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2. Anatomy and endocrinology of cow reproduction

2.1 Anatomy
2.2 Endocrinology of reproduction
2.3 References

2.1 Anatomy

The genital tract of non-pregnant cows normally lies in the pelvic cavity and consists of the vulva, vagina, cervix, uterus, Fallopian tubes (oviducts), ovaries and their supporting structures (Figure 2). Most of the reproductive structures can be palpated through the rectum; this is the basis of routine fertility work described in subsequent sections of this monograph. In general, the reproductive tract of Bos indicus cattle is smaller than that of taurine cattle. The reproductive tract is supplied by blood from the utero-ovarian and uterine arteries, of which the middle uterine artery is the largest.

Figure 2. The reproductive tract of the cow (lateral view) showing its position inside the pelvic and abdominal cavities

Source: Peters and Lamming (1987).

The uterus is a muscular organ consisting of a body, about 4 to 5 cm long, and two uterine horns (cornua), each 15 to 25 cm in length and 1 to 3 cm in diameter. The uterus is suspended by the broad ligament in a coiled or curled manner. Its size varies with breed, age, parity, pregnancy and disease.

The cervix is a sphincter-like structure with a thick wall and a narrow lumen. This lumen is tightly closed, except during oestrus and at parturition, and the cervix forms a barrier between the uterus and the outside environment. The length of the cervix varies from 1.5 cm in heifers to 8 cm in multiparous cows of larger breeds. Interlocking ridges and complex folding of the lumen mucosa can hamper insertion of a pipette or tube for intra-uterine insemination or infusions.

The vagina extends backwards from the cervix and opens into the vulva. Its length varies with breed and stage of pregnancy. The vaginal epithelial cells near the cervix secrete mucus, especially around the time of oestrus.

The ovaries are oval-shaped structures, 1 to 4 cm long and 1 to 3 cm in diameter; their size depends on the stage of the reproductive cycle. They are linked to the uterus by the Fallopian tubes which open anteriorly into the fimbriae - funnel-shaped structures close to, but not attached to the ovaries. The fimbriae guide unfertilized eggs from the ovary into the Fallopian tubes.

Figure 3 is a schematic representation of the mammalian ovary. The ovary has two major functions: gametogenesis (the production of female gametes) and steroidogenesis (the production of steroid hormones, which play vital roles in the reproductive cycle).

Figure 3. The morphology and architecture of a mammalian ovary during the reproductive cycle

Source: Erickson et al (1985).

The ovary of a new-born heifer may contain up to 100,000 primordial follicles (Erickson, 1966). However, only a few of these mature and release an ovum. From birth to shortly before puberty, the primordial follicles are in a state of arrested development (dictyotene; the resting stage). Shortly before puberty, many primordial follicles start to grow and develop in response to hormone (gonadotrophin) stimulation. The presence of developing follicles indicates active gametogenesis and steroidogenesis. During each oestrous cycle, several follicles may develop to the Graafian stage, but usually only one reaches full maturity and ruptures to release the ripe ovum (ovulation): the others become atretic (also known as degenerating, luteinising or anovulatory follicles) (McDonald, 1980).

Ovulation involves changes in steroid, gonadotrophin and prostaglandin secretions along with alterations in ovarian neuromusculature, in particular the breakdown of the follicular wall, escape of the ovum and release of follicular fluid. The ovulation fossa formed after ovulation fills with blood to become the corpus haemorrhagicum and eventually the corpus luteum (yellow body), which protrudes from the surface of the ovary. For details on the histology of the corpus luteum, see, for example, Corner (1915); McNutt (1924); Foley and Greenstein (1958); Gier and Marion (1961); Donaldson and Hansel (1965); Parry et al (1980); Alila and Hansel (1984); and Braden et al (1988). The corpus luteum is part of the endocrine system.

2.2 Endocrinology of reproduction

2.2.1 Endocrine changes in the prepubertal heifer
2.2.2 Endocrinology of the oestrous cycle
2.2.3 Endocrinology of pregnancy
2.2.4 Maternal recognition of pregnancy
2.2.5 Endocrinology of the postpartum period

The development of radio-immunoassay (RIA), competitive protein binding (CPB) and enzyme-linked immunosorbent assay (ELISA) methods since the 1960s has allowed rapid, accurate and sensitive measurement of the concentration of several pituitary, ovarian and adrenal hormones in blood, tissue, milk and urine. Ovarian tissue can now be studied in vitro. Cells can be broken down and the secretion and use of hormones by their organelles examined. The interrelations among hormones at various stages of the reproductive cycle in the female cow are therefore better understood, as are the physiological control mechanisms that govern reproductive function (Entwistle, 1983).

