Regression of the corpus luteum of pregnancy following parturition in the ewe

Corpora lutea (CL) of pregnancy from single-lambing ewes were examined by light and electron microscopy within 24 h and at 8, 15, 23, 31 and 41 days after parturition (2 ewes per stage). Within 24 h of parturition the structure of the CL was well preserved and both large and small luteal cells, characteristic of this species, were present in substantial numbers. However, both types of luteal cell contained numerous cytoplasmic lipid droplets, and smooth endoplasmic reticulum and secretory granules in large luteal cells were less prominent than in normal functional CL of cyclic ewes. Leucocytic infiltration, and death of some luteal and endothelial cells, were also observed at this stage. Further regression of the CL progressed slowly, and lipid-rich large luteal cells were still readily recognisable 15 days after parturition. The size of the CL declined progressively, and the proportion of tissue occupied by intercellular substances increased. Corpora albicantia approximately equal to 3-4 mm in diameter were still recognisable 41 days after parturition. It was concluded that luteal regression post partum progresses much less rapidly than at the end of the oestrous cycle.     

Evidence that the corpus luteum of pregnancy contributes to the control of tonic secretion of LH in the ewe

Concentrations of LH and FSH were measured in blood samples collected from the jugular vein at 20-min intervals for 7 h (09:00-16:00 h) on Days 60, 80, 100 and 120 of pregnancy in 5 intact ewes and 5 from which the CL had been excised on Day 70. In the 5 intact ewes, plasma LH concentrations remained low and unchanged between Days 60 and 120. During this period, pulsatile release of LH occurred irregularly and infrequently. Removal of the CL resulted in an increase in the basal values of LH and in the frequency and amplitude of LH pulses. Concentrations of FSH were relatively constant in all stages of pregnancy examined and were similar in both groups of ewes. These results show that (1) LH concentrations are low during the second half of pregnancy; and (2) LH, but not FSH, increases after CL excision, presumably by removing some luteal factor inhibitor of LH secretion.     

Resumption of gonadotrophin release during the post-partum period in suckling and non-suckling ewes

The post-partum secretion of LH, FSH and prolactin was monitored in 15 suckling and 6 non-suckling Préalpes du Sud ewes lambing during the breeding season by measuring plasma hormone concentrations daily at 6-h intervals and also weekly at 20-min intervals for 6 h from parturition to resumption of regular cyclic ovarian activity. There was a constant phenomenon in the resumption of normal patterns of FSH and LH secretion: there was a rise in FSH values culminating on average on Day 4 post partum and returning subsequently to values observed during the oestrous cycle, and concurrently an increase in the frequency and amplitude of LH pulses more progressive in suckling than in non-suckling ewes which led to an elevation of LH mean concentrations and occurrence of an LH surge. Since neither the FSH secretory pattern nor FSH mean values differed between suckling and non-suckling ewes, the results suggested that LH pulsatile pattern was a major limiting factor for the resumption of normal oestrous cycles. Before regular oestrous cycles resumed other changes in preovulatory LH surges also occurred: (i) they increased in duration and probably in amplitude; (ii) they were preceded by an acceleration in LH pulse frequency and a large decrease in FSH values as in normal cyclic ewes; and (iii) at least in non-suckling ewes they occurred concurrently with a prolactin surge.     

Ultrastructure of the corpus luteum of antarctic seals during pregnancy

Summary The fine structure of granulosa lutein cells from three crabeater seals, Lobodon carcinophagus, and two leopard seals, Hydrurga leptonyx, has been studied from early through mid-pregnancy. Analysis of the arrangement and modifications of the cytoplasmic organelles and inclusions has revealed three types of lutein cells throughout the corpus. Type I cell typically possesses a central nucleus and cytoplasm containing very large amounts of smooth and/or fenestrated endoplasmic cisternae which frequently extend from the juxta-nuclear to the periphery of the cell. Type II cell contains a central or eccentric nucleus, moderate amounts of peripheral, smooth and fenestrated cisternae which often form large and concentric membranous whorls, numerous mitochondria and small lipid droplets. Frequently these cells show polarity in the arrangement of the cytoplasmic organelles and inclusions. Type III cell contains predominant large lipid droplets, many mitochondria, and small amounts of smooth and fenestrated cisternae. In light microscopy the type I cell is evenly granular, while the type III cell is highly vacuolated. Type II cells have both granular and vacuolated conditions. Ultrastructural features of type I and II cells suggest that they probably secrete most of the steroids, whereas the primary role of the type III cells appear to be lipid storage.This research was supported by National Science Foundation, Grant No. 1325 from the Office of Antarctic Biology.     

