Lack of between-follicle interactions in the sheep ovary

Relationships among the corpus luteum, follicles and conceptus in sheep

This study aimed to investigate the intra- and interovarian relationships among the corpus luteum (CL), the largest follicle (LF) and follicular population in non-pregnant and between the conceptus and ovarian structures in pregnant ewes. In experiment 1, the follicular and luteal structures were examined in 538 reproductive systems of non-pregnant Awassi ewes. The follicular population was categorised into small (SF), medium (MF) and large (LF) groups. Inter-relationships between CL and follicular population and between LF and subordinate follicles were determined. In experiment 2, the location and number of conceptuses were identified and correlated with the ovarian structures in 58 reproductive systems of pregnant ewes. Effects of pregnancy status, stage of pregnancy, pregnancy side and conceptual number on follicular population were determined. The results showed that the right ovary was more active than the left ovary. CL had intraovarian positive effect on the number of medium and large follicles. LF had no local suppressive effect on the subordinate follicles. Side and stage of pregnancy and the conceptual number did not affect the follicular population. Accordingly, it can be concluded that the LF has no local suppressive effect on the subordinate follicles. The CL has intraovarian positive effect on the follicular population. Follicular population does not show remarkable changes during the first term of pregnancy. The present study probably provides information which may help in the understanding of the ovarian dynamics during pregnancy in sheep.

A comparison of alternative genetic models for litter size in pigs

Predictions of two models were compared. The models relate ovulation rate (OR) and prenatal survival (PS) to litter size (LS): the uterine capacity model (UCM), where maximum LS is limited by uterine capacity (UC), and the threshold model (TM) whereby PS is modelled as a binary threshold trait. Records were simulated according to both models using statistics from Trench Large White gilts. Both models were able to reproduce closely the observed curvilinear relationship between OR and LS, with LS reaching a plateau at high OR. Several genetic correlations (ρg) fulfilling the conditions h20R = 0·34 and h2LS = 0·12 (the residual maximum likelihood estimates in the population) were studied by means of stochastic computer simulation. The genetic correlation between OR and LS was very sensitive to changes in h2uc, whereas ρgLS,PS was always positive, and ρgOR,PS was always negative. The correlation between PS and UC was larger than 0·90, except for very small h2UC This suggests that genes affecting PS have a strong influence on UC and that PS can be a good indirect criterion to select for UC. Both models predicted that the advantage of an index combining OR and LS with respect to direct selection on LS diminishes in successive generations of selection and that the size of the experiment needed to detect significant differences is very large. Records were also simulated by halving the mean and variance of UC, so as to mimic unilateral hysterectomy-ovariectomy (UHO). If the UHO treatment results in halving UC, LS of UHO females should behave very much as half the UC of intact females.

Ovarian follicle growth and consequences for fertility in sheep

Understanding the pattern of ovarian follicle development is seen as an important step leading to the development of techniques that maximise fertility in sheep. Repeated observations of the growth of individual follicles have led to the understanding that follicles develop in a wave-like pattern during the oestrous cycle, with two to four waves per cycle being the most common. The ease with which follicle waves are described seems to depend on the their frequency and the number of follicles per wave. There is evidence for the largest follicle(s) of a follicle wave inhibiting the development of other follicles; however, in some cases this is not apparent as other follicle waves emerge when a previous large, healthy follicle is still present. Follicle development can be manipulated using exogenous gonadotrophins or progestagens and these have been shown to alter the number or age profile of developing follicles. The ovulation of aged follicles in cattle clearly has a detrimental effect on fertility, but this relationship is less clear and seems to be less critical in sheep. Recent findings at the molecular level show that the bone morphogenetic proteins (BMP) and their receptors are critically involved in the control of ovulation rate, but fully understanding their mechanism remains to be described. This highlights the potential for the integration of molecular and physiological findings to better develop methods to manipulate follicle development and reproduction in sheep.