The endocrine system comprises a series of ductless glands, each of which secretes one or several hormones that integrate body functions. Hormones are secreted directly into the blood and act on tissues elsewhere in the body.

The reproductive cycle of the cow is mainly coordinated by hormones produced by the hypothalamus, pituitary and ovary. Gonadotrophic releasing hormone (GnRH), secreted by the hypothalamus, stimulates the anterior pituitary to secrete two gonadotrophic hormones - follicle stimulating hormone (FSH) and luteinising hormone (LH). Both of these hormones control ovarian function: FSH initiates maturation of follicles, and LH induces ovulation and luteinisation of granulosa and thecal cells. The major hormones produced by the ovaries are oestrogens (primarily oestradiol-17 b), which are produced by the follicles, and progesterone, secreted by the corpus luteum. Oestrogens play important roles in oestrus manifestation, and progesterone in maintenance of pregnancy. Both regulate the reproductive cycle through a series of feedback mechanisms acting on the hypothalamus and pituitary glands. In addition to these hormones, prostaglandins, which are produced by several tissues, including the uterus, also control the cow reproductive cycle in various ways. Details of the functions of these hormones at puberty, during normal oestrous cycles, during pregnancy, and in the postpartum period are given in the following sections. Other hormones, produced by the thyroid, parathyroid and adrenal glands, the placenta and the pancreas are also important in regulating reproduction.

For information on hormone action at the cellular level, see, for example, Eisenfeld (1972), Calandra et al (1974), Kato et al (1974), Catt et al (1975), Saxena (1976), Greenstein (1978), Cermak et al (1983), Spellberg et al (1983), Saxena et al (1984), McCracken and Okulicz (1984), Smith et al (1985) and Bergh et al (1985).

There are differences in the hypothalamic, pituitary and ovarian relationships in zebu and taurine cattle (Rollinson, 1955; Plasse et al, 1970; Griffin and Randel, 1978; Randel, 1976,1984; Rhodes et al, 1979). These differences probably account for differences in fertility between the two species even when similarly fed and managed (Rhodes et al, 1982).

This section highlights the endocrinology of reproduction in Bos indicus cattle. Data from taurine cattle and other species are used for emphasis and where information for zebu cattle is not readily available.

2.2.1 Endocrine changes in the prepubertal heifer

Little information is available on the hormonal control of puberty in the zebu. Early studies tended to compare the endocrine patterns in heifers with those of more mature animals. It was originally thought that the pituitary of prepubertal animals was incapable of elaborating sufficient gonadotrophic hormones to stimulate the ovaries. This was based on observations, such as those by Macfarlane and Worrall (1970) among Boran zebus, that administering gonadotrophins stimulated follicular growth and ovulation. However, follicle growth starts soon after birth (Desjardins and Hafs, 1968), as does the release of gonadotrophins, FSH and LH (Peters and Ball, 1987). The ovaries of 2-month old calves can respond to gonadotrophin therapy (Onuma et al, 1970) and calf follicles can secrete oestrogens. Prepubertal ovaries also respond when transplanted to mature animals (Russell and Douglas, 1945) and injecting oestradiol results in LH release in calves as young as 3 months old (Schillo et al, 1983). The possible causes of sexual maturation at puberty appear to be an increase in pituitary hormones output culminating in increased size and activity of the ovaries (Hunter, 1980), and maturation of the hypothalamo-pituitary axis, resulting in secretion of gonadotrophins (Ramirez, 1973). Luteinising hormone

LH levels fluctuate before puberty (Desjardin and Hafs, 1968) but tend to increase as puberty approaches (Swanson et al, 1972). By taking samples more frequently from Angus heifers, Gonzalez-Padilla et al (1975a) confirmed that prepubertal heifers do not lack LH as such, but there is no cyclic pattern to its release. Two LH peaks were observed prior to puberty, the first (priming) peak at 9 to 11 days before first oestrus. This diphasic profile was also observed among Brown Swiss heifers by Schams et al (1981). In heifers attaining puberty at 10 months old, LH and FSH levels increased from birth to 3 months, declined to a nadir at 5-6 months and then increased to a second peak at about 9 months (Figure 4).

