Cycle In Sheep Research Articles

Role of intrafollicular regulators and FSH in growth and development of large antral follicles in sheep.

Manipulation of circulating concentrations of hormones and ovarian follicle status was carried out on Day 11-12 of the oestrous cycle in sheep. All follicles visible on the ovary were ablated by cautery and ewes were treated with oestradiol or ovine follicular fluid (oFF) to suppress FSH or with PMSG to increase circulating gonadotrophic activity. One group underwent unilateral ovariectomy which greatly increased endogenous FSH and was the only treatment which significantly affected LH pulse frequency. The size distribution of antral follicles, the extent of atresia and the mitotic index of granulosa cells of follicles on Day 15 showed that (a) treatment with oFF inhibited the growth of follicles beyond 2 mm diameter by suppressing the mitotic index of the granulosa cells and (b) the concentration of FSH in peripheral plasma was related to the ability of small antral follicles to grow during the late luteal-early follicular phase of the cycle. Subsequently, it was demonstrated that oFF inhibits, in a dose-dependent manner, folliculogenesis sustained by PMSG in ewes on Days 12-15. Inhibition of folliculogenesis was represented by a decrease in those follicles greater than 4 mm, an increase in the relative proportion of follicles less than 2 mm, and minimal change in the average number of follicles visible on the ovarian surface, and a decrease in the mitotic index of granulosa cells of follicles less than 2 mm. There was no change in the extent of atresia.(ABSTRACT TRUNCATED AT 250 WORDS)

Follicular dynamics throughout the oestrous cycle in sheep. A review.

Whatever the stage of the cycle, follicles of various sizes are apparent on the surface of the ovary of sheep. No evidence of waves of follicular growth at fixed-time intervals has been found during the luteal phase, and growth probably occurs at random until the follicle attains a size of 4 to 7 mm. This shows that follicles can enlarge under a wide range of hormonal levels. The recruitment of a crop of follicles, including the one which will ovulate, occurs at variable times around luteolysis due to the interaction of endocrine and follicular factors (FSH priming, sensitizing the follicle to increased LH pulsatility). All healthy follicles greater than 2 mm in diameter are recruited. Selection of the follicle due to ovulate can be defined by morphological criteria (size) or by its "killing" ability. In all cases, the timing of follicle selection is highly variable, and it is due to this variability that the mechanisms of selection are not fully known. They are presumably intraovarian, one of the follicles actively inhibiting the growth of the others. The dominant follicle is probably maintained because of its high oestradiol content, while the others undergo atresia. The main feature of terminal follicular development is its flexibility in terms of hormonal requirements, follicle size and the timing of the main events.

Hormone receptor control of pulsatile secretion of PGF 2α from the ovine uterus during luteolysis and its abrogation in early pregnancy

Studies on the control of PGF 2α secretion by hormone receptors in the ovine uterus based on in vivo infusion experiments in the autotransplanted uterus are summarized. In addition, direct measurement of hormone receptor dynamics for oestradiol-17β, progesterone and oxytocin in the ovine uterus under experimental conditions are reported. Investigation of the antiluteolytic effect of early pregnancy in sheep was approached in two ways: (a) studies on the antiprostaglandin secreting effect of the embryo and (b) studies on a putative luteoprotective effect of the embryo. In the former study, the measurement of PGF 2α in samples of uterine venous plasma collected hourly from 5 sheep on days 12 to 17 postmating failed to reveal the presence of any major peaks of PGF 2α secretion. In the latter approach, several putative luteoprotective substances were examined for their ability to block the luteolytic effect of PGF 2α given as 5 separate hour-long pulses over 25 h into the arterial supply of the autotransplanted ovary. These substances included: day 14–15 uterine venous plasma from pregnant sheep, PGE 2, ovine prolactin, ovine LH, ovine placental lactogen and porcine relaxin. In no case was a convincing luteoprotective effect demonstrated suggesting that the antiprostaglandin secreting effect of the embryo may be the more important of these two antiluteolytic mechanisms. Experiments were performed to investigate circulating levels of endogenous oxytocin by monitoring intramyometrial pressure on a continuous basis in conscious sheep. These studies revealed the presence of hour-long periods of intense uterine activity at intervals of 5.5 to 6.5 hours on days 14 to 16 in the cyclic animal, but not in the early pregnant animal. Based on these results and on recent reports on the production of oxytocin by the corpus luteum, a hypothesis is presented to explain the hormone receptor control of episodic pulsatile secretion of PGF 2α in the cyclic sheep, including a possible mechanism for its abrogation during early pregnancy.

