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G. R. FOXCROFT and H. HASNAIN

Embryonic mortality in domestic rabbits ovulating in response to post-partum matings was investigated initially by Hammond (1925). In non-suckling does or does with small litters, pregnancies were successfully established soon after parturition and mortality appeared to be at a normal level. In does suckling larger litters, however, there was a high incidence of embryonic mortality especially in the early postimplantation period and this was associated with luteal regression. The elevated mortality may have resulted from an inadequate plane of nutrition resulting in reduced FSH secretion and follicular development, associated with luteal regression and absorption of embryos (Adams, 1967). The fertility of post-partum matings has been more recently investigated in Dutch Belted (DB) and Large Albino (LA) strains of rabbits (Foxcroft & Hasnain, 1973) and the levels of embryonic mortality in

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G. R. FOXCROFT and H. HASNAIN

Summary.

The occurrence of mating, ovulation and fertilization of ova was established in Large Albino (LA) and Dutch Belted (DB) does following parturition. The effects of increasing intervals to mating, and presence or absence of suckling in the case of the LA does, on the above parameters of fertility were determined.

No depression in successful mating was found in lactating does compared to weaned controls in the early period after parturition. A significant increase in induced ovulation with increased time to mating was established for DB but not for LA does. In the LA animals, suckling significantly depressed the ovulation response to mating.

The percentage of successfully fertilized ova was significantly lower in some of the post-partum mated DB rabbits compared to controls but no treatment effects on fertilization of ova were established for the LA does.

Patterns of ovarian and uterine involution were observed in the LA does. Ovarian involution was characterized both by rapid regression of corpora albicantia and by a decrease in follicular development. This decrease in follicular development suggests a decrease in pituitary gonadotrophin secretion following parturition, and the resultant decline in follicular oestrogens may be responsible for the formation of smaller CL with increased time to mating after parturition.

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S. Edwards and G. R. Foxcroft

Summary. The timing and dosage of oestradiol benzoate injected after weaning was critical with respect to the pattern of behavioural oestrus and the ovarian stimulation achieved; treatment on the day of weaning (Day 0) and Day 1 with 60 μg oestradiol benzoate/kg body wt produced poor ovulatory responses and abnormal oestrous behaviour. Treatment on Day 2 with 30 μg oestradiol benzoate/kg resulted in consistent oestrus and ovulatory responses although the ovulation rates (10·6 ± 1·1 in 3-week and 12·2 ± 1·7 in 5-week weaned sows) were below those expected in fertile untreated sows weaned at these times. The timing of the preovulatory LH surge (53·6 ± 2 h after oestradiol benzoate) was closely synchronized in all sows and a similar synchronous rise in plasma progesterone concentrations 100 ± 4 h after oestradiol benzoate suggests a similar synchrony of ovulation. The magnitude of the LH and FSH responses to oestradiol benzoate were similar to those that occur in untreated sows and similar differences also existed in gonadotrophin secretion related to the length of lactation.

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S. Edwards and G. R. Foxcroft

Summary. Plasma concentrations of LH, FSH, prolactin, oestradiol-17β and progesterone were determined in 18 multiparous sows at 4-h intervals for 15–18 days around weaning at 3 or 5 weeks post partum. Sampling at 10-min intervals for 6 h occurred every 2 days throughout the same period. Shortening lactation significantly reduced the preovulatory LH surge and altered the pattern of FSH release. However, there was no significant effect on ovulation rate or interval from weaning to oestrus between groups. Weaning was consistently associated with a significant rise in basal LH concentrations whilst FSH secretion remained unaffected. Lactation length did not appear to affect the characteristics of episodic LH secretion before weaning, nor were any consistent changes in LH secretion apparent until the preovulatory rise in LH. Plasma prolactin values declined rapidly at weaning and remained low thereafter. These results indicate that the 'trigger' controlling the return to cyclic ovarian activity after weaning in the pig is complex, but it is suggested that lactational anoestrus and anovulation result primarily from a lack of LH stimulation to the ovary.

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H. J. Shaw and G. R. Foxcroft

Summary. Blood samples were collected from primiparous sows via indwelling jugular cannulae at 15-min intervals for 12 h before and for 24 h (2 sows) or 48 h (10 sows) after weaning and then every 4 h until behavioural oestrus. Weaning to oestrus intervals ranged from 3 to 10 days and 2 sows showed no signs of oestrus and had not ovulated by Days 11 and 16 after weaning.

Prolactin concentrations in plasma decreased significantly (P < 0·001) and reached basal levels 1–2 h after weaning in all sows whilst plasma progesterone concentrations remained basal until ~30 h after the preovulatory LH surge in sows that ovulated. Elevated concentrations of prolactin or progesterone during the post-weaning period were, therefore, not responsible for delayed restoration of cyclicity.

Overall, mean LH concentrations rose significantly (P < 0·001) from 0·22 ± 0·02 during the 12-h period before weaning to 0·38 ± 0·03ng/ml during the 12-h post-weaning period. After weaning, pulsatile and basal LH secretions were markedly increased for sows that showed an early return to oestrus (≤ 4 days) compared with sows showing a longer weaning to oestrus interval but a correlation did not exist between either of these LH characteristics and the time taken to resume cyclicity. Mean LH concentrations before weaning were, however, inversely related (r = − 0·649; P < 0·05) to the weaning to oestrus interval.

Overall, mean FSH concentrations rose significantly (P < 0·001) from 151·1 ± 6·2 (s.e.m.) ng/ml in the 12-h period immediately before weaning to 187·7 ± 9·7ng/ml in the subsequent 12-h period but there was no correlation between FSH concentrations, before or after weaning, and the interval from weaning to oestrus. However, a significant correlation was apparent between ovulation rate and peak concentrations of the rise in FSH after weaning (r = 0·746; P < 0·05) and overall mean FSH values (r = 0·645; P < 0·05).