LH secretion in prepubertal heifers is probably suppressed through an inhibitory feedback (gonadostat) effect (Anderson et al, 1985). As noted above, components of the endocrine system can apparently function soon after birth (Ramirez and McCann, 1963). LH is secreted from the pituitary gland and stimulates ovarian follicles to produce oestradiol-17 b. However, the hypothalamus-pituitary axis is highly sensitive to the negative feedback effect of oestradiol, and further LH release is inhibited. Ovariectomy of immature rats significantly increases the concentration of plasma LH (Ramirez and McCann, 1963; Caligaris et al, 1972) and FSH (Kragt and Masken, 1972). The same is true in the calf (Odell et al, 1970). The sensitivity of the hypothalamus-pituitary axis to oestradiol must thus decrease prior to puberty in the heifer (Schillo et al, 1982). This allows LH to stimulate follicular growth and leads to increased oestrogen production (Foster and Ryan, 1981) and ovulation. Oestradiol and progesterone

There are few reports on the plasma concentration of oestradiol-17 b in prepubertal heifers. Glencross (1984), using a sensitive and fully validated radio-immunoassay, found that the plasma oestradiol-17 b levels of four British-Friesian heifers varied randomly within the range 1 to 4 ng/litre between 59 and 15 days before puberty. About 8 days prior to puberty, oestradiol-17 b levels increased significantly (P<0.02) to a mean of 6.3 ▒1.3 ng/litre, comparable to the normal preovulatory peak in post-pubertal heifers. It was not clear, however, if this induced an LH surge and ovulation. Progesterone levels subsequently rose (P<0.001) to a peak of 1.0 ▒0.1 m g/litre on the fourth day, indicating some luteinisation. After the return of the progesterone to basal levels, oestradiol-17 b again rose significantly (P<0.001) to a second peak of 9.0 ▒1.0 ng/litre on the day of first oestrus. Following this second peak, concentration of progesterone in the plasma remained high and pregnancy was confirmed in three of the heifers. The second peak in oestradiol-17 b concentration had therefore been followed by ovulation. A third oestradiol-17 b peak (P<0.02) of 4.3 ▒0.8 ng/litre occurred 4 days later, when progesterone levels were rising sharply due to the formation of a corpus luteum. The changes in oestradiol-17 b and progesterone on or after the day of first oestrus were similar to those observed in post-pubertal heifers (Glencross and Pope, 1981; Glencross et al, 1981) and mature cows (Echternkamp and Hansel, 1973; Glencross et al, 1973; Smith et al, 1975).

Figure 4. Plasma LH and FSH concentrations (▒ SEM) of female calves from birth to 12 months of age

Source: Adapted from Schams et al (1981).

Studies among Bos taurus heifers showed that progesterone concentration is low through most of the prepubertal period with two rises before puberty (Gonzalez-Padilla et al, 1975a). The first occurred between 18 and 11 days before the LH peak and was thought to be of adrenal origin. The second, from 9 days before until the day of the LH peak, was assumed to be of ovarian origin. Schams et al (1981) also observed an increase in progesterone concentration for 8-12 days before first oestrus in four Brown Swiss heifers that attained puberty at about 10 months old (Figure 5). A fifth heifer, which showed first oestrus at 14 months, exhibited a progesterone secretion pattern resembling that of a normal corpus luteum during the 18 days before first oestrus. Prior to this rise, levels were elevated for 8 days, but only slightly (Figure 5). Similar elevations in progesterone concentration were reported by Berardinelli et al (1979) who attributed them to small luteal tissues, deeply embedded in the ovary, which could not be palpated. This agrees with Ojeda et al (1980), who stated that, in general, there is no compelling evidence for a role of adrenal sex steroids in the onset of puberty. These initial rises in progesterone may establish a phasic pattern to LH release and/or sensitise the ovaries to LH (Berardinelli et al, 1979) as in some postpartum cows.

Figure 5. Mean (▒ SEM) plasma progesterone levels before and after the first oestrus in heifers

a = 4 heifers attaining puberty at 303 + 11 days of age
b = 1 heifer attaining puberty at 414 days of age

The arrows indicate the time of the preovulatory LH surges
Ě indicates the time of oestrus

Source: Schams et al (1981).