Ovulation of aged follicles does not affect embryo quality or fertility after a 14-day progestagen estrus synchronization protocol in ewes

The aim was to examine the effect of ovulation of aged follicles on embryo quality and fertility in ewes. In Experiment 1, ewes (n = 39) received a prostaglandin analogue on Day 6 of the cycle and then received either a progestagen sponge from Day 6 to 20 after estrus (Single sponge) or a progestagen sponge on Day 6 that was replaced on Day 11 and 16 and removed on Day 20 (Multiple sponges). In a subgroup of ewes, the growth of ovarian follicles was characterised using ultrasonography. Fertile rams were introduced 48 hours after sponge withdrawal; we slaughtered the ewes on Day 5 of pregnancy and recovered the embryos. The mean age of the ovulatory follicles was greater in ewes that received a single sponge compared with multiple sponges (8.7+/-0.8 days, range 4 to 14, versus 4.5+/-0.7 days, range 3 to 6; P<0.05). However, the groups did not differ (P>0.05) in ovulation rate (2.4+/-0.3 corporal lutea per ewe) or the proportion of good quality embryos recovered (71 to 82%; developed to the early morula stage or further). In Experiment 2, ewes (570 in total) received treatments similar to those in Experiment 1 but were kept until lambing. Ewes that received a single sponge came into heat earlier (P<0.05) than those that received multiple sponges, but > or = 97% of ewes in all groups (P>0.05) were bred by 48 to 72 hours after ram introduction. There was no difference (P>0.05) between groups for the proportion of ewes that lambed to first service (80 to 86%) or the number of lambs per ewe (1.94+/-0.08 lambs). We conclude that when luteolysis occurs at the beginning of progestagen synchronisation, ewes will ovulate aged follicles, but that compared to shorter duration follicles, these follicles produce oocytes that are equally competent to be fertilised and develop into good quality embryos and full-term lambs.

Pre-ovulatory follicular characteristics and ovulation rates in different breed crosses, carriers or non-carriers of the Booroola or Cambridge fecundity gene

Terminal follicular dynamics and ovulation rates (OR) were compared in different local breeds after introducing fecundity genes of different origin. Crossbred ewes which were carriers (F+) or non-carriers (++) of Booroola (BFec) or Cambridge genes (CFec) were included: CambridgexCambridge (CC), CambridgexSuffolk (CS), CambridgexTexel (CT), BooroolaxTexel (BT) and BooroolaxGerman Mutton Merino (BGM). The numbers of small (diameter 2-3.5 mm), medium (diameter >3.5-5.0 mm) and large (diameter >5.0 mm) growing follicles, the maximum diameter before ovulation and the regression and artesia rates of ovarian follicles >/=2 mm in diameter were studied laparoscopically and repeatedly during the last 5 days of an induced oestrous cycle. The ORs were determined one cycle before and two cycles after the repeated laparoscopy. BFec and CFec significantly enhanced the OR of all crossbreeds. Carriers of BFec or CFec did not have significantly different ORs due to any crossbreeding effect. The same observation was made for non-carriers of both Fec gene types. Whatever the crossbreed, the number of small, medium and large growing follicles were similar between carriers and non-carriers in spite of a higher number of ovulating follicles in carriers of both Fec gene types. The diameter of ovulatory follicles did not differ among crossbreds, or between carriers and non-carriers except in the BT (5.2+/-0.2 vs. 6.5+/-0.8 mm, respectively) and CC (6.6+/-0.2 vs. 5.6+/-0.3 mm) ewes.The higher OR in the presence of the Booroola gene was associated with a low atresia rate of large follicles in all crossbreeds (BT: 52+/-8% (F+) vs. 61+/-7% (++); BGM: 51+/-6% vs. 75+/-5%). The high OR of the carriers of the CFec gene seemed to be associated with a lower number of large growing follicles with a lower (P<0.05) atresia rate as compared with Booroola crossbreeds. In conclusion, follicular features were similar between purebred Cambridge and its crossbred CS and CT. In ewes carrying the BFec or CFec gene, the reduction in follicular atresia seemed to be one of the main follicular features implicated in the higher OR.