It is concluded that both LH and FSH concentrations in peripheral blood rose in response to removal of the suckling stimulus at weaning. The increase in LH pulse frequency associated with weaning was not directly related to the weaning to oestrus interval although a specific pattern of LH secretion was observed in sows showing an early return to oestrus ( ≤ 4 days). It is proposed that the characteristics of LH secretion after weaning may reflect the ovarian status at the time of weaning and that this may, in part, be dependent on steroid feedback. The weaning-associated rise in FSH may be involved in the determination of ovulation rate.

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T. Wiesak, M. G. Hunter and G. R. Foxcroft

Summary. 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.

Keywords: follicle; gonadotrophins; steroids; pig

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S. A. Grant, M. G. Hunter and G. R. Foxcroft

Summary. Ovaries were recovered from groups of naturally cyclic pigs (N = 5) on each of Days 16, 18, 20 and 21 of the oestrous cycle. Follicular diameter, follicular fluid volume and concentrations of oestradiol, testosterone and progesterone, and granulosa cell number were determined in all follicles ⩾2 mm in diameter (n = 511). In alternate follicles either granulosa cell aromatase activity and theca testosterone content or 125I-labelled hCG binding to granulosa and theca were determined. The mean total number of follicles recovered per animal decreased as the follicular phase progressed and a strong positive relationship (P < 0·001) existed between follicular diameter and volume on all days. The number of granulosa cells recovered per follicle was variable, and not related to oestrogenic activity of the follicles. Mean follicular fluid oestradiol, testosterone and 125I-labelled hCG binding all increased until Day 20 and decreased on Day 21, whereas mean theca testosterone content, 125I-labelled hCG binding to theca tissue and aromatase were all maximal on Day 21. On Days 20 and 21 a subset of 14–16 large follicles was readily distinguishable from the remaining smaller, less oestrogenically active population in each animal. Yet, consistently within these subsets there was a difference in follicular diameter of ∼ 2·0 mm and also a considerable range of biochemical development even among follicles of equal size. These results indicate asynchrony at the time of recruitment and selection among follicles destined to ovulate and suggest that heterogeneity continues into the immediate preovulatory period.

Keywords: follicle; development; pig; heterogeneity

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M. G. Hunter, S. A. Grant and G. R. Foxcroft

Summary. Ovaries were collected from naturally cycling gilts during the preovulatory period and the stage relative to the LH surge estimated by measurement of oestradiol and progesterone concentrations in follicular fluid. Many of the follicles recovered had become flaccid with an associated increase in follicular fluid viscosity. Marked infolding of both the granulosa and theca tissue in some follicles suggested early luteinization. However, these morphological changes did not necessarily occur simultaneously in the same follicle, or in all follicles within an ovary. Moreover, they were not consistently related to characteristic differences in the concentration of follicular fluid steroids, suggesting either that the morphological and biochemical aspects of the luteinization of follicles may be independently controlled, or may respond at different rates to the same signal.

Keywords: pig; follicle; histology; heterogeneity

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W. Haresign, G. R. Foxcroft and G. E. Lamming

Summary. Examination of hormonal changes occurring in farm species at the onset of puberty, during the follicular phase of the oestrous cycle, and at those times when ovarian activity is re-established after periods of seasonal or lactational anoestrus, provides circumstantial evidence that the final phases of follicular development are dependent on a pattern of tonic (episodic) LH secretion. A suppression of episodic LH secretion is associated with periods of anovulation. Stimulation of tonic LH secretion by repeated injections of small doses of synthetic Gn-RH or purified LH restores normal reproductive function in all but deeply anoestrous animals. Continuous infusion of Gn-RH is as effective as repeated injections. It is suggested that an additional inadequacy, possibly endocrine, contributes to the anovulatory state in deep anoestrus.

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T. Wiesak, M. G. Hunter and G. R. Foxcroft

Summary. Luteal cells were obtained by digestion of luteal tissue of cyclic (day 12) and early pregnant (days 12, 20 and 30) pigs. Suspensions of the dispersed luteal cells (5 × 104 cells ml−1) were incubated for 2 h in minimum essential medium (MEM) alone (control) and MEM with different concentrations of prostaglandin F (PGF) and PGE2 (0·01, 0·1, 1, 10, 100 and 1000 ng ml−1) and luteinizing hormone (LH) 100 and 1000 ng ml−1, or with combinations of LH + PGFand LH + PGE2. Net progesterone production was measured in the incubation media by direct radioimmunoassay.

The overall response pattern of the luteal cells to exogenous hormones on day 12 of the oestrous cycle and pregnancy differed (P < 0·5) from treatment on day 20 and 30 of pregnancy. In general progesterone production was higher (P < 0·5) and the response to PGF and PGE2 treatment was most obvious on day 12 of the oestrous cycle and pregnancy. Overall, PGF stimulated progesterone production in a dosedependent manner (P < 0·05). The response to PGE2 was of a quadratic nature (P < 0·05) in which the lowest and the highest doses of PGE2 were associated with a greater production of progesterone than were the intermediate doses. Treatment of luteal cells with PGF + LH or PGE2 + LH caused overall inhibition (P < 0·05) of progesterone production compared with treatment with each hormone alone. This interaction was not affected by the dose of LH used.

These findings indicate that PGF and PGE2 are involved in the autocrine control of corpus luteum function.

Keywords: pig; luteal cells; prostaglandins; pregnancy