The above observations have led to attempts to stimulate puberty. Most efforts have tried to simulate the transient rise in progesterone prior to first oestrus using implants or daily injections of progesterone combined with oestrogen or pregnant mare serum gonadotrophin (Gonzalez-Padilla et al, 1975b; Rajamahendran et al, 1982). Generally the treatments have been more successful in animals approaching puberty.

2.2.2 Endocrinology of the oestrous cycle

Figure 6 illustrates how the concentrations of the main reproductive hormones in the plasma change during the cow oestrous cycle. Hypothalamic GnRH induces the release of both LH and FSH from the pituitary (Kaltenbach et al, 1974; Schams et al, 1974). LH is released in pulses (Rahe et al, 1980, 1982) (Figure 7). Each LH pulse appears to be in response to a release of GnRH from the hypothalamus and LH secretion can be stimulated by GnRH injections. LH induces ovulation and luteinisation of the granulosa and thecal cells. It also appears to be the principal luteotrophic factor in the cow (Peters and Lamming, 1983). Luteinising hormone

LH concentration is low during most of the luteal phase of the oestrous cycle, with one pulse every 4 or more hours. It begins to rise a few days prior to oestrus. Pulse frequency increases to one or more per hour; pulse amplitude, however, falls. The large preovulatory LH peak or surge that occurs near the beginning of oestrus is preceded by a rise in the concentration of oestradiol one or 2 days before oestrus.

Figure 6. Schematic representation of the changes in plasma hormone concentrations during the bovine oestrous cycle

Day O = day cow first shows oestrus

Source. Peters and Lamming (1983).

Randel (1976) estimated the interval between oestrus onset and the LH surge to be 0.4 3.4 hours in Brahman cows, 6.8 ▒2.1 hours in Brahman x Hereford cows and 5.3 ▒1.3 hours in pure Herefords. Randel and Moseley (1977) recorded intervals of 2.0 ▒1.3 in Brahman cows, 3.0 ▒1.0 in Brahman x Hereford cows and 6.5 1.8 hours in Hereford cows. The preovulatory LH surge therefore seems to occur sooner after the onset of behavioural oestrus in zebu than taurine cows or their crosses. In addition, Randel (1976) estimated the interval between the LH surge and ovulation to be 18.5 ▒3.1 hours in Brahman cows, 22.2 ▒2.6 hours in Brahman x Hereford cows and 23.3 ▒2.1 hours in Hereford cows. Zebu cows thus appear to ovulate sooner after the LH surge than Bos taurus cows Oestradiol

Oestradiol-17 b is the principal biologically active oestrogen. Randel (1980) measured total serum oestrogen (TSO) from 72 hours before oestrus until 24 hours after oestrus in Brahman, Brahman x Hereford and Hereford heifers. TSO did not differ significantly between breeds prior to oestrus. The highest pro-oestrous TSO level occurred 24 hours before oestrus in Brahmans, 8 hours before oestrus in Herefords and 16 hours before oestrus in the crossbreds. The pattern was similar for the lowest levels after oestrus: 24 hours after oestrus TSO levels were lower in Brahmans than the other two genotypes (P<0.05) and these lower values coincided with ovulation. This finding agreed with data showing that Brahman cows tend to ovulate within 24 hours of the onset of heat, earlier than taurine cattle (Plasse et al, 1970; Randel, 1976). Randel's (1980) data also indicate that the oestradiol surge has two peaks. Echternkamp and Hansel (1973) proposed that the pre-oestrus rise in oestrogen mediates the LH release from the bovine pituitary, which in turn might stimulate the second oestradiol rise.

Figure 7. Pattern of plasma LH concentration on day 3 and 11 and the preovulatory LH surge on day 19 in the cow

Source : Rache et al (1982)

Glencross and Pope (1981), working with taurine cattle, found that oestradiol-17 b levels are low in peripheral plasma for most of the oestrous cycle and rise as the concentration of progesterone begins to fall, reaching a peak 3 to 4 days later. Probably the drop in progesterone concentration following luteal regression allows the preovulatory follicle to increase its secretion of oestradiol-17 b (Karsh et al, 1978). In the Holstein heifers used by Glencross and Pope (1981), plasma oestradiol-17 b concentration was low at the start of luteal regression (2.2 ▒0.5 pg/ml), increased to 3.8 ▒0.6 pg/ml the next day and reached 6.6 ▒0.9 pg/ml when the concentration of progesterone in the blood had fallen to a minimum. The highest concentration of oestradiol-17 b (10.1 pg/ml) was recorded one or 2 days after complete luteolysis. A similar trend was reported by Wettemann et al (1972).

