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.

In viviparous teleosts, the lack of oviducts defines intraovarian gestation, with the ovary being the site for oogenesis but also the site for insemination, fertilization, and gestation. Consequently, intraovarian gestation is a complex and exceptional type of reproduction among vertebrates. The analysis of the morphological and physiological components of intraovarian gestation documents the evolutionary process of nutrition in viviparous species. Two types of embryonic nutrition may occur during gestation: (a) lecithotrophy, when most nutrients for the embryo come from the abundant yolk stored during oogenesis, and (b) matrotrophy, when nutrients for the embryo with scarce yolk must be obtained during gestation by additional maternal provisioning, developing a placenta. Then, investment of maternal nutrients for the embryo is greater during oogenesis in lecithotrophic species, and investment of maternal resources for the nutrition of the embryo is greater during gestation in matrotrophic species. Microscopic techniques allow for proper observation of maternal and embryonic structures involved in both types of nutrition during the development of embryos. Specifically, we focused on the morphology of placental structures associated with embryonic nutrition at different stages of development, which are the yolk sac and the pericardial sac. The oocytes of Poecilia latipinna contain a large amount of yolk (an average oocyte diameter of 1.9 mm); in contrast, the oocytes of Heterandria formosa contain extremely reduced amounts of yolk (an average oocyte diameter of 0.4 mm). Therefore, these species represent appropriate models for studying the strategy of two different types of embryonic nutrition, lecithotrophy and matrotrophy, in viviparous teleosts.