Lipoprotein lipase activity in the bovine corpus luteum during the estrous cycle and early pregnancy.

Lipolytic activity measured at pH 8.6 in bovine corpora lutea exhibited classical properties of lipoprotein lipase (LPL) in terms of serum and heparin stimulation and NaCl inhibition. LPL activity was measured in 23 corpora lutea collected at different stages of the estrous cycle and early pregnancy. The LPL activity in cyclic corpora lutea (mumole FA released/hr/100 mg acetone powder) was low at Days 4-8 of the estrous cycle (3.1 +/- 1.5: mean +/- SE) and at Days 19-20 (1.6 +/- 0.6). However, high activity of the enzyme was found at Days 12-15 of the cycle (11.8 +/- 1.8); these concentrations were significantly (P less than 0.01) elevated over those found at Days 4-8 and 19-20. The enzyme activity began to decline at Days 16-18 of the estrous cycle (5.1 +/- 1.7). Low enzyme activity was found in the corpora lutea removed from two cows at Day 22 of pregnancy. Progesterone concentrations were measured in 16 of the 23 corpora lutea and a good correlation (r = 0.75, P less than 0.01) was found between lipoprotein lipase and progesterone concentrations of the tissue. The data suggest that LPL may be involved in controlling the transfer of fatty acids, including arachidonic, from plasma lipoproteins to luteal tissue.     

Does inadequate luteal function limit the establishment of pregnancy in the early post-partum ewe?

In Exp. 1 the effect of lactation versus early weaning on luteal function was examined in seasonally anoestrous Finn Dorset ewes that were induced to ovulate at 21 (N = 14) or 35 (N = 14) days post partum by using a CIDR device and PMSG. Prolactin concentrations were significantly higher (P less than 0.001) in lactating compared with early weaned ewes throughout the study. The proportion of lactating ewes with inadequate luteal function (as assessed by daily progesterone concentrations) in the 21-day group was 0.43 (3 or 7) compared with 0.67 (4 of 6) for those weaned within 2 days after parturition. Corresponding values for the 35-day group were 0 (0 of 4) and 0.14 (1 of 7) respectively. There was no evidence of abnormal luteal function in standard ewes (N = 8) for which the interval from parturition was greater than 150 days. In Exp. 2 we examined whether pregnancy can be successfully established during the breeding season following transfer of embryos into lactating or early weaned ewes in the early post-partum period. Embryos were donated from Border Leicester x Scottish Blackface ewes for which the interval from previous parturition was greater than 150 days. These embryos were transferred synchronously on Day 5 after behavioural oestrus to recipient ewes with the same breeding history as the donors (standard ewes, N = 15) or to lactating or early weaned recipients that had been induced to ovulate on Day 21 (N = 16) or 35 (N = 24) post partum. In the 21-day group inadequate luteal function was observed in 2 of 7 (0.28) lactating and 4 of 9 (0.44) early weaned ewes compared with corresponding values of 1 of 13 (0.08) and 2 of 11 (0.18) in the 35-day post-partum group. Luteal function was normal in all standard ewes. The proportion of successful pregnancies in the standard ewes was 0.80 (12 of 15) compared with 0 in lactating and early weaned ewes in the 21-day group and 0.08 (1 of 13) and 0.36 (4 of 11) respectively in the 35-day group. The incidence of inadequate luteal function is therefore independent of the suckling stimulus and is higher in ewes induced to ovulate on Day 21 than Day 35 post partum during breeding and non-breeding seasons.(ABSTRACT TRUNCATED AT 250 WORDS)     

A morphometric analysis of the corpus luteum of the cow during the estrous cycle and early pregnancy