Glencross et al (1973), Smith et al (1975), Glencross and Pope (1981) and Hansel and Convey (1983) observed a second, postovulatory, peak in oestradiol 5 to 7 days after oestrus (Figure 8). The last authors observed this peak in non-pregnant cattle and those inseminated but failing to conceive. It appeared to be related to the presence of a large follicle. Its physiological significance is not clear but the follicle is not destined to ovulate (Peters and Ball, 1987). Progesterone

Mukasa-Mugerwa et al (1989), using the ELISA method, found that the concentration of progesterone in the plasma of Ethiopian highland zebu cattle was less than 1.0 ng/ml from 2 days before oestrus to 3 days after oestrus. Llewelyn et al (1987) referred to this as the "basal progesterone" period. Progesterone concentration gradually increased from 4 days after oestrus (the "rising progesterone or early luteal" period), as the corpus luteum became functional. It reached a maximum of 8.0 to 10.0 ng/ml at 11 to 15 days after oestrus (the "plateau progesterone" period) and then declined (the "falling progesterone" period) to basal levels before the next oestrus and ovulation (Figure 9). A similar pattern in progesterone concentration was reported by Adeyemo and Heath (1980) in White Fulani cattle, Eduvie and Dawuda (1986) in Bunaji cattle, Coetzer et al (1978a) in Africander cattle, Vaca et al (1983) in InduBrazil cattle, Llewelyn et al (1987) in Boran cattle, Randel (1980) in Brahman cattle and their taurine crosses and Hansel (1981) in Holstein heifers.

Coetzer et al (1978a) observed a drop in the concentration of progesterone in the blood of zebu cows about 13 days after oestrus. Similar observations were made by Erb et al (1971) and Schams et al (1977) in taurine cows. Coetzer et al (1978a) associated the decrease with mid-cycle follicle growth and development. They also found a small, but consistent, peak in progesterone concentration, 18-25 hours after the preovulatory LH surge, the time of ovulation. If consistent, this could be used to determine more precisely the time of ovulation.

Figure 8. Follicular growth and endocrine changes from luteal regression to the resumption of luteal function

Source: Hansel and Convey (1983).

Figure 9. Mean (▒ SD) plasma progesterone levels during the oestrous cycle of Ethiopian zebu cows

Source: Mukasa-Mugerwa et al (1989).

Randel (1980) noted that progesterone concentration was generally lower in Brahman and Brahman x Hereford crosses than in purebred Hereford heifers. Between-breed differences in progesterone concentration have sometimes been suspected to arise from differences in ovarian size (Adeyemo and Heath, 1980; Irvin et al, 1978; Segerson et al, 1984a). The exact relationship is, however, not clear. Randel (1984), for example, suggested that the lesser responsiveness of the ovaries of Brahman cows to gonadotrophic hormone during formation of a corpus luteum might also result in a smaller corpus luteum. Nevertheless, the corpora lutea of Brahman and Brahman x Hereford cows had similar total progesterone contents. The Brahman corpora lutea seemed to have compensated for the small size. In fact, the activity of 3 b -hydroxysteroid dehydrogenase, the enzyme responsible for converting pregnenolone to progesterone, was greater in corpora lutea from Brahman cows. Prostaglandins

Oestrus usually occurs 1 to 5 days after the corpus luteum starts to regress. The regression of the bovine corpus luteum is brought about by the action of prostaglandin F2 a (PGF2 a) (Knickerbocker et al, 1988). Dobson and Kamonpatana (1986) suggested that the variation in this interval is due partly to differences in the time ovulatory follicles take to develop and mature. Views on the growth, distribution and selection of follicles in the ovary differ. Choudary et al (1968), Donaldson and Hansel (1965) and Marion and Gier (1971) suggested that follicular growth is continuous and independent of the phases of the oestrous cycle. Matton et al (1981) thought that numbers of follicles do not vary between stages of the cycle. In contrast, Rajakoski (1960) suggested that antral follicles grow and regress throughout the oestrous cycle in two waves: the first ends around day 12 and is followed by atresia; the second culminates in oestrus.

