Sexual Differentiation- Reproduction I PDF

Sexual Differentiation 1  Learning Objectives Understand the current concept of gonadal differentiation Sexual differentiation of the brain 2  What is Sex? Need for 2 individuals: produce germ cell...

Sexual Differentiation 1  Learning Objectives Understand the current concept of gonadal differentiation Sexual differentiation of the brain 2  What is Sex? Need for 2 individuals: produce germ cells which fuse to produce an offspring Difference (sexual dimorphism) carried by genes = sex chromosomes (mammals: XX=F, XY=M; birds: ZW=F, ZZ=M). During early fetal life, gonads are undifferentiated 3  Embryonic Sex Differentiation Formation of primordial germ cells (P.G.C.’s) – Extra-embryonic origin – Circulate in vasculature, migrate through tissues Colonize the genital ridge to form indifferent gonads Differentiation of cortex into ovary or medulla into testis – testis organization through transient expression of genes (testes differentiation factor - TDF) from the SRY (Sex-determining region Y) 4 Fig. 4-4 5 (mesonephric duct) (paramesonephric duct) 6 Leydig cells Sertoli cells (Mullerian duct) (Wolffian duct) 7  Development of the testes FYI formed from the deep part of the sex cords connects to the rete tubules form testicular chords with PGCs 8 FYI PGCs  spermatogonia Coelomic epithelium  sertoli cells (produce MIS) Mesonephric mesenchyme  connective tissue (tunica albuginae), blood vessels, leydig cells (produce testosterone) Testosterone stimulates the development of the mesonephric 9  Development of the ovary FYI forming a rudimentary rete Coelomic epithelium will form the cortex Second generation of cord epithelium form primordial follicle Mesonephric tubules regress to form a rudimentary rete 10  Absence of MIS: development of paramesonephric duct Absence of testosterone: regression of mesonephric duct FYI Development of the female tract 11  Sex Differentiation of the Brain Hypothalamus has 2 centers with GnRH neurons (Tonic center: responsible for basal secretion; Surge center: responsible for ovulatory cycles and female behaviour) During perinatal period: – Estradiol released by placenta and embryonic adrenals binds to a carrier in blood and does not enter the brain – Testosterone released by embryo testes diffuses into the brain where it is converted to estradiol – Presence of estradiol in the brain inactivates the surge center = male brain For review: Morris JA et al. (2004). Sexual differentiation of the vertebrate nervous system. Nature Neuroscience 7:1034-1039. 12 13