Sexual dimorphism in the neonatal gonad

The neonatal gonad has two distinct forms (i.e., is sexually dimorphic), as judged by morphological and endocrine characteristics. The dimorphic proce ss begins early in embryogenesis. It is well established by the time of bir th, by which time the genital ridge has developed into either a testis or a n ovary. The mechanisms involved in sex determination involve the Y chromos ome, autosomal genes, transcription factors and possibly other unidentified control networks. This review paper describes the morphological changes th at occur and the endocrine functions in the developing gonads. It highlight s a number of important differences in fetal and neonatal gonadal function. The testis has early histological definition, several determining genes, d elayed germ cell maturation, early autonomous steroid secretion, luteinizin g hormone (LH) receptor and steroid enzyme expression, high fetal testicula r testosterone content, prominent postnatal Leydig and Sertoli cells and hi gh postnatal serum testosterone levels. The ovary has a prolonged monomorph ic state, probably one determining gene, germ cells in early meiotic arrest , delayed expression of LH receptor and aromatase, low ovarian oestradiol c ontent, prominent postnatal follicles and low postnatal serum oestradiol le vels.