Induction of transient functional luteolysis in cyclic sheep by a 3 β-hydroxysteroid dehydrogenase inhibitor

The mechanism by which prostaglandin F2 alpha terminates luteal function in the sheep is unclear even though it is used extensively in animal husbandry. At the time of luteal regression, a decrease in 3 beta-hydroxysteroid dehydrogenase (3 beta-HSD) activity is apparent in the corpus luteum, but it is not known whether the decrease in enzyme activity is the primary cause of structural luteolysis. The effect of trilostane, a 3 beta-HSD inhibitor, on luteal function and morphology has therefore been investigated. Intravenous injection of trilostane in the mid-luteal phase of the oestrous cycle caused a decrease in ovarian tissue progesterone content. A transient decrease in peripheral and utero-ovarian vein plasma progesterone was observed but there was no significant effect on the length of the luteal phase of the cycle. There was no significant change in plasma 13,14-dihydro-15-oxo-prostaglandin F2 alpha during the period when plasma progesterone was depressed. Morphological examination of the corpora lutea revealed a decrease in the concentration of electron-dense granules without any other features of impending luteal regression. When plasma progesterone was reduced for more than 10 h by two injections of trilostane 4h apart, there was again no subsequent effect on the length of the oestrous cycle or on the return to oestrus. Plasma progesterone returned to preinjection levels within 24 h of injection. This evidence suggests that competitive inhibition of 3 beta-HSD activity, per se, is ineffective in bringing about structural luteolysis.

Prostaglandin F 2α and PGE 2 in uterine lymph during the oestrous cycle in sheep

The concentration of prostaglandin F 2α (PGF 2α) in uterine lymph remained low (<1 ng/ml) during the first two-thirds of the oestrous cycle in the non-pregnant sheep. However increased amounts of PGF 2α were present from day 12 onwards. The concentration of prostaglandin E 2 in uterine lymph remained low (< 0.43 ng/ml) throughout the cycle.

Effects of systemic administration of indomethacin to cyclic ewes on endometrial concentrations of prostaglandins effects on estrous cycle lenght and on progesterone, luteinizing hormone and prolactin patterns

Experiments were designed to evaluate in cyclic sheep the effects of systemic administration of a prostaglandin synthetase inhibitor (indomethacin). Indomethacin (100 mg, 3 times daily, S.C.) was administered from day 7 of the estrous cycle for 16 days to five ewes in which the cycle was synchronized as well as the cycles of five control ewes. All control ewes had cycles of approximately 17 days duration, but three of five Indomethacin treated ewes showed no estrous behavior before their slaughter at 23 days after induced ovulation. Autopsy revealed normal corpora lutea which had not undergone luteolysis, as confirmed by progesterone determination in blood. The two remaining Indomethacin treated ewes showed an estrous behavior on day 19 and 20 respectively together with a “prevulatory surge” of luteinizing hormone and prolactin which was not followed by follicular rupture. These results show that inhibition of PGF 2α synthesis by systemic administration of Indomethacin to the ewe is able to prevent luteolysis. When luteolysis did occur however, it was not followed by an ovulation despite a normal gonadotropin surge, indicating that inhibition of prostaglandin synthesis by systematic administration of Indomethacin interferes with the luteolysis and follicle rupture processes.

Sodium appetite and the oestrous cycle in sheep: effect of oestrogen, progesterone and changes in food intake.

Previous studies of voluntary sodium intake and electrolyte excretion in sheep suggested that the pattern of intake associated with the oestrous cycle was not a response to alterations in sodium balance. The pattern thus seemed likely to be dictated by changes in either hormonal levels or food intake. These possibilities were examined in experiments with ovariectomized sheep. Physiological doses of oestrogen or progesterone had no effect on sodium preference, whether given in isolation or succession. Depression of food intake by 25% (as at oestrus) did affect preference slightly (though significantly) but in the opposite direction to the change at oestrus. It is concluded that none of these 3 factors is responsible for the varying sodium preference during the oestrous cycle and that direct hypothalamic control of this pattern is an interesting possibility.

Water and Electrolyte Excretion During the Oestrous Cycle in Sheep

Electrolyte excretion was observed during 24 oestrous cycles in housed sheep, together with mixed salivary Na/K ratio during 10 additional cycles. 1. The sharp fall in food and fluid intake at oestrus accompanied a peak of sodium excretion which changed to peak retention 3 days later, both in faeces and urine. 2. Potassium excretion declined with food intake at oestrus but subsequently failed to recover to pre-oestrous levels dispite full recovery of dietary intake. 3. Curiously, water intake also recovered completely whereas urinary and faecal water retention continued; faecal loss actually exceeded renal excretion on these liberal water intakes. 4. Changes in salivary, urinary and faecal Na/K indicated an aldosterone peak neither during the luteal phase nor at oestrus but three days later. The data raise questions concerning the regulation of water and electrolyte balance within the normal cycle. They also provide a baseline for the investigation of renal effects of gonadal steroids. Possible roles for aldosterone, ADH and progesterone in maintaining fluid and electrolyte balance are discussed, emphasising problems confronting species which have evolved with heavy obligatory potassium excretion but undependable supplies of sodium and water.

