning electron microscopy of vascular corrosion casts even demonstrated that cortical blood vessels are subjected to continuous remodeling. SEM showed different morphological types of vascular plexuses which supplied antral follicles (Types 1-2), atretic follicles (Types 3-4), peri-ovulatory follicles (Type 5), growing pseudopregnant corpora lutea (Type 6) regressing pseudopregnant corpora lutea (Type 7) and pregnant corpora lutea (Type 8). In estrous rabbit, growing to mature follicles (Types 1-2) showed a gradual enlargement and proliferation of the theca capillaries. These changes, associated with capillary hyper-permeabilization were observed in ovulatory and post-ovulatory follicles (Type 5), after hCG stimulation. The corpus luteum formation (Type 6 and 8) was accompanied by additional capillary dilation, diffuse angiogenetic sprouting and organization of conspicuous venous drainage which appeared more enhanced in pregnant (type 8) than in pseudopregnant (type 6) corpora lutea. The regression of the corpus luteum (Type 7) was characterized by the appearance of avascular areas within the gland and by regression of vascular dilatation. Finally, in estrous rabbit the atretic follicles (Types 3-4) wall showed large interruptions (avascular areas) and focal invasion of the central cavity by newly formed capillaries arranged randomly. The hCG stimulation did not affect consistently the interstitial microvasculature. It is concluded that the gradual increase of ovarian blood flow occuring during follicle growth and corpora lutea formation is provided by a functional adaptation of major ovarian vessels and by capillary functional (vasodilation) and structural (angiogenesis) support as well