Corpora lutea were collected from cows on Days 6, 8, 10, 12, 14, 16, 18 and 19 of the estrous cycle and early pregnancy (n=2/d) and were examined by light microscopy. Mean lutein cell diameter was significantly (P<0.05) greater in pregnant than in cyclic cows on Days 6, 8, 10, 12, 16, 18 and 19 (cyclic versus pregnant: Day 6: 13.9 +/- 0.22 vs 14.9 +/- 0.24; Day 8: 13.8 +/- 0.20 vs 15.4 +/- 0.2; Day 10: 14.8 +/- 0.24 vs 17.4 +/- 0.24; Day 12: 13.2 +/-0.25 vs 17.9 +/- 0.31; Day 16: 13.9 +/- 0.28 vs 16.5 +/- 0.31; Day 18: 13.0 +/- 0.22 vs 16.5 +/- 09.36, and Day 19: 15.0 +/- 0.23 vs 17.6 +/- 0.33 mum, respectively). The distribution of cell sizes was leptokurtotic throughout the estrous cycle and the first 10 d of pregnancy, but tended towards bimodality after Day 14 of pregnancy. The proportion of lutein cell cytoplasm occupied by vacuoles was lower in pregnant than in cyclic cows from the 12th day post estrus, but there was a marked (P<0.05) increase in vacuolation of cells from cows undergoing luteolysis. Stainable intercellular collagen was also less abundant in pregnant than cyclic cows from the 12th day post estrus. The higher rate of progesterone secretion of pregnant, compared with cyclic cows may be attributed to the greater numbers and greater contribution to luteal mass of large lutein cells in the corpus luteum of pregnancy.     

Rescue of the corpus luteum in human pregnancy

Rescue of the corpus luteum from its programmed senescence maintains progesterone production required for pregnancy. In primates, chorionic gonadotropin produced by the developing conceptus acts as the primary luteotrophic signal. The purpose of this research was to assess corpus luteum rescue by examining changes in daily urinary progesterone metabolite levels during the first week after implantation. We determined the variability in progesterone metabolite profiles and evaluated its relationship to early pregnancy loss in 120 naturally conceived human pregnancies, including 43 early pregnancy losses. In other primates, an abrupt increase in the progesterone metabolite occurs at the time of implantation. This pattern occurred in an estimated 45% of the pregnancies in the present study. In the remaining pregnancies, there was a delayed rise (18%), neither a rise or decline (22%), or a decline (15%) during the week after implantation. The estimated rate of early pregnancy loss increased across these categories (from 5% loss with an abrupt rise at implantation to 100% loss with progesterone metabolite decline). Low urinary hCG levels in early pregnancy were significant determinants of a decline in postimplantation progesterone metabolite. However, preimplantation steroid metabolite levels were not significant, suggesting no inherent problem with the corpus luteum. Examination of individual progesterone metabolite profiles in relation to hCG profiles also indicated that few losses were caused by corpus luteum failure. Delineating the functional importance of an abrupt progesterone rise at the time of implantation may provide new strategies for promoting successful implantation in assisted reproduction.     

Expression of prostaglandin synthesis pathway enzymes in the porcine corpus luteum during the oestrous cycle and early pregnancy

Prostaglandins (PGs) of luteal origin may have paracrine and/or autocrine actions on the functions of the corpus luteum (CL). Previously, we have shown that enzymes of PG synthesis pathway such as prostaglandin E synthase (mPGES-1), prostaglandin F synthase (PGFS) and prostaglandin 9-ketoreductase (CBR1) are important in regulation of PG production in the conceptuses and endometrium of cyclic and pregnant pigs. Therefore, localization and expression patterns of these enzymes were determinated in porcine CL. The PGFS protein content was lower in metestrus and higher around luteolysis, and then decreased in late regressing CL. PGFS protein levels were lower on days 5-8 of pregnancy and did not differ between days 10 and 25. Elevated expression of mPGES-1 mRNA was found in early luteal phase. The mPGES-1 protein content, similarly to PGFS, was higher during luteolysis. mPGES-1 mRNA and protein levels were constant between days 5 and 25 of pregnancy. PGFS and mPGES-1 expression was down-regulated on days 16-17 of the oestrous cycle when compared to the corresponding days of pregnancy. Enhanced mPGES-1/PGFS ratio occurred during early luteal phase and days 5-8 of pregnancy. Expression of CBR1 mRNA and protein was constant during the cycle and pregnancy. Our studies revealed higher mPGES-1/PGFS ratios in the CL during early luteal phase and corresponding days of pregnancy that could favor PGE(2) synthesis and may be important in the control of luteal development. However, PG synthesis in the endometrium/conceptus rather than in the CL could be involved in luteolysis and maternal recognition of pregnancy in pigs.     

