Abstract
Viviparity in teleosts involves, invariably, the ovary in a gestational role. This type of viviparity is due to the combination of unique aspects, different from those found in the rest of vertebrates. These aspects are: The ovary has a saccular structure; the germinal epithelium lines the ovarian lumen; the absence of oviducts; and the intraovarian insemination, fertilization, and gestation. The communication of the germinal zone of the ovary to the exterior is via the caudal zone of the ovary—the gonoduct. The germinal epithelium is composed of oogonia and oocytes scattered individually or in cell nests among somatic epithelial cells. In the ovarian stroma the follicles are included which are formed by the oocyte, which is surrounded by follicular cells and the vascularized theca. The oogenesis comprises three stages: chromatin-nucleolus, previtellogenesis, and vitellogenesis. There is no ovulation, as the oocyte is retained in the follicle. During the insemination, the spermatozoa enter into the ovarian lumen and the intrafollicular fertilization occurs, followed by intrafollicular gestation. The intraovarian gestation of poeciliids involves morphological characteristics associated with the intrafollicular embryogenesis and types of nutrition, such as lecithotrophy and matrotrophy. In lecithotrophy, the nutrients come from the yolk reserves stored during oogenesis, whereas in matrotrophy the nutrients are provided by supplies from maternal tissues to the embryo during gestation. The maternal–embryonic metabolic interchanges converge through the development of the association of maternal and embryonic blood vessels, establishing a follicular placenta.
Highlights
Ovarian Adaptations of Viviparous TeleostsAmong more than 32,672 species of teleost fishes [1], approximately 510 are viviparous [2,3,4]
Viviparity in teleosts involves, invariably, the ovary in a gestational role
During the adult life of non-teleost vertebrates, the ovarian cortex, which is lined by the germinal epithelium with oogonia for oogenesis, remains in contact with the coelomic cavity where ovulation occurs
Summary
Among more than 32,672 species of teleost fishes [1], approximately 510 are viviparous [2,3,4]. A distinctive feature of almost all viviparous teleosts occurs when, during the early embryonic development, the right and left ovaries fuse, forming a single ovary [8,11,12] This fusion establishes a single and saccular ovary, with the germinal epithelium lining the internal lumen. In accordance with the amount of yolk deposited in the oocyte during oogenesis, the nutrition of the embryo requires the transfer of nutrients from the maternal tissue, developing different levels of matrotrophy: incipient, middle, or high [19] The understanding of this complex process of nutrition during gestation requires the analysis of the specialized structures related with this process. As it is considered by Blackburn and Starck [20], morphology has a central role in helping to explain the function and evolution of patterns of fetal nutrition in viviparous teleosts
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