Fortune J E, Sirois J, Turzillo A M, Lavoir M
Department and Section of Physiology, College of Veterinary Medicine, Cornell University, Ithaca, New York 14853.
J Reprod Fertil Suppl. 1991;43:187-98.
Ultrasound imaging has shown that cattle exhibit 2 or 3 waves of follicular development during an oestrous cycle. The waves consist of the contemporaneous appearance, about every 7 days, of a group of follicles > or = 5 mm in diameter. One follicle gradually becomes larger than the rest (i.e. dominant). There are several lines of evidence suggesting that the waves occur regularly under conditions of basal LH and FSH. (1) Cycles with 3 waves of follicular development are longer and have longer luteal phases than do cycles with 2 waves, indicating that the number of waves in a cycle is determined by the time of luteal regression. (2) Cycles with 4 or 5 waves of follicular development can occur when the luteal phase is artificially prolonged with exogenous progesterone. (3) Waves of follicular development occur during pregnancy. However, the secondary surge of FSH may be important in initiating new follicular recruitment after ovulation, since suppression of the secondary surge delays the first wave of follicular development. Follicles are functionally dominant (capable of ovulating after luteal regression) while they are still growing and early during their plateau in growth. Functional dominance is lost some time between the early and late plateau phases, while the follicle is still morphologically dominant (i.e. the largest follicle). The factors that lead to dominance of one follicle and the mechanisms that suppress the growth of subordinate follicles are not well understood. When the luteal phase is artificially extended with low doses of exogenous progesterone, the normal pattern of follicular development is altered and the ovulatory follicle grows for a prolonged period of time. This finding indicates that subtle changes in the hormonal milieu can dramatically alter follicular dynamics and that the experimental model of prolonged dominance may be useful in studying the mechanisms of follicular dominance. In contrast, patterns of follicular development in sheep must be assessed in more indirect ways, but sheep offer the advantage of breeds that differ in ovulation rate. Correlation of the endocrine environment with ovulation rate in this species provides a valuable approach to understanding the mechanisms controlling follicle selection and ovulation rate. It has been suggested that in some species a high ovulation rate is achieved by increased recruitment, whereas in others there is increased selection. There is evidence that oestradiol is involved in regulating the number of dominant follicles in sheep. Follicular recruitment requires the presence of gonadotrophins, particularly FSH. In general, the mechanisms that regulate follicular selection and dominance in domestic ruminants are not well understood. Further experiments may determine the relative roles of paracrine factors and ovarian-pituitary-hypothalamic interactions in regulation of follicular selection and dominance in cattle and sheep.
超声成像显示，母牛在发情周期中会出现2或3个卵泡发育波。这些波由一组直径≥5毫米的卵泡大约每隔7天同时出现组成。其中一个卵泡会逐渐比其他卵泡更大（即优势卵泡）。有几条证据表明，在基础促黄体生成素（LH）和促卵泡生成素（FSH）的条件下，卵泡发育波会有规律地出现。（1）卵泡发育有3个波的周期比有2个波的周期更长，黄体期也更长，这表明一个周期中卵泡发育波的数量由黄体退化时间决定。（2）当用外源性孕酮人为延长黄体期时，会出现卵泡发育有4或5个波的周期。（3）卵泡发育波在怀孕期也会出现。然而，FSH的第二次高峰在排卵后启动新的卵泡募集过程中可能很重要，因为抑制第二次高峰会延迟卵泡发育的第一个波。卵泡在仍在生长以及生长平台期早期时在功能上具有优势（在黄体退化后能够排卵）。在生长平台期早期和晚期之间的某个时间，卵泡在功能上的优势丧失，而此时卵泡在形态上仍占优势（即最大的卵泡）。导致一个卵泡占优势的因素以及抑制从属卵泡生长的机制尚不清楚。当用低剂量外源性孕酮人为延长黄体期时，卵泡发育的正常模式会改变，排卵卵泡会延长生长一段时间。这一发现表明，激素环境的细微变化会显著改变卵泡动态，并且优势延长的实验模型可能有助于研究卵泡优势的机制。相比之下，绵羊的卵泡发育模式必须通过更间接的方式进行评估，但绵羊具有排卵率不同的品种这一优势。该物种内分泌环境与排卵率的相关性为理解控制卵泡选择和排卵率的机制提供了一种有价值的方法。有人提出，在某些物种中，高排卵率是通过增加募集实现的，而在其他物种中则是通过增加选择实现的。有证据表明，雌二醇参与调节绵羊优势卵泡的数量。卵泡募集需要促性腺激素的存在，尤其是FSH。一般来说，调节家养反刍动物卵泡选择和优势的机制尚不清楚。进一步的实验可能会确定旁分泌因子以及卵巢 - 垂体 - 下丘脑相互作用在调节牛和绵羊卵泡选择和优势方面的相对作用。
