Gonadal ridge

In embryology, the gonadal ridge (or urogenital ridge) is the precursor to the gonads. The gonadal ridge initially consists mainly of mesenchyme and cells of underlying mesonephric origin. Once oogonia enter this area they attempt to associate with these somatic cells. Development proceeds and the oogonia become fully surrounded by a layer of cells (pre-granulosa cells). In embryology, the gonadal ridge (or urogenital ridge) is the precursor to the gonads. The gonadal ridge initially consists mainly of mesenchyme and cells of underlying mesonephric origin. Once oogonia enter this area they attempt to associate with these somatic cells. Development proceeds and the oogonia become fully surrounded by a layer of cells (pre-granulosa cells). The gonadal ridge appears at approximately five weeks, and gives rise to the sex cords. Genes associated with the developing gonad can be categorized into those that form the sexually indifferent gonad, those that determine whether the indifferent gonad will differentiate as male or female, and those that promote differentiation into male or female parts. Genes that form the sexually indifferent gonad are SF1 and WT1. Genes that determine sex are SRY, SOX9, and DAX1. Genes driving the differentiation into male or female structures are SF1, WT1, and WNT4. SRY is the only gene expressed solely in the developing gonad. The other genes have roles in development that aren’t exclusively sex-related. (Raymond 1999) Evidence suggests that a DM domain gene, DMRT1, is involved in sexual development. This gene is located on chromosome 9. Its location suggests that it’s required for the development of testis. XY humans hemizygous for the chromosome 9p, where DMRT1 is located, are often feminized. This feminization can range from ambiguous genitalia to XY sex reversal.The DMRT1 homolog from chicken has been localized on the Z chromosome. Birds have heterogametic females (ZW) and homogametic males (ZZ). The avian Z chromosome is conserved synteny with chromosome 9 of humans. ZZ embryos have a higher dose of DMRT1 and therefore have the potential to have a higher expression. It’s been suggested that embryos with a higher expression of Dmrt1 expression develop into males while embryos with a lower expression are led to female development. (Raymond 1999)