The genetic sex of the embryo is determined at fertilization and depends on the presence or absence of the Y chromosome in the fertilizing sperm. The short arm of the Y chromosome contains SRY gene (SRY stands for sex-determining region of the Y chromosome). The protein product of this gene is a transcription factor called testis-determining factor (TDF). Under the influence of TDF the genital system develops into male type, whereas in the absence of TDF the developing genital system assumes female characteristics.
The genital system begins to develop during the 5th week of development. However, during the 5th and 6th weeks all the three components of the genital system (gonads, genital ducts and external genitalia) in the male as well as female embryos are similar, and distinction of sex can not be made by the inspection of the genital system during this period, which is called indifferent stage of sexual development. The male and female morphological characteristics begin to develop by the 7th week of development.
Development Of the epithelial cords, which are called pri-
^ mary sex cords. The gonocytes be-
come embedded within the primary sex cords. The indifferent gonad is now The Indifferent Gonad supposed to consist of an outer region As discussed in the beginning of or cortex and an inner re9ion or me~ this chapter, the ventromedial division dulla. Both regions are present in the of the urogenital ridge is called genital Qonads of the male and female em-ridge (also called gonadal ridge). In bry°s during the indifferent stage, the 4th week of the development the However, after the 6th week they pur-gonadal ridges are seen to be located sue different fates in the male and fe-on the medial aspect of the meso- rna^e- ln tne n^'6 embryos the medulla nephroi. In both sexes the gonadal differentiates into testis and the cortex ridges are formed by cells derived from regresses. On the other hand, in the the following four sources: (1)coelomic female embryos the cortex differenti-epithelium, (2) mesonephric meso- ates into ovary and medulla regresses, derm, (3) local mesenchyme, and (4)
primordial germ cells or gonocytes, Development of Testis
which originate in the endoderm of the In the male embryos the
yolk sac and migrate into the develop- coelomic epithelial cells of the sex
ing gonads. cords contain an XY sex chromosome
In the beginning of the 5th week complement. As already mentioned,
the coelomic epithelium covering the presence of SRY gene on the Y chro-
gonadal ridge proliferates and be- mosome results in the production of
comes many layers thick. The mesen- TDF (testis determining factor) within
chyme underlying the coelomic epithe- the cells of the primary sex cords. Un-
lium becomes condensed. Cells de- der the influence of this factor the pri-
rived from the mesonephros also mi- mary sex cords become more promi-
grate into this mesenchyme. During the nent and penetrate deeper into me-
6th week of development primordial dulla. The epithelial cells in the me-
germ cells (gonocytes) reach the geni- dullary portions of the primary sex
tal ridges. These cells differentiate in cords differentiate into Sertoli cells,
the endoderm of the posterosuperior whereas the cells of the cortical parts
wall of the yolk sac during the 4th week of the sex cords degenerate. The sex
and migrate along the dorsal mesen- cords of the medullary region of the
tery of the hindgut to the genital developing gonad are now called testis
ridges, where they reach during the 6th cords. The testis cords consist of Ser-
week. Arrival of the gonocytes induces toll cells and gonocytes. Near the hilum
the coelomic epithelium covering the of the developing testis, the testis cords
gonadal ridge to produce finger-like give rise to a network of cell strands
that will later become rete testis. Septa grow in from the tunica albu-ginea to divide the testis into compartments.
As the development proceeds, the testis cords become separated form the coelomlc epithelium by a dense layer of connective tissue called tunica albuginea. The surface epithelium of testis flattens to form the meso-thelium on the external surface of testis.
Some cells of the original mesenchyme of the gonadal ridge differentiate into interstitial cells (of Leydig) during the 6th and 7tfvweeks of development. These cells begin to secrete androgenic hormones (testosterone and androstenedione) by the 8th week. These hormones induce the differentiation of the genital ducts and external genitalia toward a male pattern. The testis cords remain solid and do not acquire any lumen during the fetal life and childhood. When puberty arrives, lumina begin to develop within the solid cords, which now become seminiferous tubules (and rete testis). The walls of the seminiferous tubules are composed of two varieties of cells: (1) Sertoli cells, and (2) male gonocytes, which are called spermatogoina.
After becoming canalized, the seminiferous tubules join the rete testis tubules, which become connected to 15 to 20 mesonephric tubules that become the efferent ductules of the testis. These ductules open into the mesonephric duct, which gives rise the
ductus epididymis and ductus deferens.