In the chick embryo, estrogen can induce chromosomally male ZZ left gonad epithelial cells to form an ovarian cortex which supports oogenesis

In chicken, as in most amniotes, the first morphogenetic event of ovary differentiation is the formation of two distinct domains: a steroidogenic core, the medulla, overlain by the germ cell niche, the cortex. This process normally starts immediately after sex determination in the ZW embryos, substantially before the progression of germ cells into meiosis. In order to address the extent to which the cortical domain depends on intrinsic or extrinsic factors, we generated models of gonadal intersex by mixing ZW (female) and ZZ (male) cells in gonad chimera, or by altering estrogen levels of ZZ and ZW embryos in ovo. We found that both female and male cells can become part of the cortical domain and that this can form relatively independently of the sex of the medulla as long as estrogen is provided. We also show that the cortex promoting activity of estrogen signalling is mediated via Estrogen Receptor alpha within the left gonad epithelium. Either a ZW or ZZ cortical domain thus provides an adequate niche which allows the germ cells to progress into meiosis. However, the presence of a medulla with an intersex or male phenotype may compromise this progression, causing cortical germ cells to remain in an immature state in the embryo. This makes meiotic entry a checkpoint for a coordinated sexual differentiation along the female pathway of cortex and medulla.