Comparison of lambing outcome in ewes after synchronization of oestrus by progestagen or prostaglandin treatment

It has been shown (Gordon, 1975) that certain progestagens administered intravaginally can be highly effective in the control of oestrus, ovulation and fertility in ewes. Prostaglandin (PGF2α) when infused into the uterine vein ipsilateral to the ovary containing the corpus luteum has been shown to result in luetolysis and a return to oestrus in a high proportion of ewes (Hearnshaw, Restall &amp; Gleeson, 1973). Synthetic analogues of PGF2α have also been shown to have a marked luteolytic effect in ewes (Baird &amp; Scaramuzzi, 1975). Variable fertility has been reported following the use of prostaglandin or its analogue for oestrous synchronization (Jennings, 1975; Haresign, 1976; Fairnie, Cumming &amp; Martin, 1976; Lightfoot, Croker &amp; Marshall, 1976; Trounson, Willadsen &amp; Moor, 1976). Low fertilization has been reported after the use of a prostaglandin analogue and artificial insemination at a predetermined time in ewes (Boland, Gordon &amp; Kelleher, 1978). The purpose of the present experiment was to extend these observations, using a ram in natural service after employing a double prostaglandin injection regime or an intravaginal progestagen treatment for the control of oestrus in cyclic sheep.

The use of skin autografts to study wool growth cycles in sheep

The aim was to produce regions in which the wool growth cycle was out of phase with that in the surrounding skin. Four experiments were carried out involving 60 full-thickness autografts on the flanks of Wiltshire and Soay sheep. In two experiments skin was stored at 70°C for six months from summer to winter, and vice versa, but whatever the season, and whether the graft had been stored or replaced immediately as a control, wool growth on the graft was the same in each ease. In two other experiments, grafts made in autumn grew wool during the winter when the surrounding follicles were mostly inactive. The wool grew for at least the usual six months, after which some of the fibres shed, as in a normal autumn moult. In other animals growth continued for about a year, when the graft wool was considerably longer than that of the surround. Although after a year the graft became in phase with the surround again, the synchrony was not complete. In the first two experiments where follicle loss was considerable almost entire follicle groups seemed to have disappeared. In later experiments trio groups could usually be recognised, and so there was no evidence of primary follicle loss. However, secondary/primary follicle ratio fell by about 0.5 to 1, suggesting that the secondary follicles are more vulnerable. Although there was no significant difference in mean wool fibre diameter on the graft compared with the surround, the modal diameter was invariably and significantly greater. This is explained by preferential loss of small secondaries, rather than by a change in absolute diameter. The experiments showed that it is possible to provoke activity in seasonally quiescent follicles, but not possible to restrain activity by transplantation to a season of quiescence. The results support control by a combination of systemic and local factors, with the follicle, apparently able to override systemic effects.

A new concept for control of the estrous cycle of the ewe based on the temporal relationships between luteinizing hormone, estradiol and progesterone in peripheral serum and evidence that progesterone inhibits tonic LH secretion.

The temporal relationships between LH, estradiol and progesterone in peripheral serum of the ewe were characterized throughout the estrous cycle. Between successive preovulatory LH sruges, serum concentrations of LH fluctuated markedly in a manner indicative of pulsatile discharges. Mean serum LH and progesterone concentrations were inversely related, LH being highest during the early and late luteal phases of the estrous cycle and lowest in the mid-luteal phase. A progressive, 5-fold increase in serum LH concentrations occurred between the onset of the precipitous fall in circulating progesterone attendant to luteolysis and the initiation of the preovulatory LH surge. Two major increments in circulating estradiol were observed in each cycle, both occurring when serum LH concentrations were relatively high. One estradiol increment occurred during the early luteal phase, the other during the 2-3 days prior to onset of the preovulatory LH surge. The latter estradiol increment thus accompanied the progressive, 5-fold increase in circulating LH which precedes the LH surge. The observations are inconsistent with the view that tonic LH secretion in the ewe is solely a function of a negative feedback action of estradiol. Rather, the temporal relationships between circulating hormones, in conjunction with recent findings that progesterone can inhibit tonic LH secretion in the ewe, lead to the conclusion that progesterone plays a major role in the regulation of tonic LH secretion during the estrous cycle of sheep.

