Laboratorio de Biología de la Reproducción, Departamento de Biología Comparada, Facultad de Ciencias, Universidad Nacional Autónoma de México, Mexico City, Mexico.
Department of Biology and Electron Microscopy Center, Trinity College, Hartford, Connecticut, USA.
J Morphol. 2021 Nov;282(11):1575-1586. doi: 10.1002/jmor.21407. Epub 2021 Aug 10.
In viviparous Mexican fishes of the family Goodeidae, embryos develop in the maternal ovarian lumen. They typically absorb maternal nutrients during gestation by means of "trophotaeniae," that is, specialized, elongated extensions of the hindgut that are exposed to the fluids, which occupy the ovarian lumen. The sole exception is Ataeniobius toweri, whose embryos lack trophotaeniae but are nevertheless matrotrophic. Thus, how its embryos obtain maternal nutrients is unclear. We studied a series of non-pregnant and pregnant ovaries of A. toweri using histology to identify the mechanism of maternal-embryo nutrient transfer. By early-gestation, embryos have depleted their yolk supplies. Yolks are released into the ovarian lumen and are ingested by the developing embryos, as shown by yolk material in their digestive tracts. The embryonic gut is lined by an epithelium consisting of columnar cells with apical microvilli, providing a means for nutrient absorption. Contrary to statements in the literature, embryos develop minuscule trophotaenial rudiments that extend slightly into the ovarian lumen. These structures are formed of an absorptive epithelium that overlies a vascular stroma, similar to the trophotaeniae of other goodeids. Through late gestation, vitellogenic follicles form and oocytes are discharged into the ovarian lumen, contributing to embryonic nutrition. Thus, histological evidence suggests that embryos chiefly obtain nutrients from ingestion of yolk and maternal secretions released into the ovarian lumen. This function possibly is supplemented by uptake via the small hindgut protrusions and other absorptive surfaces (e.g., the skin and the gill epithelium). Our observations are consistent with two evolutionary interpretations of the hindgut protrusions: (a) that they are rudimentary, evolutionary precursors of trophotaeniae formed by exteriorized hindgut; and (b) that they are vestigial remnants of trophotaeniae that were lost during a switch to a form of matrotrophy involving nutrient ingestion.
在胎生的墨西哥胎鱂科鱼类中，胚胎在母体的卵巢腔中发育。它们通常通过“滋养体”吸收母体的营养物质，即延伸到暴露于充满卵巢腔的液体中的后肠的专门的延长部分。唯一的例外是 Ataeniobius toweri，它的胚胎没有滋养体，但仍然是母源营养的。因此，其胚胎如何获得母体营养物质尚不清楚。我们使用组织学研究了一系列非怀孕和怀孕的 A. toweri 卵巢，以确定母体-胚胎营养物质转移的机制。在早期妊娠时，胚胎已经耗尽了卵黄供应。卵黄被释放到卵巢腔中，并被正在发育的胚胎吸收，因为它们的消化道中存在卵黄物质。胚胎肠道由一层柱状细胞组成，细胞顶端有微绒毛，为营养吸收提供了一种途径。与文献中的说法相反，胚胎发育出微小的滋养体雏形，略微延伸到卵巢腔中。这些结构由覆盖在血管基质上的吸收上皮组成，类似于其他胎鱂科的滋养体。在晚期妊娠期间，形成卵黄生成滤泡，卵母细胞排入卵巢腔，为胚胎提供营养。因此，组织学证据表明，胚胎主要通过摄入卵黄和释放到卵巢腔中的母体分泌物来获取营养。这种功能可能通过小后肠突起和其他吸收表面（例如皮肤和鳃上皮）的摄取来补充。我们的观察结果与后肠突起的两种进化解释一致：（a）它们是原始的，是由外部化后肠形成的滋养体的进化前体；（b）它们是在涉及营养物质摄入的母源营养形式转变过程中失去的滋养体的退化残余物。
