Genital ridge

Gonadal ridge
Section of the fold in the mesonephros of a chick embryo of the fourth day. ("Genital ridge" labeled at left.)
Details
Precursor	urogenital folds
Gives rise to	sex cords
Identifiers
Latin	crista gonadalis
Code	TE E5.7.1.0.0.0.5
Anatomical terminology []

In embryology, the gonadal ridge (or genital 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 Wnt-4. 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)

In the mouse gonadal primordium, the genital ridge, which forms from intermediate mesoderm, becomes morphologically distinct at E10.5. By E12, sexual differentiation of the gonad is apparent, indicating that genes involved in the formation of the bipotential gonad is expressed before E10.5 and E12. Before E10.5, Dmrt1 is expressed at similar levels in the genital ridges of XX as well as XY embryos. By E12.5 and E13.5, Dmrt1 is expressed differentially as sex specific structures start to form. By E14.5 and E15.5, Dmrt1 expression is maintained in the testis while it has begun to decrease in the ovary. (Raymond 1999)