Follicular dynamics in adult cows

The time required for ovarian follicles to grow from the primordial and preantral stages to ovulation is several months and several weeks, respectively (LUSSIER et al. 1987). Results of extensive real-time ultrasonographic studies have documented that follicular growth during the bovine estrous cycle is characterized by two or three follicular waves [Fig. 6] (SAVIO et al. 1988; SIROIS and FORTUNE 1988). Each follicular wave consists of three phases: a) recruitment, b) selection and c) dominance [Fig. 7] (DRIANCOURT 2001). Recruitment is defined as the initiation of gonadotropin-dependent folliculogenesis of a cohort of healthy follicles. Wave emergence is also used to describe this process (GINTHER et al. 1996). During the mid-follicular phase, at the time of selection, a dominant follicle appears while the other follicles of the cohort regress by atresia. No recruitment occurs during the dominance phase (LUCY et al. 1992; DRIANCOURT 2001). Onset of the first follicular wave is described as the sudden ultrasound-detectable emergence of a cohort of small follicles from 3 mm just before the day of ovulation (GINTHER et al.

1996). GINTHER et al. (1997) examined Holstein heifers with ultrasound and

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detected a mean of 24 growing follicles ≥3 mm at the emergence of the wave. Each follicular wave was preceded by a FSH surge (ADAMS et al. 1992). Appropriate in vivo models to investigate the actual FSH requirements of antral follicles after completion of first growth phase, are those in which endogenous FSH rises are abolished using GnRH immunization or long-term GnRH agonist treatment. Such studies showed that development from 4 mm essentially required a rise in FSH from baseline levels (CROWE et al. 2001; GONG et al. 1996; PRENDIVILLE et al. 1995).

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FOLLICULAR DYNAMICS DURING THE ESTROUS CYCLE IN CATTLE

DAYS OF THE ESTROUS CYCLE

FOLLICULAR DYNAMICS DURING THE ESTROUS CYCLE IN CATTLE

DAYS OF THE ESTROUS CYCLE

CLASS 4 LUTEOLYSIS

Fig: 6. Follicular dynamics during of estrous cycle in cattle. Reproduced from LUCY et al. 1992.

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Fig: 7. Main events that occur during a follicular wave.

At recruitment, a cohort of follicles begins its final growth phase as it enters gonadotropin-dependent folliculogenesis. At selection, one follicle is selected. The ovulatory follicle becomes dominant and the other follicles of the cohort regress by atresia. Reproduced from DRIANCOURT 2001.

A histological study, of follicles from cows injected with follicular fluid, indicated that the resulting depressed FSH levels did not diminish the number of follicles < 4 mm, suggesting that these follicles did not require elevated FSH (LUSSIER et al. 1994).

However, expression of FSHr mRNA was confirmed in primary follicles with only one to two layers of granulosa cells, presumably before formation of the theca interna and the basement membrane (BAO and GARVERICK 1998). This could suggest that FSH stimulates granulosa cell proliferation and follicular development at early stages of growth.

In animals with two waves, growing follicles are detected on days -0.2 and 9.6 (ovulation is day 0) and in animals with three waves on days -0.5, 9 and 16 of estrous cycle, respectively (SAVIO et al. 1988). Selection is the process by which a single follicle is chosen from a group of medium-sized growing follicles for further

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when the future dominant follicle reaches a diameter of 8.5 mm (GINTHER et al.

1997). This involves several aspects: First, there is a final decline in the FSH surge followed by maintenance of FSH at basal levels. This induces atresia of the subordinate follicles and delays the onset of the next follicular wave. Concentrations of FSH remain low for the next 3 or 4 days until the increase associated with the next surge (GINTHER et al. 1996). Second, an increase of LH concentration and mRNA expression of LH receptors (LHr) as well as 3β-hydroxysteroid dehydrogenase (3β-HSD) in the granulosa cells which triggers a greater responsiveness to LH of the selected follicle and conversion of pregnenolone to progesterone which could be utilized by thecal cells to produce androgen (BAO et al. 1997; BERGFELT et al.

2000). An increase in mRNA expression of cytochrome P450 side-chain-cleavage (P450scc) and cytochrome P450-aromatase (P450arom) has been observed in theca and granulosa cells (BAO and GARVERICK 1998). These enzymes are involved in the increasing amounts of 17β estradiol. Third, an enhanced capacity of the selected follicle to produce estradiol and inhibin precursors and to maintain low amounts of IGFBP-2, -4 and -5 allows an increase in levels of free IGF-I in follicular fluid (AUSTIN et al. 2001; GINTHER et al. 2003b). Following selection, follicles continue to gain size. The estrogen activity and lifespan of the dominant follicle are controlled by the LH pulse pattern. The other follicles cease growing and undergo atresia (ADAMS et al. 1992). The increased estradiol and inhibin production leads to a decline in FSH concentration which avoids new follicular recruitment (EVANS et al.

1997; KANEKO et al. 2002). However, basal FSH concentrations are essential to support survival of the first DF of the cycle. Suppression of FSH below normal basal levels using bovine follicular fluid or estradiol predisposes the DF to atresia (ADAMS et al. 1992; BERGFELT et al. 2000; GINTHER et al. 2000). Results of a recent study suggest that subtle differences in the blood concentrations of Inhibin A and FSH during nonovulatory and ovulatory follicular waves underlie the mechanisms regulating the number of waves of ovarian follicle development that occur during the bovine estrous cycle (PARKER et al. 2003). The putative first DF emerges approximately 6 h earlier than the first largest subordinate follicle and increases in size daily until a maximum diameter is reached on day 6-8 of the estrous cycle. It

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remains stable in size for 2-4 days (days 7-10 of the estrous cycle) and then gradually declines (GINTHER et al. 2001b; ROCHE and BOLAND 1991). The fate of dominant follicles that reach maximum size during the luteal phase of the estrous cycle and of subordinate follicles that enter as cohorts during the initiation of each wave of follicular growth is atresia. If luteal regression occurs during the growing phase of the dominant follicle, the follicle ovulates (KO et al. 1991).