There is little information about prostaglandin concentration in zebu cows. Studies in the buffalo showed increases in prostaglandin concentration in blood plasma and milk from 250 to 900 pg/ml over the 2 or 3 days prior to oestrus (Batra and Pandey, 1983). Similar increases (150-750 pg/ml) were reported for the taurine cow by Kindhal et al (1976). Edquist and Kindhal (1981) give more information on the use of prostaglandins in animal reproduction.

2.2.3 Endocrinology of pregnancy

When a cow conceives, plasma and milk progesterone levels rise as in a normal oestrous cycle but instead of declining at about 15 to 18 days after oestrus, remain high for the rest of the gestation period, preventing further ovarian cycles (Peters and Lamming, 1983). Working with Holstein heifers, Hansel (1981) noted that jugular plasma progesterone concentrations were higher (P<0.05) in pregnant than cyclic non-pregnant animals 10 days after oestrus (Figure 10), indicating that the bovine blastocyst is able to stimulate progesterone synthesis by as early as the 10th day after conception.

In a study of the blood progesterone levels during the gestation period of Ethiopian zebu cows, Mukasa-Mugerwa and Azage Tegegne (1989) (Figure 11) observed a trend similar to that found by Agarwal et al (1977, 1980) in Haryana Zebu and Coetzer et al (1978b) in Africander cows. Progesterone levels in Ethiopian zebu cows were high (over 5 ng/ml) until the last 12 to 18 days of pregnancy. This was followed by a decline to 3.7-8.2 ng/ml one to two days before parturition. Hashmat Shehata (1982) observed that progesterone levels declined to 1.2-2.0 ng/ml during the last 12-24 hours before calving and were less than 1 ng/ml 24-48 hours after delivery in local Egyptian cattle. The differences in progesterone concentration among the studies probably reflect breed and/or assay-technique variability.

Figure 10. Mean (▒ SEM) jugular plasma progesterone concentrations in pregnant, cyclic and inseminated non-pregnant heifers during an 18-day period

Source: Hansel (1981).

Although the corpus luteum remains active throughout pregnancy its weight and progesterone content do not perfectly reflect changes in jugular plasma progesterone, indicating an extra-ovarian source (Erb et al, 1971). Abortion can be induced using prostaglandin F2 a (PGF2 a) prior to 120 days of pregnancy (Dobson and Kamonpatana, 1986). After this time, both the placenta (Melampy et al, 1959) and adrenals (Wendorf et al, 1983) can produce progesterone. Progesterone from these sources probably maintains pregnancy after mid-term ovariectomy (Estergreen et al, 1967). These sources probably also account for the low abortion rates after administering prostaglandins at 120 to 250 days of gestation, and the administration of PGF2 a is not very successful until after 250 days of pregnancy (Johnson, 1981).

Figure 11. Mean (▒ SEM) jugular plasma progesterone concentration in zebu cattle during pregnancy

Source: Mukasa-Mugerwa and Azage Tegegne (1989).

Coetzer et al (1978b) reported that the level of unconjugated oestrogens was low (193 to 267 pg/ml) and fairly constant during pregnancy in Africander cattle. Between 2 and 6 days prior to delivery the level increased sharply to 271 to 523 pg/ml. The upper limit is comparable to the 501 pg/ml obtained by Stellflug et al (1978) for Angus and Hereford beef cows. Two days after parturition the level stabilised at around 110 pg/ml, with little variation between individuals. Erb et al (1971) had noted the same trend in taurine cattle. Substantial amounts of oestrogens are also produced by the bovine placenta after 100 days of pregnancy (Veenhuizen et al, 1960; Ainsworth and Ryan, 1966; Robertson and King, 1979; Evans and Wagner, 1981; Shelton and Summers, 1983).

2.2.4 Maternal recognition of pregnancy

The exact mechanisms involved in maternal recognition of pregnancy are not fully understood. However, it appears necessary that (i) corpus luteum function is maintained and (ii) the cyclic release pattern of LH must be terminated, prostaglandins must be stopped from reaching the corpus luteum, or some substance must be secreted to check the cyclic action of prostaglandins. It is suspected that interactions between the developing conceptus and maternal system are involved. The mechanisms regulating the establishment and maintenance of pregnancy in cattle must be understood before techniques can be developed to reduce the incidence of early embryonic mortality (Segerson et al, 1984a).