Gonadotropin-releasing hormone secretion during the postpartum anestrous period of the ewe

An increase in episodic release of LH is putatively the initial event leading to the onset of postpartum ovarian cyclicity in ewes. This experiment was conducted to determine the relationship between hypothalamic release of GnRH and onset of pulsatile secretion of LH during postpartum anestrus. Control ewes (n = 7) were monitored during the postpartum period to determine when normal estrous cycles resumed. In controls, the mean interval from parturition to the first postpartum estrus as indicated by a rise in serum progesterone greater than 1 ng/mg was 25.8 +/- 0.6 days. Additional ewes (n = 4-5) at 3, 7, 14, and 21 days postpartum (+/- 1 day) were surgically fitted with cannula for collection of hypophyseal-portal blood. Hypophyseal-portal and jugular blood samples were collected over a 6- to 7-h period at 10-min intervals. The number of GnRH pulses/6 h increased (p less than 0.05) from Day 3 postpartum (2.2 +/- 0.5) to Days 7 and 14 (3.6 +/- 0.2 and 3.9 +/- 0.4, respectively). A further increase (p less than 0.05) in GnRH pulse frequency was observed at Day 21 postpartum (6.4 +/- 0.4 pulses/6 h). Changes in pulsatile LH release paralleled changes observed in pulsatile GnRH release over Days 3, 7, 14, and 21 postpartum (0.83 +/- 0.3, 2.8 +/- 0.4, 2.9 +/- 0.6, and 4.0 +/- 1.1 pulses/6 h, respectively). GnRH pulse amplitude was higher at Day 21 than at Days 3, 7, or 14 postpartum. These findings suggest that an increase in the frequency of GnRH release promotes the onset of pulsatile LH release during postpartum anestrus in ewes.     

Effect of active immunization against oxytocin on gonadotrophin secretion and the establishment of pregnancy in the ewe

A group of 14 ewes was actively immunized against oxytocin coupled to carrier protein, and comparisons of the reproductive status of these animals were made against ewes immunized against carrier protein only (N = 5) and untreated controls (N = 6). The last two groups were indistinguishable and were therefore combined as a single control group for analysis of the results. Oestrous cycle lengths were significantly extended in oxytocin-immunized ewes (P less than 0.005) with 42% of cycles lasting greater than 18 days. Cloprostenol treatment in the mid-luteal phase resulted in apparently normal luteal regression and re-ovulation, but luteal phase FSH concentrations and follicular phase LH concentrations were elevated in the immunized ewes, although surge levels of both hormones were unaffected. Measurements of free oxytocin concentrations in the blood suggested that these were significantly raised in treated animals. Progesterone concentrations in peripheral plasma were not altered by treatment. Mating resulted in a conception rate of 91% in control ewes compared with only 28% in oxytocin-immunized animals (P less than 0.01). There was no evidence of any conceptus material in the uteri of non-pregnant immunized ewes 25 days after service. Some had re-ovulated, whereas the ovaries of others contained mature corpora lutea which had been maintained. Ovarian histology appeared normal. We conclude that active immunization against oxytocin influences gonadotrophin secretion and reduces fertility. The site(s) of action for both of these effects needs to be determined.     

