2 and 5 sex cordsA depiction of the migration of the cells which will give rise to the sex cords into the genital ridge where they will become the gonads of the embryo
Sex cords are embryonic structures which eventually will give rise (differentiate) to the adult gonads (reproductive organs).[1] They are formed from the genital ridges - which will develop into the gonads - in the first 2 months of gestation (embryonic development) which depending on the sex of the embryo will give rise to male or female sex cords.[2] These epithelial cells (from the genital ridges) penetrate and invade the underlying mesenchyme to form the primitive sex cords.[3] This occurs shortly before and during the arrival of the primordial germ cells (PGCs) to the paired genital ridges.[3] If there is a Y chromosome present, testicular cords will develop via the Sry gene (on the Y chromosome): repressing the female sex cord genes and activating the male.[4][5] If there is no Y chromosome present the opposite will occur, developing ovarian cords.[6][7] Prior to giving rise to sex cords, both XX and XY embryos have Müllerian ducts and Wolffian ducts.[2] One of these structures will be repressed to induce the other to further differentiate into the external genitalia.[2]