Differences in follicular morphology, steroidogenesis and oocyte maturation in naturally cyclic and PMSG/hCG-treated prepubertal gilts

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Abstract
Ovaries were obtained from naturally cyclic pigs on Days 16-17, 18, 19, 20 and 21 of the oestrous cycle and on the basis of observed follicular characteristics were assigned as representative of the early (Group 1), mid- (Groups 2 and 3) or late (after LH: Group 4) follicular phase. Follicular development in cyclic gilts was compared with that in ovaries obtained from late prepubertal gilts 36 (Group 5) or 72 (Group 6) h after treatment with 750 i.u. PMSG alone, or with a combination of 500 i.u. hCG 72 h after PMSG and slaughter 30-40 h later (Group 7). After dissection of all follicles > 2 mm diameter, follicular diameter, follicular fluid volume, follicular fluid concentrations of progesterone, oestradiol and testosterone, and the stage of oocyte maturation were determined. Combined PMSG/hCG treatment of immature gilts resulted in a pattern of follicular development different from that in naturally cyclic gilts during the follicular phase. Overall exogenous gonadotrophin treatment also increased (P < 0.001) the variability in follicular diameter and fluid volume. Comparisons between appropriate groups also established differences in the variability of both morphological (diameter and volume, Group 1 vs Group 5; P < 0.05) and biochemical development (follicular fluid oestradiol, Group 3 vs Group 6 and Group 4 vs Group 7; both P < 0.05). Such differences in both morphological and biochemical characteristics between cyclic and PMSG hCG-treated gilts were particularly evident in the population of larger (> 6 mm) follicles. These results indicate that the pattern of follicular development in naturally cyclic and in PMSG/hCG-treated gilts is dissimilar and suggests that the ovaries of gonadotrophin-treated prepubertal gilts are functionally different from the ovaries of mature females.