Trophoblastic factors and the maternal recognition of pregnancy in sheep and cattle

The establishment of pregnancy in domestic ruminants depends upon the continued secretion of progesterone by the corpora lutea. In non-pregnant cycles the corpora lutea regress between days 12-15 after oestrus in the sheep; this process must be blocked to ensure continued exposure of the uterus to progesterone. This review discusses the evidence that embryonic products are involved in the maintenance of corpus luteum function, the identification of factors which may be responsible for this maintenance and the probable mechanism of action. The discussion centres on the recent identification of a trophoblast interferon which is thought to be the major trophoblastic factor preventing luteolysis in sheep and cattle.     

Myogenesis in sheep is altered by maternal feed intake during the peri-conception period

The effect of varying short-term maternal feed intake during the peri-conception period on the development of ovine fetal muscle at mid-gestation was investigated. Superovulated donor Merino ewes (n = 24) were fed a roughage/grain pelleted diet (10.1 MJME/kg dry matter) at either 1.5x maintenance (H; high) or 0.5x maintenance (L; low) from 18 days before until 6 days after ovulation. Embryos were transferred to recipient ewes (n = 60) on day 6. Singleton fetuses were collected on day 75 of gestation and placental weights, fetal body dimensions and fetal organ and muscle weights recorded. The number, type and size of muscle fibres and the dry matter, RNA, DNA and protein content in the semitendinosus muscle were determined. Maternal feed intake did not influence body dimensions, organ development or muscle weights in the fetus. However, L feed intake decreased total muscle fibre number in the fetus by approximately 20% (P = 0.06) compared to H feed intake. This resulted from a reduced secondary to primary fibre ratio (P < 0.05) and indicated that secondary fibre formation occurred at a reduced rate in L fetuses. In addition, protein:DNA ratio tended to be lower in muscles of L fetuses (P < 0.1). It is concluded that restricting feed intake over the peri-conception period reduces or delays myogenesis in fetal sheep. The potential mechanisms by which nutritional availability during this period may influence subsequent myogenic development are discussed.     

The critical period for the maternal recognition of pregnancy in pony mares.

Two experiments were performed to deterine the critical time at which the equine blastocyst must be present within the uterus of the mare to prevent regression of the corpus luteum, and thus establish the critical time for the maternal recognition of pregnancy. A non-surgical blastocyst collection technique was developed to study this relationship between the blastocyst and the maternal ovary. Results from these experiments demonstrated that the cyclic life-span of the corpus luteum is not affected by the presence of the blastocyst within the mare's uterus until after Day 14 after ovulation. Luteal function was prolonged when blastocysts were removed on Day 15 or later. The critical period for the maternal recognition of pregnancy in the Pony mare appears to be confined to the period between Days 14 and 16 after ovulation.     

Ovarian oxytocin and the maternal recognition of pregnancy

The secretion of oxytocin by the corpus luteum is thought to stimulate the episodic release of PGF-2 alpha from the uterus, thereby contributing to luteolysis. In pregnancy corpus luteum function is maintained, and secretion of oxytocin, or its actions on the uterus, appear to be inconsistent with the successful establishment of gestation. Protection against the effects of oxytocin is ensured by a number of mechanisms, including the cessation of luteal oxytocin secretion, which is evident by Day 20 after mating in sheep, and the maintenance of low levels of the oxytocin receptor in the uterus.     

The maternal recognition of pregnancy in mammals

Progesterone secreted by the corpora lutea stimulates uterine secretion of nutrients required for embryonic growth. But progesterone also blocks maturation of ovarian follicles and oestrous behaviour. When conception fails, therefore, the corpora lutea must lyse, in order to allow a second ovulation and oestrus, while in pregnancy the corpora lutea must be maintained. A number of different endocrine mechanisms exist to ensure the corpora lutea are controlled appropriately depending on pregnancy status; among the best studied of these are the secretion by the developing conceptus of interferons (in ungulates) or chorionic gonadotropins (in primates). These and other mechanisms are reviewed.     

Conceptus interferons and maternal recognition of pregnancy

Ovine trophoblast protein-1 (oTP-1) and bovine trophoblast protein-1 (bTP-1) are secreted by ovine and bovine conceptuses for a restricted period when the conceptus acts to block regression of the corpus luteum. Both are considered to be antiluteolytic, acting locally on the endometrium to prevent the production of the uterine luteolysin prostaglandin F2 alpha. Molecular cloning of cDNAs for oTP-1 and bTP-1 has shown that both are structurally related to alpha interferons (IFN-alpha s). In particular, they resemble members of the 172-residue long IFN-alpha II (or IFN-omega) subfamily. Both proteins possess the antiviral and antiproliferative properties of IFN-alpha s and appear to bind IFN-alpha receptors. A novel and previously unsuspected role for IFNs in early pregnancy is apparent.     

Stimulation of immunoreactive inhibin production by preimplantation embryos during early pregnancy in the marmoset monkey (Callithrix jacchus).

