Progenitor cells that eventually form into the penis or clitoris originate in compartmentalised regions that are brought together as the embryo develops, a study of chick embryos in Scientific Reports finds. Identification of the precise origins of these cells may provide insights into the events that cause genital defects to develop.
Although some of the processes of external genital development are beginning to be identified, the origin of cells that produce external genitalia has been elusive. By mapping the fate of cells in a developing chick embryo, Ana Herrera and Martin Cohn identify the progenitor cells that contribute to the formation of a structure called the genital tubercle, the precursor of the penis and clitoris. They find that the genital tubercle arises from two populations of cells on the left and right side of the embryo, located near ‘buds’ that give rise to hind limbs and the tail. These pools of cells are brought together during body wall closure, an event that ‘zips’ together the left and right sides of the embryo, transforming it from a flat sheet to a three-dimensional tube. Incomplete closure of the body wall results in birth defects; thus the authors conclude that the discovery that genital tubercle origins lie in two compartmentalised sets of cells helps to explain why genital defects are often associated with disruption of body wall closure. The authors also suggest that failure to fuse the left and right genital progenitor pools might explain how some animals, such as snakes and lizards, evolved paired penises.