Plasma progesterone levels are significantly (P<0.05) higher in pregnant than cyclic non-pregnant Holstein heifers by as early as 10 days after fertilisation (see Figure 10). Shemesh et al (1979) found that the bovine blastocyst can produce progesterone, some testosterone and limited amounts of oestradiol-17 b by day 13 to 16. Blastocysts 15-17 days old are also able to convert androstenedione to oestrogen in vitro (Eley et al, 1979). Oestrogens have a luteolytic action in the cow (Wiltbank et al, 1961; Eley et al, 1979).

Homogenates of sheep embryos (Martal et al, 1979) and sheep conceptus secretory proteins (Godkin et al, 1984) can extend corpus luteum function and cycle length when administered into the uterine lumen of cyclic ewes. Thus, in sheep the pre-implantation embryo appears to produce a luteotrophic substance that contributes to the maintenance of early pregnancy by directly stimulating progesterone secretion by the corpus luteum. In cattle too, Knickerbocker et al (1986) found that treating cycling Holstein cows with conceptus secretory proteins extended the life-span of corpora lutea and inter-oestrus interval. An evaluation of spontaneous prostaglandin response suggested that proteins synthesised and secreted by the bovine conceptus accommodate luteal maintenance during early gestation via an attenuation of prostaglandin production. In sheep and cows, oestrogen, which is luteolytic in the late luteal phase, may indirectly induce uterine prostaglandin synthesis. The role of the embryonic hormone, which the above substance appears to be, may be to counter the lytic action of the prostaglandins. Further details on the subject and the proteins secreted are presented in Roberts and Parker (1974, 1976), Laster (1977), Segerson et al (1984a, 1984b), Thatcher et al (1984) and Roberts et al (1985).

2.2.5 Endocrinology of the postpartum period

The interval between calving and conception depends on the reestablishment of normal ovarian cycles after calving, the occurrence of oestrous behaviour at the appropriate time in the cycle, and the pregnancy rate following service (Peters, 1984).

The interval between parturition and ovulation is characterised by sexual quiescence (postpartum anoestrus). The duration of this interval varies with breed, milk yield level, animal age, suckling or lactating status, nutritional level before and after calving, season and associated photo-periodism, climate, health status and calving difficulty. Of these factors, nutrition and suckling appear to be very important.

Schallenberger et al (1978) noted that pregnancy reduces the sensitivity of the pituitary to GnRH. Sensitivity of the pituitary to GnRH increases only gradually after calving. The resumption of ovarian cyclicity depends on the establishment of a pulsatile pattern of LH secretion (Peters and Lamming, 1983). The observed delay is due, probably, to their being insufficient oestradiol to induce the pre-ovulatory LH surge (Peters et al, 1981). Luteinising hormone

Peters and Lamming (1983) estimated that a pulsatile pattern of LH secretion with a frequency of 0.25 to 1 per hour appears to be a prerequisite for the first ovulation postpartum. This results in gradually increasing LH concentration before the first LH surge. Hansel and Alila (1984) stated that the frequency of LH pulses is due to increased frequency of pulsatile releases of GnRH and that the factors affecting the duration of postpartum anoestrus also affect the time taken to establish the pulsatile pattern of LH release. Peters and Lamming (1986) thought that changes in gonadotrophin concentration may be brought about by changes in pituitary responsiveness to GnRH during the postpartum period. The time at which the pulsatile releases of LH appear and pituitary sensitivity increases varies among breeds and is also affected by suckling.

The exact mechanisms by which suckling interferes with the hypothalamus-hypophyseal axis are not well defined (Hanzen, 1986), but it is unlikely that teat manipulation alone can alter LH release patterns in the cow (Williams et al, 1984). Other factors, such as the presence of the calf or social interactions, might be necessary before teat stimulation has an effect (Hanzen, 1986). Suckling reduces the frequency and amplitude of LH release (Carruthers and Hafs, 1980), pituitary sensitivity to GnRH (Carruthers et al, 1978) and the pulsatile release of GnRH by the hypothalamus (Carruthers et al, 1980).

Suckling probably inhibits LH and GnRH release and their action rather than their synthesis (Hanzen, 1986). Both the hypothalamic concentration of GnRH and the pituitary concentration of LH are similar in milked and suckled cows (Carruthers et al, 1978; Saiduddin et al, 1967; Graves et al, 1968). Suckling can also inhibit the positive effect of endogenous or exogenous oestradiol on the release of pituitary LH (Short et al, 1979, Stevenson et al, 1983). Temporary weaning (48 hours) may, however, increase plasma LH concentration (Walters et al, 1982a), but LH concentration falls to previous levels within 4 hours of calf return (Walters et al, 1982b). Suckling delays LH release and reduces the amount of LH released in response to GnRH injection. Temporary calf removal can, however, enhance the total amount of LH released in response to GnRH injection (Dunn et al, 1985).