Pituitary responsiveness to synthetic LH-RH and pituitary LH content at various reproductive stages in the sheep

In sheep the basal concentration of LH in jugular vein plasma was significantly higher during the first 50 days of gestation in late pregnancy or at parturition. The pituitary response to a single i.v. injection of 200 microng synthetic LH-RH was determined at different stages of gestation and compared with that of anoestrous and cyclic sheep. Pituitary response to LH-RH decreased progressively with advancing gestation: by 56 days after mating the response had declined to 35% and by parturition to 14% of the value in anoestrous sheep. The pituitary response to LH-RH increased after parturition and the pattern of recovery differed in non-lactating and lactating sheep. By 63 days postpartum the response to LH-RH in non-lactating and lactating animals had returned to values similar to those in sheep during anoestrus and sheep during the luteal phase of the oestrous cycle. A decrease in pituitary responsiveness during pregnancy was associated with a decrease in pituitary content of LH. The quantity of LH released in response to a standard injection of LH-RH was linearly related to pituitary LH content.

Proceedings: Role of progesterone in regulating the length of the oestroud cycle in sheep

Proceedings: Role of progesterone in regulating the length of the oestroud cycle in sheep

RELEASE OF PROGESTERONE FROM SILICONE RUBBER IMPLANTS IN VITRO, AND THE EFFECTS OF THE IMPLANTS ON PLASMA PROGESTERONE LEVELS IN SHEEP

Ministry of Agriculture, Fisheries and Food, Central Veterinary Laboratory, New Haw, Weybridge, Surrey KT15 3NB (Received 26th November 1974) The effects of silicone rubber progesterone implants on plasma progesterone levels in anoestrous ewes have been reported by Symons, Cunningham, Saba & Millar (1974). Plasma progesterone rose from a mean preimplantation value of 0·18 ng/ml to 0·58 ng/ml 24 hr after implantation. The mean level then declined and by 10 days was not significantly different from the preimplantation value. Such short-lived elevations of plasma progesterone would probably not be effective in suppressing ovulation in cyclic sheep. For the synchronization of oestrus in cyclic ewes by inhibition of ovulation, or the induction of oestrus in anoestrous ewes, it would be desirable to maintain plasma progesterone levels above 1 ng/ml for 10 to 14 days. Thus, we have studied the release of progesterone from various silicone rubber implants in vitro, and have investigated

Influence of 2-Br-alpha-ergocryptine on serum levels of prolactin and the estrous cycle in sheep.

In a preliminary study 1 mg per day of 2-Br-a-ergocryptine (CB-154) was shown to be effective in reducing serum levels of prolactin. In a second experiment 1 mg CB-154 was injected twice daily for 20 days, which included one complete estrous cycle, into 5 ewes. Serum levels of luteinizing hormone, follicle-stimulating hormone and progesterone were not different between CB-154-injected and control ewes. Prolactin levels were reduced to <0.5 ng⁄ml in CB-154-treated ewes throughout the treatment period. Prolactin levels ranged from 10 to 100 ng⁄ml in control ewes. It was concluded that luteal function as judged by regression of marked corpora lutea, estrous cycle length, or serum levels of progesterone was not different between CB-154- treated and control ewes. These data suggest that: 1) very low levels of prolactin are adequate to insure normal luteal function; or 2) prolactin does not play a role in the regulation of luteal function in normal cycling sheep. (Endocrinology 94: 612, 1974)

The kinetics of oestradiol -17 metabolism in the sheep.

SUMMARY The metabolic clearance rate of oestradiol-17β from whole blood and its conversion to oestrone and oestradiol-17α were measured in pregnant and non-pregnant (cyclic, lactating and anoestrous) sheep using tracer kinetic techniques. The metabolic clearance rate was much the same in sheep in different reproductive states (mean in all animals (±s.e.m.) = 2·449 ± 0·155 1/min). The highest values were reached on days 143–145 of pregnancy (3·287 ± 0·3351/min). The mean conversion ratio of oestradiol-17β to oestrone was 17·0 ± 2·2%, and that of oestradiol-17β to oestradiol-17α was 14·1 ± 1·1%. The production rate of oestradiol-17β increased from 0·007 to 0·103 μg/min up to 3 days before parturition to 0·035–1·257 μg/min during the last 12 h of pregnancy. During the continuous infusion of [3H]oestradiol-17β and when a steady state had been reached, approximately 20–30% of the radioactivity in whole blood could be extracted with ether. Of this radioactivity more than 90% was attributable to labelled oestradiol-17β, oestrone and oestradiol-17α in jugular blood, though only about 70% could be accounted for as these oestrogens in uterine venous blood.

