Comparative Histological Investigation of the Ovarian and Placenta Structure in the Viviparous Fish Genus Poeciliopsis

Schimmel/Conrades Science Center Atrium

Across the fish genus Poeciliopsis there are varying degrees of placentation, which is determined by the amount of continued maternal nourishment provided to the embryo after fertilization. The degree of placentation can be studied using a morphological approach to examine the placenta structures involved in maternal-fetal nutrient exchange across species. Features used for comparison include the maternal follicle that surrounds the developing embryo, the amount of yolk present in the yolk sac of embryos, embryo structures potentially utilized for nutrient absorption, and sperm storage structures. Recent comparative morphological studies in Poeciliopsis have used scanning electron microscopy techniques, finding surface features of the maternal follicle and embryos that vary across species. However, there has not been a comprehensive comparative histological study of the Poeciliopsis gestating ovary. We present the first stages of a histological analysis that compares the gestating ovaries from four different species of Poeciliopsis representing placentation variation. Histological slides were previously prepared from fixed, embedded, sectioned and hematoxylin and eosin stained gestating ovaries of Poeciliopsis gracilis, P. infans, P. prolifica, and P. turneri. Slides were observed at 40x under light microscopy and digital images were captured using the program Infinity Analyze. We are in the process of creating an extensive digitized collection and panoramic images of entire ovaries. With these panoramas we have begun to grasp a clearer picture of the overall features of the ovary, embryos, maternal tissues and cellular features. We are currently using these images to determine which ovarian and placenta structures to compare across species. Once features are identified throughout the ovary, future microscopy will compare the structures at higher magnification across species, work which will enhance our understanding of the evolution of placentation in Poeciliopsis.