Endocrine Hormonal Changes during Estrus, Pregnancy and Parturition in Cattle
Rashmi and Tamilmahan P.
- PhD scholar, Indian veterinary research institute, division of veterinary surgery and radiology, bareilly
- Assistant professor, Tamil Nadu veterinary and animal sciences university division of veterinary surgery and radiology
Endocrinology at the time of estrus
Cattle are polyestrous animals and display estrous behaviour approximately every 21 days. Various hormones control the oestrous cycle like GnRH or gonadotropin-releasing hormone released by hypothalamus, FSH or follicle-stimulating hormone and LH or luteinizing hormone by the anterior pituitary, progesterone (P4), estradiol (E2) and inhibin secreted by ovaries and the prostaglandin F2α (PGF) secreted by uterus. These hormones function through a system of positive and negative feedback to govern the oestrous cycle of cattle. GnRH, released by hypothalamus, controls oestrous cycle by acting on the anterior pituitary which regulates the secretion of gonadotrophins LH and FSH (Schally et al., 1971). The FSH and LH release is controlled by two functionally separate system that is episodic/tonic system and surge system. The episodic system is responsible for continuous basal secretion of gonadotropin and stimulate growth of germinal and endocrinal part of ovary. This release centre is influenced by negative feedback effect of estradiol and progesterone. Surge system control the short lived massive secretion of gonadotropin particularly LH responsible for ovulation.
Two or three waves of follicles growth rises per estrous cycle on average in cattle. FSH exerts signalling effects that included increased cellular growth and proliferation of follicles (Richards et al., 1998). In this event, one follicle is selected from the group of follicles, the diameter increases and it is recognized as the largest healthy follicle in the cohort (Gougeon, 1993). Dominance occurs when the follicle reaches 9 mm in diameter, and it actively suppresses FSH, thus preventing further follicle wave emergence until the Dominant Follicle (DF) either undergoes atresia or ovulated.
Estradiol is produced only by those antral follicles that have potential for ovulation (Baird et al., 1991) and inhibin is produced by all antral follicles. Estradiol reaches a discrete peak around day 6 of cycle due to first wave of follicular growth and there is sudden rise in plasma estrogens (estradiol) before the onset of behavioural oestrus. Peak value occur at beginning of estrus with a subsequent decline to basal concentration at the time of ovulation. During rest of the cycle, there are fluctuations in concentration. The increase in estradiol concentrations along with inhibin exert negative feedback on the anterior pituitary gland by signalling to supress FSH to basal concentrations (Sunderland et al., 1994; Ginther et al., 2000). The selected DF becomes increasingly responsive to LH (Ginther et al., 2000) and continues to grow in the face of decreasing FSH concentrations. LH-receptor are localised to the theca and granulosa cells of healthy follicles, at different stages of follicle development (Camp et al., 1991). During the early luteal phase amplitude and frequency of LH surge are insufficient for final maturation and subsequent ovulation of the DF resulting in atresia of this follicle (Rahe et al., 1980). This causes removal of negative feedback block to the hypothalamus/pituitary thus FSH secretion increases and a new follicle wave emerges. During the follicular phase of the estrous cycle, large concentration of estradiol produced by the pre-ovulatory DF induces the release of LH and resulting LH surge of sufficient amplitude causes final maturation and ovulation of the DF (Sunderland et al., 1994).
LH level is very much high just before ovulation because of LH surge. Serum FSH levels on the day of estrus tended to be higher than follicular or luteal phase levels. The rise in FSH on the day of estrus in cows was 1.2 times compared to follicular or luteal phase levels, which is of similar magnitude to that reported in ewes during estrus (L’Hermite et al., 1972). The changes in progesterone concentration mimic closely the physical changes of the CL. At the time of ovulation progesterone level is very much low. Peak values are reached by days 7 and 8 after ovulation, and decline fairly quickly from day 18.
