Animals acquire male or female characteristics and obtain the capability to make their progeny. These processes are led and orchestrated by gonad (testis or ovary). In vertebrates, gonadogenesis starts with the formation of sexually indifferent gonad. Subsequently this indifferent gonad develops into either a testis or an ovary in early embryo. The mechanisms, which regulate testicular or ovarian differentiation, have been investigated extensively. By contrast, the mechinery underlying the trigger of gonadogeneis itself remains to be elucidated.
We addressed this longstanding question by pinpoint gene-manipulation in chicken embryos. We observed an early embryonic tissue (Lateral Plate Mesoderm: LPM), which contains gonadal progenitors, and found that a part of cells shows characteristic behaviors (ingression) for the onset of gonadgenesis. This ingression and subsequent processes for the formation of indifferent gonad seems to be caused by the interaction with a gut precursor. Gut precursor seems to secret Sonic hedgehog (Shh) and Shh triggers and orchestrates the onset gonadogenesis in LPM cells by BMP4 expression.
Gonad plays central roles in the sexual differentiation and germ cell development. It is expected that our finding opens a way to create a novel technology for the treatment of infertility or disorder of sex development.
Fig. 1. In early embryos, gonadogenesis seems to be triggered by an interaction with a gut precursor. Sonic hedgehog (SHH), secreted from gut precursor, causes BMP4 expression and onset of gonadogenesis in a part of lateral plate mesodermal cells.
Fig. 2. Transverse view of gonad and kidney in chicken embryos (left) and its magnification (right). When cells, which locate next to gonad and cover kidney (ki), are activated by sonic hedgehog (green), they become gonadal cells(red) and form gonadal structure (arrow heads).