GONADAL DEVELOPMENT

1 14-1 RECOMMENDED READING: Larsen, Human Embryology, 3rd Edition, pp : The male and female reproductive tracts are derived from the same embryonic/fetal tissue. The gonads and internal and external genetalia begin as bipotential tissues. Thedifferentiation of male gonad is dependent on the expression of SRY (sex reversal Y) = TDF (testesdetermining factor). This gene is expressed in the Sertoli cells of the male in a cell autonomousfashion. Its expression results in a cascade of events leading to the DEVELOPMENT of seminiferoustubules.

2 The Sertoli cells of the seminiferous tubules secrete AMH which stimulates the differentiationof Leydig cells (testosterone secreting). We shall discuss in lecture how these two hormones regulatethe further events of male differentiation . The absence of SRY expression results in the differentiationof the presumptive gonad into an ovary. DAX-1 is a gene normally expressed in both ovarian andtesticular tissue but is down regulated in the latter. DAX-1 downregulates the effectiveness of SRYor downstream elements, resulting in an ovary.

3 Over expression of DAX-1 in XY individuals causessex reversal. DEVELOPMENT of the internal and external genetalia in the male are dependent on thegonad (testes). DEVELOPMENT of female internal and external structures are gonad :5 reductase: Converts testosterone to dihydrotestosterone. If mutated, the external genitalia ofXY male are feminized. The degree of feminization depends on enzymatic activity (if any) : (anti-m llerian hormone) = MIS (M llerian Inhibitory Substance). Made by the Sertoli cellsof the testes and causes degeneration (apotosis) of the M llerian ducts in : Expressed normally in presumptive GONADAL tissue but down regulated in male.

4 Expressionpredates SRY expression by 10 days in humans. DAX duplication in XY individuals can result in : dihydrotestosterone, derived from testosterone by the action of 5 : incomplete closure of the urethral folds resulting in inappropriate urinary openingsthat occur on the inferior aspect of the cells : of the testes that secrete llerian = paramesonephric ducts. Retained in female, forms oviduct, uterus, and upper 2/3 : Steroidogenic factor-1. A transcription factor essential in GONADAL ridge 9 a gene closely related to SRY structurally, also involved in testes differentiation in in this gene can lead to sex DEVELOPMENTDr.

5 Ann-Judith SilvermanDepartment of Anatomy & Cell BiologyTelephone: 305-3540E-mail: sex reversal Y. A gene on the Y chromosome that initiates the cascade resulting in malesexual differentiation . gene absent in XY (human) females. Can be translocated to X during meiosisresulting in ducts = mesonephric ducts. Retained in male due to secretion of testosternone by Leydigcells. Forms efferent ductules, vas :Wilm s Tumor 1, a transcription factor essential in urogenital ridge OBJECTIVES:You should be able to:1.

6 Discuss the central role of SRY2. Explain how testicular DEVELOPMENT (or absence of testes) results in the male/femalepatterns of differentiation of the internal and external genitalia. What are the hormonesinvolved and which cells produce them?3. Describe the switch from bipotential gonads, internal ducts and external genitalia into themale or female Discuss the migration of primordial germ cells and the possible mechanism(s) guidingthis Discuss the role of DAX-1 (dose sensitive sex-reversal-adrenal congenital hypoplasia onthe X chromosome) in sex of components of the gonadsThe gonads develop from three sources.

7 The mesothelium (coelomic epithelium) lining theposterior abdominal wall, the underlying mesenchyme (intermediate mesoderm), and the primordialgerm cells . The mesothelium proliferates to form the genital ridge, a bulge of tissue medial to themesonephros. From this epithelum primary sex cords penetrate the mesenchyme. The indifferentgonad now consists of a medulla and cortex. In XX embryos the ovary will originate from the cortexand the medulla will decline. In the XY embryo the medulla will develop into the testes and thecortex Germ cells ( ; 14-2)The primordial germ cells (PGCs) are committed in the epiblast and have been identified inthe mouse early in gastrulation at a position posterior to the primitive streak.

8 PGCs migrate to theextraembryonic mesoderm at the angle of the amniotic membrane (Fig. 14-2A). From here the cellsmigrate to the base of the allantois and into the yolk sac endoderm (Fig. 14-2A). During hindgutfolding they migrate through the hindgut endoderm, up the dorsal mesentery to the genital ridge(Fig. 14-2B-E). PGCs (50-80 cells ) leave the hindgut and extend long (40 um) processes linking one to theother. Once the cells aggregate in the genital ridge they lose these processes and become PGCs proliferate by mitosis during their migration to approximately 30,000 when they reach thegenital ridge.

9 Two mutant mice lack primordial germ cells ( white spotted w/w and steel sl/sl) .sl encodes a growth factor - Stem Cell Factor (SCF) - and w encodes the receptor for SCF, a tyrosinekinase receptor (c-kit). SCF is necessary for the survival of the migratory PGCs. c-kit is expressedon the PGCs and SCF is found along the migratory route and is most concentrated in the developinggonad. SCF induces proliferation and guides migration through a chemoattractant/concentrationgradient mechanism. PGC migration is also controlled by TGF-beta 1 which is secreted by thegenital ridge.

10 TGF 1 acts as a chemoattractant but inhibits proliferation, thereby modulating thenumber of primary germ cells in the 14-1. Summary of time course of DEVELOPMENT of gonads and reproductive tract in both males and Gonad (Fig. 14-3)Regulated by SRY (see below), the primary sex cords enter the medulla and differentiate intothe seminiferous cords. These are the precursors to the seminiferous tubules where sperm will beproduced. The parts of the primary sex cords that extend deepest into the medulla form the retetestes, the first in a series of structures by which sperm leave the testes in adulthood.