RESUMO
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.
Assuntos
Ciprinodontiformes , Embrião não Mamífero , Animais , Feminino , Nutrientes , Oócitos , Ovário , GravidezRESUMO
Teleosts possess unique features of the female reproductive system compared with the rest of vertebrates, features that define the characteristics of their viviparity. Viviparity involves new maternal-embryonic relationships detailing the most diverse structures during gestation that include embryonic nutrition. In order to analyze the morphological features of the complex nutrition in viviparous teleosts during intraovarian gestation, this study utilizes the goodeid Xenotoca eiseni as a model. Ovarian gestation in X. eiseni, as in all goodeids, is intraluminal; the early embryo moves from the follicle to the ovarian lumen where gestation continues. The scarce yolk in the oocytes implies that the initial lecithotrophy is replaced by matrotrophy, with nutrients provided via maternal tissues. The nutrients are absorbed by the embryo mainly by trophotaenia, extensions of the embryonic intestine into the ovarian lumen. This histological study analyses the structures involved in these two types of nutrition and when they occur during gestation in X. eiseni. The morphology displayed in this study demonstrated the extended simultaneity of lecithotrophy and matrotrophy during gestation with the progressive reduction of lecithotrophy and increase of matrotrophy. Similarly, it describes the development of complex embryonic structures for metabolic exchange with the maternal tissues associated with matrotrophy; specifically the branchial placenta and mainly the trophotaenia.