The role of the embryo in promoting increased plasma concentrations of immunoreactive inhibin after conception in the marmoset monkey was determined by flushing embryos from the uterus between days 5 and 9 after ovulation (implantation commences on days 11-12). Blood samples were taken from each animal (three times a week) after ovulation until the end of the luteal phase. Plasma inhibin concentrations were measured using a radioimmunoassay based on antisera against a synthetic fragment of the alpha-subunit of human inhibin. When embryos were flushed on days 5 and 6 (n = 6) after ovulation inhibin concentrations did not exceed 250 ng ml-1 for the duration of the luteal phase. In contrast when embryos were flushed on days 7 (n = 4), 8 (n = 4) and 9 (n = 3) maximum concentrations of inhibin always exceeded 250 ng ml-1, reaching > 400 ng ml-1 when embryos were flushed on days 8 and 9. Inhibin concentrations remained high for the duration of the luteal phase, which varied in length between 20 and 32 days. Significantly (P < 0.01) higher mean plasma concentrations of immunoreactive inhibin were first recorded on days 7-8 after ovulation in animals that had embryos flushed on days 7, 8 and 9 compared with concentrations in animals that had embryos flushed on days 5 and 6. Inhibin could not be detected in the medium of embryos cultured for up to 2 weeks.(ABSTRACT TRUNCATED AT 250 WORDS)     

The effect of nutrition during pregnancy on the in vitro production of embryos from resulting lambs

It is possible to produce offspring from FSH-treated lambs using in vitro maturation and fertilisation procedures but a major constraint is the high embryo wastage after transfer. It is postulated that this wastage is associated, at least in part, with the quality of the harvested oocytes. The aim of this study was thus to determine if nutrition during pregnancy influenced the quality of oocytes collected from resulting lambs. The study was a 2x2x2 factorial that examined the effect of a low (L; 0.7x maintenance) or high (H; 1.5x maintenance) diet provided during three periods (-82 to 70, 71-100 and 101-126 days relative to the date of conception). There were eight treatments namely LLL, LLH, LHL, LHH, HLL, HLH, HHL and HHH. Oocytes were harvested from 9-week-old lambs, matured and fertilised in vitro and the percentages of oocytes and embryos that developed into blastocysts were recorded. There were significant differences between treatments in oocyte and embryo yields and these resulted from complex interactions between diet and the stage of pregnancy. The efficiency of producing blastocysts from oocytes was highest when a H diet was provided between 71 and 110 and/or 101-126 days of pregnancy. These results demonstrate the need to manage nutrition during pregnancy in programs aimed at producing offspring from juvenile animals.     

A study of some serum mineral levels before and during pregnancy and during lactation period of sheep and cattle

The purpose of this study was to determine serum Cu, Zn, Mn, K, and Mg levels before and during pregnancy and during the lactation period of sheep and cattle and to determine the relationships between periods. For this purpose, blood samples of 14 healthy, pregnant Brown Swiss cows fed under normal condition and 36 Karakaş (İvesi × Akkaraman) sheep were used. Blood samples were analyzed using an atomic absorption spectrophotometer. There were significant differences between before pregnancy and during pregnancy, and during pregnancy and the lactation period for serum Zn concentration; also before pregnancy and during pregnancy, and before pregnancy and the lactation period for serum K concentration in cows (p<0.05). There were significant differences between before pregnancy and the lactation period for serum Cu concentration, and before pregnancy and during pregnancy for serum K concentration; also pregnancy and the lactation period for serum Mg concentration in sheep (p<0.05).     

Puromycin-sensitive aminopeptidase is essential for the maternal recognition of pregnancy in mice

Maternal recognition of pregnancy in rodents requires semicircadian surges of hypophyseal PRL secretion during early gestation, which are required for the formation of the corpus luteum of pregnancy (CLP). Here we show that puromycin-sensitive aminopeptidase (Psa)-deficient mice display female infertility that results from impaired formation of CLP. Transplantation of mutant ovaries into normal females restored fertility but not vice versa. Psa-deficient females revealed no semicircadian surges of PRL induced after mating stimuli. Pregnancy in the mutant females was restored by grafting intact pituitaries to elevate circulating levels of PRL. Psa is thus required for the appearance of the semicircadian surges of PRL secretion that are crucial for maintaining pregnancy in rodents.     

Changes in concentrations of follistatin in maternal plasma and foetal fluids during pregnancy and parturition in sheep

Studies have shown that follistatin may be involved in the regulation of ovarian development, pregnancy and parturition. The aim of the present study was to measure follistatin levels in maternal plasma and foetal fluids during pregnancy and parturition in sheep. Using a previously described follistatin radioimmunoassay, we found that follistatin in foetal plasma and allantoic fluid was higher in the female than in the male at days 50-75 of gestation. Follistatin concentrations in maternal plasma declined from -9 to -3h before the completion of lamb delivery, and increased from 21 to 39 h after parturition. These results suggest that follistatin may play a role in femaleness development, and that follistatin may be involved in the mechanism of normal parturition and in regulation of the tissue repair after parturition.