Sharjah Equine Hospital, Sharjah, United Arab Emirates.
Robinson College, University of Cambridge, Cambridge, UK.
Adv Anat Embryol Cell Biol. 2021;234:181-204. doi: 10.1007/978-3-030-77360-1_9.
The female elephant shows a 3-week "follicular phase" to commence her 16-week estrous cycle at the end of which a second surge in pituitary luteinizing hormone (LH) release matures and ovulates an ovarian follicle in association with estrous behavior and mating, whereas the first LH surge at the start of the follicular phase causes luteinization of 3-5 partially developed follicles. The prolonged pregnancy of 22 months is supported by a zonary endotheliochorial placenta which secretes placental lactogen (ePL) from around 40 days of gestation in association with replacement of the lumenal epithelium of the endometrium by trophoblast and the development of large corpora lutea (CLs) in the maternal ovaries from the previously formed luteinized follicles in response to the first LH peak early in the follicular phase. The zonary placenta develops above, rather than within, the endometrium. The elephant placenta secretes neither estrogens nor progestagens throughout gestation, as pregnancy maintenance relies on 5α-dihyroprogesterone and other 5α reduced progestagens secreted by secondary CLs stimulated by ePL and the stromal tissue of the fetal gonads, which become extremely enlarged during the second half of the 22-month pregnancy. In female fetuses, this ovarian enlargement includes the development and subsequent regression of multiple primary and secondary follicles with a consequent substantial decline in primary follicle numbers at birth. During the next 8-9 years of pre-pubertal life, however, oocyte and primary follicle numbers recover to levels near those found in late gestation, which may be evidence of postnatal oogenesis occurring in the elephant.
母象经历一个为期 3 周的“卵泡期”，随后进入为期 16 周的发情周期，在发情周期结束时，垂体促黄体生成素（LH）再次大量分泌，促使一个成熟的卵泡排卵，同时伴随着发情行为和交配，而在卵泡期开始时的第一次 LH 激增则导致 3-5 个部分发育的卵泡黄体化。22 个月的妊娠期较长，由一层带内皮的绒毛膜胎盘支持，该胎盘从妊娠约 40 天开始分泌胎盘催乳素（ePL），同时子宫内膜腔上皮被滋养层取代，母体卵巢中之前形成的黄体化卵泡发育成大黄体（CLs），以应对卵泡期早期的第一次 LH 高峰。带层胎盘在子宫内膜之上而不是之内发育。象胎盘在整个妊娠期都不分泌雌激素或孕激素，因为妊娠的维持依赖于 5α-二氢孕酮和其他由 ePL 和胎儿性腺基质组织刺激的次级 CL 分泌的 5α 还原孕激素，这些孕激素在 22 个月妊娠的后半期变得极度增大。在雌性胎儿中，这种卵巢增大包括多个原始和次级卵泡的发育和随后的退化，导致出生时原始卵泡数量的大幅减少。然而，在接下来的 8-9 年的青春期前生命中，卵母细胞和原始卵泡数量恢复到接近妊娠晚期的水平，这可能是大象产后发生卵发生的证据。
