Human Biochemistry, Volume 2 - Murray R. 1993

Biochemistry of Intra- and Intercellular Communication
Sex Gland Hormones
Gonadal Sex

In the Cytology/cytology/16.html">Early stages of Pregnancy, male and female embryos are indistinguishable. In humans, primordial Germ Cells begin migrating from the yolk sac to the genital ridges between the 35th and 50th days of gestation, culminating in The formation of the primitive gonad. Initially bipotential, the undifferentiated gonad consists of primordial germ cells, connective or interstitial tissue, and a covering epithelial layer. By approximately day 56, Hormonal Influences trigger gonadal differentiation. While the exact trigger mechanism remains elusive, one fact is undisputed: in the absence of a positive signal—namely, the Differentiation of the primitive gonad into a Testis—all embryos develop as phenotypic females. The differentiation of the primitive gonad into a testis is associated with a specific male Cell-surface antigen, the so-called H-Y antigen (Fig. 50.11).

If the genetic sex is male, Leydig cells appear in the Connective Tissue, testosterone synthesis begins, and the male reproductive tract develops. In the absence of Y-chromosome signaling, the FEMALE Reproductive System develops instead. Female Organogenesis occurs with a slight time delay and is virtually complete by the 90th day of gestation. This sequence of events—in which the Structural and functional Development of the Testes precedes the male phenotype, whereas ovarian development and The Emergence of the female phenotype occur only when testicular differentiation fails to take place—is characteristic of all mammals, as illustrated schematically in Fig. 50.11. This paradigm was originally established using data from rabbits, but its extrapolation to humans and other species has since been validated.

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Fig. 50.11. The Role of Hormones in sex differentiation. MIF, Müllerian-inhibiting factor. (Modified and reproduced, with permission, from Fox SI. Page 631. In: Human Physiology, William C. Brown, 1984.)