Pig Model to Study Dynamics of Steroids During Ovarian Follicular Growth and Maturation

Abstract

The pig is a polyovular farm animal and its prolificacy differs in different breeds. The female pig has an estrous cycle of 20–21 days with a short follicular phase of only 3 days and a long luteal phase. The luteal phase starts at day 1 after ovulation to day 15 of the estrous cycle and it overlaps the early follicular phase on day 16–17. The follicular phase lasts only 3–4 days. The early follicular growth is initiated while a high concentration of progesterone (P) is secreted by active corpora lutea, circulating it in the peripheral blood. In this study, P, androstenedione (A), testosterone (T), estrone (E1), and estradiol-17β (E2) were measured in peripheral blood, ovarian venous blood, follicular fluid, and perifollicular blood at various stages of the pig estrous cycle. Concentrations of P in peripheral blood, ovarian venous blood, follicular fluid, and perifollicular blood (blood from the vasculature surrounding the follicle) during the active luteal phase of gilts were 19.0 ±2.4, 59.2 ±12.7, and 3937 ±710ng/ml, respectively. Interestingly, the high concentration of P in perifollicular blood did not equilibrate with the follicular fluid and it appears that there is a blood barrier between the two. This level of P was significantly lower in the late luteal, follicular, and estrous stages. The follicular fluid in the follicular phase contained the highest concentrations of A (163 ±37.9ng/ml) of all the stages of follicular growth. During this stage the perifollicular blood also contained a concentration of A higher than that observed at any other stage of the estrous cycle. On the day of estrus the amounts of both A and T in the follicular fluid as well as in ovarian venous blood were at their lowest. Concentrations of T in perifollicular blood were higher in the luteal phase than in the follicular phase. The high levels of E 1 (22.94 ±2.95 ng/ml) and E2 (18.96 ±1.80 ng/ml) in the follicular fluid during the follicular phase may be due to aromatization of A and T, which are also present in high concentrations in follicular fluid and are in contact with granulosa cells. On the day of estrus E1 and E2 in the follicular fluid were 7.42 ±1.45 and 6.93 ng/ml, respectively, which may precede the onset of the preovulatory surge. The lower levels of E1 and E2 in perifollicular blood than in follicular fluid suggest that the main site of aromatization could be the granulosa cells in the follicles.