Intravaginal SC-9880-progesterone treatments in the synchronization of oestrus in cyclic sheep

SUMMARYThe mating response and lambing outcome in fifty-three sheep was studied after treatment with intravaginal sponges during October (normal breeding season). Bach pessary carried a dose of 30 mg ‘Cronolone’ (9α-fluoro-11β-hydroxy-17α-acetoxyprogesterone; SC-9880, G. D. Searle) in finely dispersed form and was inserted for 14 days. Half the number of pessaries carried an additional dose of 400 mg progesterone. Breeding was by natural service, using 10% rams. Ninety per cent of the fifty ewes bred immediately after treatment conceived; the average number of lambs/conception was 1·9. Sheep treated with ‘Cronolone’/progesterone pessaries showed a marginal and non-significant advantage over those treated with ‘Cronolone’. The application was effective in ensuring the birth of most lambs (87% of total) within a period of 1 week.

Artificial insemination and natural service at a predetermined time in cyclic sheep treated with ‘SC-9880’-progesterone sponges

SUMMARYResults are given of an experiment in which 61 cyclic Finnish Landrace × Dorset Horn ewes were synchronized for oestrus by intravaginal treatment for 14 days using sponges impregnated with 30 mg ‘Cronolone’ (9a-fluro-11-hydroxy-17a-acetoxy-progesterone; SC-9880, G. D. Searle) and 400 mg progesterone. Groups of 31 and 30 sheep were inseminated twice, either artificially or naturally, at predetermined intervals of 48 and 58 hr after sponge withdrawal. All 61 ewes were in oestrus within 48 hr of removing sponges; 26 of 30 (87%) artificially inseminated and 29 of 31 (94%) hand-mated sheep conceived at the controlled oestrus. The incidence of multiple births was not significantly affected by the method of breeding.

Pituitary responsiveness to purified luteinizing hormone-releasing hormone (LH-RH) at various stages of the estrous cycle in sheep.

Pituitary responsiveness to purified luteinizing hormone-releasing hormone (LH-RH) at various stages of the estrous cycle in sheep.

Fertility following synchronization of oestrus in Romney, Corriedale and Merino ewes.

Abstract Extract Synchronization of oestrus in sheep has been achieved by the use of progesterone and progesterone analogues administered either as an intramuscular injection, or by an impregnated sponge inserted into the the vagina (Robinson, 1958 Robinson, T. J. 1958. Studies in controlled artificial insemination of Merino sheep. Aust. J. agric. Res., 9: 693–703. [Google Scholar], 1961 Robinson, T. J. 1961. The time of ovulation and efficiency. of fertilization following progesterone and pregnant mare serum treatment in the cyclic ewe. J. agric. Sci., Camb, 57: 129–135. [Google Scholar], 1964 Robinson, T. J. 1964. Synchronisation of oestrus in sheep by intravaginal and subcutaneous application of progestin impregnated sponges. Proc. Aust. Soc. anim. Prod., 5: 47–52. [Google Scholar]; Denny and Hunter, 1958 Denny, J. F. and Hunter, G. L. 1958. Synchronisation of the oestrus cycle in sheep. S. Afr. J. agric. Sci, 1: 381–388. [Google Scholar]; Brunner et al., 1964 Brunner, M. A., Hansel, W. and Hogue, D. E. 1964. Use of 6-methyl-17-acetoxyprogesterone and pregnant mare serum to induce and synchronise oestrus in ewes. J. anim. Sci., 23: 32–36. [Google Scholar]; Clark et al., 1966 Clark, J. N., Roberts, E. M., Carter, A. H. and Kirton, A. H. 1966. Hormonal synchronisation of oestrus in Romney Marsh ewes during the breeding season. Proc. N.Z. Soc. anim. Prod., 26: 107–121. [Google Scholar]). Twenty milligrams of progesterone administered every second day appears as effective as 10 mg daily (Robinson, 1958 Robinson, T. J. 1958. Studies in controlled artificial insemination of Merino sheep. Aust. J. agric. Res., 9: 693–703. [Google Scholar]) and provides satisfactory synchronization and fertility to natural service (Lamond and Bindon, 1962 Lamond, D. R. and Bindon, B. M. 1962. Oestrus, ovulation and fertility following suppression of ovarian cycles in Merino ewes by progesterone. J. Reprod. Fertil., 4: 57–66. [Google Scholar]).