Sexual differentiation between human males and females can be seen to occur in five stages that determine genetic sex, gonadal sex, internal reproductive structures, external genitalia, and some remaining phenotypic characteristics
Genetic Sex
In most mammals, female ova cells are only X while males can contribute either X or Y sperm cells to a zygote. At conception, the zygote is designated as genetically male (XY) or female (XX). The determinant of further development is the sex-determining region Y (SRY) gene located on the short arm of the Y chromosome (Stanfield & German, 2009). If the SRY gene is expressed, the offspring will develop as a male; if it is not expressed, the organism will develop as female.
Gonadal Sex
Early embryos have undifferentiated gonads made up of cortical and medullary regions which will begin to develop into either testes or ovaries around week six. In males, the SRY gene encodes for transcription factors that promote development of the medulla while the cortex regresses, resulting in a testis. Conversely, if there are no SRY transcription factors produced, the cortical region will develop as the medulla regresses and ovaries develop. After formation of the testes, they almost immediately begin to produce testosterone from Leydig cells stimulated by human chorionic gonadotropin; ovaries do not produce hormones until later (Lee & Houk, 2008).
Internal Reproductive Structures
Embryos possess a double set of genital ducts - wolffian and mullerian - which develop into the internal reproductive structures. In males, testosterone promotes the development of the wolffian ducts. Mullerian-inhibiting hormone (MIH), a glycoprotein which is produced in the testicular sertoli cells, promotes regression of the mullerian ducts. In conjunction, MIH and testosterone result in the development of wolffian ducts into the prostate gland, epididymis, seminal vesicles, and vas deferens. In the absence of testosterone and MIH, the mullerian ducts of the female will spontaneously develop into the uterus, oviducts, cervix and upper portion of the vagina while the wolffian ducts degenerate (Hadley & Levine, 2007).
External Genitalia
The external genitalia of both sexes arise from a common urogenital sinus, genital tubercle and labiogenital folds. Cells of the aforementioned structures are able to convert testosterone to dihydrotestosterone (DHT) by 5α-reductase, which promotes the development of external genitalia in males. This process starts shortly after wolffian duct development begins (~9 weeks) and is complete around the 13th week of gestation (Boron & Boulpaep, 2009). In the absence of testosterone - and consequent lack of DHT - the formation of characteristically female genitalia is seen.
Remaining Phenotypic Characteristics
Testosterone is also vital to the development of the male brain. Through an aromatase catalyst, estradiol is formed from testosterone and results in differentiation of the hypothalamus. The difference when compared to a female brain can be seen in the release of gonadotropins; females release in a cyclic fashion while males remain tonic (Hadley & Levine, 2007). Puberty results in manifestation of secondary sex characteristics and it is the point at which humans are considered sexually mature and differentially complete. While there are variances in development amongst other mammals, the overall process remains very similar.
Works Cited
- Boron, W., & Boulpaep, E. (2009). Medical Physiology. Philadelphia: Saunders Elsevier.
- Hadley, M. E., & Levine, J. E. (2007). Endocrinology. Upper Saddle River, New Jersey: Pearson Prentice Hall.
- Lee, P., & Houk, C. (2008). Disorders of Sexual Differentiation in the Adolescent. Annals of the New York Academy of Sciences, 1135, 67-75.
- Stanfield, C., & German, W. (2009). Principles of Human Physiology. San Francisco: Pearson Benjamin Cummings.
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