Using suckled beef cows, Rawlings et al (1980) noted that the maximum magnitude and frequency of LH peaks occurred 10 to 33 days before the initial increase of plasma progesterone, i.e. when there was a marked development of large follicles and a large variation in oestradiol-17 b. These and similar observations have led researchers to (i) suggest that there might be a deficiency of GnRH during the early postpartum period and (ii) attempt to stimulate ovulation and ovarian cycles by repeated injections of GnRH (Peters and Lamming, 1986) in order to simulate the events of the preovulatory period. Results from GnRH treatment have been inconsistent (Riley et al, 1981; Walters et al, 1982b; Edwards et al, 1983). The number of animals, whether milked or suckled, that respond positively to GnRH injection increases during the postpartum period. Suckled cows are, however, less likely to respond during the initial 15 days than milked cows. This difference diminishes during subsequent weeks (Hanzen, 1986). After this period the magnitude of GnRH-induced LH release appears to be directly proportional to follicular development (Smith et al, 1983). Oestradiol

It is difficult to monitor pulses of oestradiol-17 b in the jugular vein. Peters and Lamming (1986) reported unpublished work in which cannulae were inserted into the posterior vena cava, anterior to the junction of the ovarian veins, of recently calved cows. Oestradiol-17 b pulses, both naturally occurring and induced by three-hourly injections of GnRH, were detected: similar pulses could not be registered in the jugular vein, although increases in oestradiol concentration were measured. It was concluded that the early postpartum cow is sensitive to GnRH-induced gonadotrophin release and responds by secreting oestradiol. Progesterone

Two types of luteal activity have been observed in the postpartum cow. Fifty to 80% of milked or suckled dairy cows exhibit an initial luteal phase in which increases in plasma progesterone concentration are of shorter duration and progesterone concentrations are lower than in the normal cycle (Donaldson et al, 1970; Schams et al, 1978; Odde et al, 1980; Peters and Riley, 1982). This is referred to as the short luteal phase and lasts 6 to 12 days. The second luteal phase lasts about 14 days and tends to have lower than normal progesterone levels. This dual progesterone pattern occurs even after GnRH challenge (Fonseca et al, 1980), early weaning (Odde et al, 1980) or limited suckling with (Dunn et al, 1985) or without GnRH treatment (Flood et al, 1979).

Tribble et al (1973) thought that progesterone can be released from follicles that fail to ovulate. Schams et al (1978) felt that short progesterone cycles are caused by shortage of LH or its receptor. However, no differences in plasma LH concentration are observed before or after an oestrus associated with a short cycle (Ramirez-Godinez et al, 1982a). Troxel et al (1980) suggested that the short life-span may be the result of short GnRH-induced LH surges. Alternatively, the amount of LH receptor and number of granulosa cells may not be sufficient to give optimum response to this luteotrophic stimulus (Channing et al, 1981).

Hanzen (1986) pointed out that the corpus luteum from a cow with regular cycles responds positively in vitro to LH addition but that a corpus luteum formed after GnRH injection does not. This may be due to luteal tissue of the latter being unable to recognise LH (Kesler et al, 1981). It has been shown that the in vitro response of the postpartum corpus luteum during the first three cycles is related to the integrity of luteal tissue at the time of removal (Duby et al, 1985). These observations suggest that there may be premature luteolysis due to PGF2 a synthesised after calving by caruncular uterine tissue (Troxel and Kesler, 1984). It has been suggested that oxytocin can increase production of PGFM (a metabolite of PGF2 a) and lead to earlier luteolysis (Troxel et al, 1984). Whatever the cause, the abnormal short cycles can result in increased early embryonic mortality (Ramirez-Godinez et al, 1982b; Troxel et al, 1983).

In a study of 20 postpartum pluriparous Brahman cattle, Rutter and Randel (1984) reported a higher incidence of abnormal cycles after the first heat than the second (35% vs 5%; P<0.05). It was suggested that the higher incidence of abnormal luteal function following the first and second postpartum cycles contributes to lower conception rates in animals bred during the first or second postpartum heat.

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