Sexual Reproduction: Meiosis, Germ Cells, and ... - U-Cursos

PRIMORDIAL GERM CELLS AND SEX DETERMINATION IN MAMMALS 1285in the genital ridge and elsewhere in the embryo in at least four ways:1. They stimulate the newly arriving PGCs to develop along a pathway thatproduces sperm. They do so by inhibiting the cells from entering meiosisand developing along the pathway that produces eggs, as we discuss later.2. They secrete anti-Müllerian hormone, which circulates in the blood andsuppresses the development of the female reproductive tract by causingthe Müllerian duct to regress (this duct otherwise gives rise to the oviduct,uterus, and upper part of the vagina).3. They stimulate endothelial and smooth muscle cells in adjacent mesenchymaltissue to migrate into the developing gonad. These cells formcritical elements within the testis that are required for normal sperm production,which begins when the organism reaches sexual maturity.4. They help to induce other somatic cells in the developing gonad to becomeLeydig cells, which secrete the male sex hormone testosterone into theblood. The secretion of testosterone is responsible for inducing all malesecondary sexual characteristics, including the structures of the malereproductive tract, such as the prostate and seminal vesicles, whichdevelop from another duct called the Wolffian duct system. This duct systemdegenerates in the developing female because it requires testosteroneto survive and develop. Testosterone secretion also helps masculinize theearly developing brain, influencing sexual identity and sexual orientation,and thereby sexual behavior: female rats that are treated with testosteronearound birth, for example, later display malelike sexual behavior.The Sry gene encodes a gene regulatory protein (Sry) that binds to DNA andinfluences the transcription of other genes involved in Sertoli cell development.One crucial downstream gene encodes another gene regulatory protein related toSry, which is called Sox9. The Sox9 gene is not on the Y chromosome, but it isexpressed in males in all vertebrates, unlike Sry, which is found only in mammals.If Sox9 is expressed ectopically in the developing gonads of an XX mouse embryo,the embryo develops as a male, even if it lacks the Sry gene, suggesting that Srynormally acts by inducing the expression of Sox9. The Sox9 protein directly activatesthe transcription of at least some Sertoli-cell-specific genes, including thegene encoding anti-Müllerian hormone.In the absence of either Sry or Sox9, the genital ridge of an XY embryo developsinto an ovary instead of a testis. The supporting cells become follicle cellsinstead of Sertoli cells. Other somatic cells become theca cells (instead of Leydigcells), which, beginning at puberty, contribute to the production of the femalesex hormone estrogen. The PGCs develop along a pathway that produces eggsrather than sperm (Figure 21–19), and the animal develops as a female.How do Sertoli cells induce the PGCs that migrate into the developing gonadin males to follow the pathway leading to sperm production rather than to eggproduction? The mechanism depends on the small signal molecule retinoic acid(see Figure 15–13), which, in both sexes, is produced by cells in a transient tubularstructure called the mesonephros that lies adjacent to the developing gonad.In the embryonic ovary, the retinoic acid induces the proliferating germ-linecells to enter meiosis and start down the pathway leading to egg production; thecells become arrested after diplotene of prophase I, where they remain untilovulation, beginning when the female reaches sexual maturity. In the embryonictestis, by contrast, Sertoli cells produce an enzyme that degrades retinoic acid,preventing the retinoic acid from the mesonephros from inducing the germ-linecells to enter meiosis and begin egg development. Only much later, when themale becomes sexually mature, do the germ-line cells in the testis begin productingsperm.Many Aspects of Sexual Reproduction Vary Greatly betweenAnimal SpeciesAlthough meiosis is highly conserved in all sexually reproducing eucaryotes,other aspects of sexual reproduction are extremely variable. We have seen that