Microvascular morphodynamics of swine periovulatory ovarian follicles as studied by SEM of vascular corrosion casts

The growth of ovarian follicle and corpus luteum development is dependent on angiogenesis, the proliferation of new capillaries from pre-existing vessels. The morphological changes driven by angiogenesis ensure the adequate metabolic support to the follicularluteal complex and are essential for fertility. However, the morphofunctional relation between neoangiogenesis and fertility are not yet fully understood, especially in the time between the LH surge and ovulation (periovulatory period). In this stage, somatic follicular cells rapidly differentiate in luteal steroidogenic cells. The metabolic switch, from an estrogen-to-steroidogenic activity, is sustained by extensive blood vessel remodelling. The purpose of this work was to describe the sequence of vascular remodelling events, rapidly occurring during the periovulatory period. Preovulatory, early periovulatory and late periovulatory follicles were obtained at a precise timing from prepubertal gilts stimulated by a validated hormonal protocol (eCG+hCG). The swine model was chosen due to a long periovulatory window (40-44 h, as in human). The three-dimensionality of the blood vessel network, the presence of angiogenesis and the patterns of angiogenic figures (sprouting or non-sprouting angiogenesis), were analysed by means of scanning electron microscopy of vascular corrosion casts. Results showed the presence of three concentric vascular plexuses (inner, medium and outer) in all groups. In the inner network, a high angiogenic activity was evidenced in preovulatory follicles, as demonstrated by a dense carpet of capillaries, with sprouting angiogenic figures. In early periovulatory follicles, the layer compactness decreased in consequence of an elongation of capillaries. The pattern of angiogenic figures was similar to the previous group. Close to the ovulation, in late periovulatory follicles, evident modifications in the vessel architecture were observed. The inner layer became undulated, as several folds were visible on its surface. Many gaps showed the characteristics of the underlying medium layer. Sprouting and non-sprouting angiogenesis, as intussusception, were visible. From the data obtained it is possible to conclude that the follicular angiogenesis in pig is high during the preovulatory phase, while the periovulatory period is constituted by a first step of quiescence followed by a second one of active angiogenesis, characterized by sprouting and, interestingly, non-sprouting angiogenesis. The quiescence status, in early periovulatory follicles, represents the starting point for the subsequent metamorphosing process, necessary to transform them into functional corpora lutea.