Background: In viviparous fish, a considerable degree of variation in placental structures have been described. However, no distinct structures are reported in Scorpaenidae.
Results: In this study, we demonstrate a new type of folliculogenesis and follicular placentation in Sebastes schlegelii. Before copulation, the germinal epithelium gradually surrounds the oocytes and develops to individually follicles with a stalk-like structure hanging on the ovigerous lamella, which ensures each follicle have access to spermatozoa after copulation. From stage V to early gestation, the cyp17-I highly expressed accompanied by cyp19a1a signals disappearance, and 11-ketotestosterone level keeps rising and peaks at blastula stage, while 17β-estradiol declines to the bottom. Meanwhile, the theca cells rapidly proliferate and invade outwards forming a highly hypertrophied and folded microvillous placenta. This unbalance of hormone might be an important factor driving the theca cells proliferation and invasion. Additionally, some conserved genes related to mammalian placentation are significantly high expression in follicular placenta suggesting the high convergence in vertebrate placenta evolution.
Conclusions: This finding provided a new type of placentation pattern for viviparous teleost between the intrafollicular gestation and intraluminal gestation.