Ovine follicular fluid inhibits aromatase activity

"Within follicle" regulations may be important for the fine tuning of gonadotrophin action in ovarian follicles. While numerous growth factors, steroids or proteins which are present in follicular fluid have been shown to have the ability of positively or negatively affecting follicle function, the net effect of follicular fluid of the dominant follicle on its function is unclear.A bioassay measuring aromatase activity of follicular walls was used (1) to check whether follicular fluid from dominant follicles can alter aromatase activity (2), to check how follicle size, atresia and specific gonadotrophins alter the effects of follicular fluid (3), to identify the nature (steroid or protein) of the active compound(s), and (4) to check whether the inhibition is specific of aromatase. Dominant follicular fluid had the ability to reduce aromatase activity. This effect was dose dependent and was obvious whether or not a protease inhibitor was added to the incubation medium. There was no difference in the magnitude of the inhibitory effect of follicular fluid when FSH (2 ng/ml) or no FSH was added to the incubation medium. LH, however, could potentialise the inhibitory effects of follicular fluid. Dominant follicular fluid was more potent to inhibit aromatase than follicular fluid from atretic follicles. Medium conditioned by granulosa cells, but not by theca cells could inhibit aromatase activity when added to the incubation medium. Charcoal treatment of dominant follicular fluid did not remove its inhibitory potential. Fractionation of dominant follicular fluid by a desalting column demonstrated that the inhibition was related to a compound(s) > 10 kDa. Finally, the effect of dominant follicular fluid on aromatase appears specific of this enzyme as follicular fluid does not affect androgen output by thecal shells or progesterone output by luteal cells. Further research is required to check whether the activity observed in dominant follicular fluid is related to compounds known to affect aromatase activity (inhibin, mullerian inhibiting substance, heat shock protein 90, superoxyde dismutase) or to another peptide/protein.

Waves of follicle development during the estrous cycle in sheep

The pattern of ovarian follicle development in maiden cyclic lambs was characterized using the definition of a follicle wave as the changes in the number of follicles among the days of the estrous cycle, as originally defined in cattle by Rajakoski in 1960. We also examined the steroid content relationships among follicles on Days 5 (Wave 1) and 14 (Waves 2 and 3) of the estrous cycle. In Experiment 1, the ovaries of 20 cyclic lambs (40 to 45 kg) were examined daily using transrectal ultrasonography for 1 or 2 estrous cycles (n = 31 cycles). The number of small (2 and 3 mm in diameter), medium (4 and 5 mm) and large (> or = 6 mm) follicles were aligned with the beginning and end of the average length estrous cycle and then compared among days. Identified follicles were defined as those that grew to > or = 4 mm and remained at > or = 3 mm for > or = 3 d. The number of identified follicles emerging (retrospectively identified at 2 or 3 mm) per ewe per day was also aligned with the average length estrous cycle. In Experiment 2, ewe lambs were ovariectomized on Day 5 (n = 6) or 14 (n = 5) of the estrous cycle, then follicle diameters and follicular fluid concentrations of estradiol and progesterone were compared among follicles. Data were analyzed by repeated measures ANOVA and compared among days using Fisher's LSD. In Experiment 1, either 2 (n = 10 cycles), 3 (n = 20 cycles) or 4 (n = 1 cycle) periods of emergence of identified follicles occurred during individual cycles, with estrous cycle lengths of 15.6 +/- 1.6, 16.1 +/- 1.1 and 17 d respectively. In animals with 2 or 3 periods of emergence of identified follicles, the total number of small, medium and large follicles differed (P < 0.05) among days of the estrous cycle showing a wave-like pattern. In Experiment 2, a single follicle collected on each of Days 5 and 14 of the cycle (6.2 +/- 0.2 and 3.9 +/- 0.2 mm in diameter) had a higher (P < 0.05) concentration of follicular fluid estradiol (36.2 +/- 4.4 and 50.9 +/- 21.6 ng/mL) than other follicles collected on the same day (next largest follicle: 4.3 +/- 0.3 and 3.5 +/- 0.4 mm; 4.3 +/- 0.9 and 18.2 +/- 6.7 ng/mL estradiol). The results showed that 1) there was a synchronous emergence of follicles associated with fluctuations in the number and size of follicles during the estrous cycle; 2) within a wave there was a hierarchy among follicles for diameter and steroid content; 3) ovarian follicle growth in ewe lambs occurred in 2 or 3 organized waves during the estrous cycle.

