Mammalian Mechanisms

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A developing mammalian embryo's gender is determined by two sequential processes known as primary and secondary sex determination.

Primary Sex Determination. Early in an embryo's development (four weeks after fertilization, in humans), two groups of cells become organized into the gonad rudiment that will eventually develop into either the ovaries or testicles. These gonads will eventually be the source of gametes in the adult. However, at this early stage they are unstructured organs that lack sex-specific features but have the potential to develop into gonads.

The first visible indication of sex-specific development, occurring in week seven in humans, is in males, with the gonads restructuring into two distinct compartments: the testicular cords and the interstitial region. In females, the gonads appear to lack distinct structures until later in development. Primary sex determination, including the differentiation of an embryo's gonads, is dependent on genetic factors associated with the embryo's sex chromosomes.

Secondary Sex Determination. Secondary sex determination involves the development of additional sex-specific characteristics, such as the genitalia. This secondary pathway is controlled by sex-specific hormones that are

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Early in development, every fetus has two sets of primitive ducts. Under the influence of testosterone and the hormone AMH, the Wolffian ducts develop into the male vas deferens and accessory structures. Without testosterone or AMH, the Wolffian ducts degenerate and the Mullerian ducts develop into the female fallopian tubes, uterus, cervix, and upper vagina.

secreted by the differentiated gonad. These hormones influence the sex differentiation of other parts of the body, including two pairs of ducts present in all developing embryos: the Mullerian ducts and the Wolffian ducts.

Testicles produce Mullerian inhibiting substance, a hormone that causes the Mullerian duct to degenerate. They also produce testosterone, which causes the Wolffian duct to develop into the internal male genitalia, such as the seminal vesicles and the vas deferens. Testosterone also promotes the development of the external male genitalia, including the penis, and it reduces the development of the breasts.

In females, where there are no testicles and where there is therefore no Mullerian inhibiting substance, the Mullerian duct differentiates into internal female genitalia: the fallopian tubes, uterus, and cervix.

Discovering the Testis-Determining Factor. The different effects of the primary and secondary sex determination pathways was demonstrated by embryological transplant experiments carried out by Alfred Jost in the 1940s at the Coll├Ęge de France in Paris, France. When Jost placed an undifferentiated gonad from a male rabbit next to an undifferentiated gonad inside a female fetus, the gonad from the female developed into an ovary, and the gonad from the male developed into a testicle, as it would have done inside the male. Hence, the sex of the gonad was dependent upon its genotype (XX or XY) and is a result of primary sex determination. The genitalia of these experimental animals revealed the influence of secondary sex determination mechanisms. Under the conflicting signal of the two gonads, the Mullerian duct, which normally would have developed into female genitalia, degenerated partially, and the Wolffian duct, which normally would have degenerated, began to develop into male genitalia.

Jost's experiment indicated that the sex differentiation of a gonad is determined by its sex chromosomes, and that the sexual characteristics of other tissue are determined by the gonads, not by the chromosomes in the tissues themselves. Jost also showed that in the absence of either gonad, the fetus develops as a female. Female development, then, is apparently a "default" pathway that can be overridden to produce a male.

fallopian tubes tubes through which eggs pass to the uterus sperm

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