Endocrinology at the time of pregnancy
For maintaining pregnancy, the luteal phase of the oestrous cycle is necessary in the most domestic species. It got prolonged by the persistence of a single corpus luteum (CL) or a number of corpora lutea (C.Ls) which causes progesterone concentrations remain elevated. This results in an inhibition of follicular development and ovulation because of negative feedback on the hypothalamus and anterior pituitary. In many species, the placenta subsequently replaces or supplements the luteal source of progesterone but CL is the main structure producing progesterone for the maintenance of pregnancy in the cow, the placenta producing only small amounts. Bovine conceptus produces a protein, called bovine trophoblast protein which exerts its antiluteolytic effect by inhibiting release of PGF2. Ablation of the CL either surgically or with the use of PGF2α, usually results in abortion up to about 200 days of gestation however, after this stage until just before term, pregnancy usually continues.
Progesterone concentrations in the peripheral circulation of pregnant animals during the first 14 days of gestation are similar to those of diestrus of non pregnant. But progesterone level thereafter decline sharply from about the 18th day after ovulation. In the pregnant cow, there is normally only a slight decline at this stage with a rapid recovery. Thereafter the concentration increases slightly during pregnancy until it starts to decline at about 20–30 days prepartum. Oestrogen concentrations during early and midgestation are being low, less than 100 pg/ml but level increases towards the end of gestation after day 250 to reach peak values of 7 ng/ml oestrone sulphate (Thatcher et al., 1980). Hormone level rapidly decline from about 8 hours prepartum to low levels immediately postpartum. Both FSH and LH concentrations remain low during gestation and show no significant fluctuations. Prolactin level increases from basal concentrations of 50–60 ng/ml to peak values of 320 ng/ml twenty hours before delivery of calf followed by a subsequent decline to basal concentrations by thirty hours after birth of calf. Bovine placental lactogen starts to appear in circulation in dam at about 160 days of gestation and increasing to maximum concentrations of 1000 ng/ml between 200 days and term (Bolander et al., 1976).
Endocrinology at the time of parturition
Several hormones are involved during late bovine pregnancy to maintain and successful delivery of a live calf. These hormones are e.g., progesterone originating from the corpus luteum, maternal adrenals and placenta. For the onset of normal parturition process, a change from progesterone to oestrone synthesis is crucial. Various hormones like prostaglandin F(2alpha), cortisol and oxytocin are involved in process of parturition.
In domestic ruminants such as sheep and cows, the fetus triggers labour at term which involves the activation of the fetal hypothalamic-pituitary- adrenal (HPA) axis leading to a surge in adrenal cortisol production. Fetal cortisol then acts to up-regulate directly the activity of placental 17alpha-hydroxylase/17,20-lyase (CYP17) enzyme, which catalyzes the conversion of pregnenolone to 17 beta-estradiol. Oestrogens stimulate the production and release of prostaglandin F2 alpha by cotyledon-caruncle complex and also exert direct effect upon the myometrium increasing its responsiveness to oxytocin, softening of the cervix by altering the structure of the collagen fibres. Prostaglandins participate in initiating parturition by causing, luteolysis, smooth muscle contraction and the softening of cervical collagen. PGF2α is considered to be the intrinsic stimulating factor of smooth muscle cells (Csapo, 1977) and the resulting myometrial contractions push the foetus towards the cervix and vagina. This process will stimulate sensory receptors and initiate Ferguson’s reflex, with the release of large amounts of oxytocin from the posterior pituitary. Oxytocin will stimulate further myometrial contractions and the release of PGF2α from the myometrium. The pattern of oxytocin release during late pregnancy and parturition has been studied in the cow (Schams and Prokopp, 1979). It is observed that in all these species the oxytocin levels during late pregnancy and the early stages of parturition remain fairly low and increase to reach peak values at the time when the fetal head emerges from the vulva, and when the fetal membranes are expelled. In cows, just before calving a polypeptide hormone called relaxin is released from corpus luteum. Increasing value of this hormone during parturition causes relaxation of the pubic symphysis.
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