Endocrine changes during pregnancy, parturition and the early post-partum period in the llama (Lama glama)

Mean (+/- s.d.) pregnancy length for the 14 llamas in this study was 350 +/- 4.5 days. Plasma progesterone concentrations increased by 5 days after mating and remained elevated (greater than 2.0 ng/ml) throughout most of pregnancy. At about 2 weeks before parturition, plasma progesterone concentrations began to decline, dropped markedly during the final 24 h before parturition, and returned to basal concentrations (less than 0.5 ng/ml) by the day of parturition. The combined oestrone + oestradiol-17 beta and oestradiol-17 beta concentrations varied between 6 and 274 pg/ml and 4 and 114 pg/ml, respectively, during the first 9 months of pregnancy. Concentrations increased between 9 months after mating and the end of pregnancy with peak mean concentrations of 827 +/- 58 (s.e.m.) pg oestrone + oestradiol-17 beta/ml (range: 64-1658) and 196 +/- 10 pg oestradiol-17 beta/ml (31-294) during the last week of pregnancy. Concentrations then declined to 87 +/- 14 pg oestrone + oestradiol-17 beta/ml (7-488) and 25 +/- 5 pg oestradiol-17 beta/ml (2.5-142) during the first week post partum. Plasma cortisol concentrations varied between 2.6 and 51.9 ng/ml (14.0 +/- 0.5) from mating until 2 weeks before parturition when the concentrations began to decline. Only a slight increase in plasma cortisol concentrations was observed in association with parturition. Plasma triiodothyronine concentrations varied between 0.5 and 4.5 ng/ml (1.9 +/- 0.1) throughout pregnancy and the periparturient period.(ABSTRACT TRUNCATED AT 250 WORDS)     

The size of the corpus luteum during pseudopregnancy and pregnancy in the rat.

The corpus luteum in mature Sprague Dawley rats was weighted at the various stages of pseudopregnancy and pregancy. The average size of these corpora lutea was 1.0 +/- 0.10 mg, 1.61 +/- 0.69 mg, 1.90 +/- 0.25 mg, 3.69 +/- 0.36 mg, and 4.37 +/- 0.50 mg on day 2 of diestrus, on days 10-15 of psuedopregnancy, on days 9-10, 14, and 20 of pregnancy, respectively. The fact that the average size of the corpus luteum on days 10-15 of pseudopregnancy was larger than that on day 2 of diestrus is thought to drive from prolonged exposure of the corpus luteum to prolactin. The average size of the corpus luteum on days 9-10 of pregnancy had a tendency to be larger than that on days 10-15 of pseudopregnancy and this seems to demonstrate that the placenta secreted placental lactogen by this stage of pregnancy. The average size of the corpus luteum on day 14 of pregnancy was larger than that on days 9-10 of pregnancy. This phenomenon might be attributed to the presence of large amounts of placental lactogen secreted from the placenta between days 10 and 14 of pregnancy. Furthermore, it was noted that the size of the corpus luteum on day 20 of pregnancy was larger than that of day 14, which suggests that further secretion of placental lactogen continued after day 14 of pregnancy. As there was a remarkable decrease in the number of fetuses on day 20 of pregnancy when overiectomy was performed on day 14 of pregnancy, the ovary was considered indispensable in maintaining pregnancy in the rat.     

Morphological characteristics of the bovine corpus luteum during the estrous cycle and pregnancy

The corpus luteum, one of the biological clocks of the estrous cycle and pregnancy, is known foremost for its production of progesterone that blocks the pituitary release of gonadotropins and prepares the uterus for a pregnancy. The cellular sources of this progesterone are the steroidogenic small and large luteal cells. Other luteal cells that are not steroidogenic, but are believed to have an important role in the function of this gland are the fibroblast, macrophages and endothelial cells. The most prominent luteal cell is the large steroidogenic cell characterized by an abundance of smooth endoplasmic reticulum and densely packed spherical mitochondria that are indicative of its contribution to most of the circulating progesterone believed to be constitutively secreted and not under the control of LH. Other distinguishing features of the large luteal cell are the presence of rough endoplasmic reticulum, prominent Golgi, and secretory granules that are indicative of endocrine cells. This cell undergoes dynamic changes across the estrous cycle and pregnancy, believed to reflect a change in progesterone and protein secretion that will eventually influence a successful pregnancy or another ovulation if pregnancy fails. The morphological characteristics of the bovine luteal cells are the focus of this review.