Selection of the dominant follicle in cattle occurs in the absence of differences in the expression of messenger ribonucleic acid for gonadotropin receptors

Mechanisms that allow selection of a dominant ovarian follicle from a cohort of growing follicles are unknown. Large healthy, estrogen-active follicles contain more LH receptors than atretic estrogen-inactive follicles, and levels of messenger RNA (mRNA) for LH receptor increase in the granulosa cells of dominant follicles as growth progresses. The aim of the present study was to test the hypothesis that changes in the temporal pattern of expression of mRNA for LH and FSH receptors are associated with selection of dominant follicles in cattle. Based on size, the dominant and two largest subordinate follicles were collected from the ovaries of heifers on days 2 (n = 3) or 3 (n = 3) of a follicular wave. On day 2, the dominant follicle was 1 mm larger than the largest subordinate follicle, but by day 3 of the wave the dominant follicle was 2-4 mm larger than the largest subordinate. Follicular fluid concentrations of estradiol and estradiol secretion in vitro by pieces of follicle wall (granulosa and theca cells) were greatest by the dominant compared with the subordinate follicles (P < 0.05). These data indicate that selection of a dominant follicle had occurred by the second day of the follicular wave. By in situ hybridization, mRNAs for LH and FSH receptors, P450 aromatase and P450 17alpha-hydroxylase (17alpha-OH) were localized in frozen sections from each follicle. The expression of mRNA for LH receptor in granulosa cells was always at or near background and was not different between days or follicle types (P = 0.63). In contrast, the expression of mRNA for LH receptor in theca cells of the same sections was readily detectable; there was no difference between follicle types on the second day of the follicular wave, but by the third day expression in the subordinate follicles had decreased (P < 0.05). The expression of mRNA for FSH receptor was highest in granulosa cells of dominant follicles collected on day 3 of the follicular wave (P < 0.05) and was not different between dominant and subordinate follicles on day 2 of the wave (P > 0.05). The expression of mRNA for aromatase in granulosa cells was similar (P > 0.05) between the dominant follicles on days 2 and 3 and the largest subordinate follicle on day 2 of the follicular wave and was much lower in the remaining follicles (P < 0.01). On day 2 of the wave, the expression of mRNA for 17alpha-OH was not different between the dominant and subordinate follicles, but by day 3 the dominant follicles had more mRNA for 17alpha-OH than the subordinate follicles (P < 0.05). These data show that the dominant follicle had been selected by the second day of the follicular wave (based on diameter and estradiol secretion) and that selection occurred in the absence of detectable levels of mRNA for LH receptor in the granulosa cells or differences between dominant and subordinate follicles in mRNA for LH receptor in theca cells or FSH receptor in granulosa cells. However, the divergent pattern of growth between dominant and subordinate follicles (after follicle selection) was associated with higher levels of mRNA for gonadotropin receptors and steroidogenic enzymes in dominant compared with subordinate follicles. Therefore, selection of the dominant follicle in cattle does not appear to involve the regulation of expression of mRNA for gonadotropin receptors, although such regulation may be important at other stages of differentiation of the dominant follicle.