Andrew Siegel MD 12/10/2016
What is it that most distinguishes a male from a female? The obvious answer is the genitals, with the penis/scrotum having a vastly different appearance from a vagina/vulva. Despite the male and female genitals being the feature that most characterizes the difference between a male and female, there are striking similarities. The genitals of both sexes are biologically homologous– similar in structure and having a common embryological origin–with development into male versus female based simply on the hormonal environment at the time of development. Today’s entry discusses the similarities (as opposed to the differences) between the genitals and the “homologues,” the specific anatomical structures that are of common embryological origin and are more alike than are commonly recognized.
Whether one develops a penis or a vagina is determined at the moment the sperm penetrates the egg. The egg contains an X chromosome and the sperm either an X or Y chromosome. When the coupling results in an XX, the blueprint for female development is established; when the coupling results in an XY, the blueprint for male development is established. The bottom line is that the father determines the sex of the child.
Several weeks later, when the fertilized egg has turned into an embryo, the external genitals are identical. Female genitals are the “default” model, which will remain female, absent the presence of the male hormone testosterone (T). T is activated to dihydrotestosterone (DHT) that causes conversion of what would be a vulva and vagina into a penis and scrotum. Biochemical magic! The bottom line is that the developing embryo will remain female unless T/DHT are available to masculinize the external genitals.
In the young embryo there are three key genital structures: the “tubercle,” the “folds” and the “swellings.” In the absence of T/DHT, the genital tubercle (a midline swelling) develops into a clitoris. The urogenital folds (two vertically-oriented folds of tissue below the genital tubercle) become labia minora (inner lips). The labio-scrotal swellings (two vertically-oriented bulges outside the urogenital folds) fuse to become labia majora (outer lips). In the presence of T/DHT the genital tubercle morphs into a penis, the urogenital folds become the urethra and part of the penile shaft and the labio-scrotal swellings fuse to become a scrotum.
Genital Homologues
The penis is the homologue of the clitoris. Both structures are highly sensitive organs with a tremendous concentration of nerve fibers and contain erectile tissue (corpora) that enables them to expand in size and rigidity with stimulation. Both the penis and clitoris have a head (glans) and shaft and deep internal roots. Both are covered with a layer of skin that can be pulled back to expose the underlying anatomy. In the male this is referred to as the foreskin, which is the homologue of the female clitoral hood.
Comparison of penis (left) and clitoris (right)–note similar shape and internal structure, Attribution: Esseh, Wikipedia Commons
The male scrotal sac is the homologue of the female labia majora. The raphe (the seam that runs vertically up the perineum, scrotum and penis) is the homologue of the pudendal cleft (the slit between the labia) in the female.
Comparison of vulva (left) and scrotum (right); note similarity of outer labia to scrotum and female pudendal cleft to male raphe, by Richiex (Own work) [CC BY-SA 3.0 (http://creativecommons.org/licenses/by-sa/3.0) or GFDL (http://www.gnu.org/copyleft/fdl.html)%5D, via Wikimedia Commons
The male prostate gland is the homologue of the female Skene’s glands. Both produce fluid that is released at the time of sexual climax. The male Cowper’s glands are the homologue of the female Bartholin’s glands, both of which secrete fluid at the time of sexual stimulation, pre-ejaculate fluid in the male and vaginal lubrication fluid in the female.
Male anatomy, note prostate gland and Cowper’s glands, by Male_anatomy.png: alt.sex FAQ derivative work: Tsaitgaist (Male_anatomy.png) [GFDL (http://www.gnu.org/copyleft/fdl.html) or CC-BY-SA-3.0 (http://creativecommons.org/licenses/by-sa/3.0/)%5D, via Wikimedia Commons
Note Skene’s gland and Bartholin’s glands openings below and to side of urethra and vagina respectively, by Nicholasolan (Skenes gland.jpg) [GFDL (http://www.gnu.org/copyleft/fdl.html), CC-BY-SA-3.0 (http://creativecommons.org/licenses/by-sa/3.0/) or CC BY-SA 2.5-2.0-1.0 (http://creativecommons.org/licenses/by-sa/2.5-2.0-1.0)%5D, via Wikimedia Commons
Bottom Line: As different as female and male anatomy are, so they are similar. The study of comparative genital anatomy and embryological origin is fascinating. Next week’s entry addresses when this process of differentiation into male versus female goes awry, leading to “ambiguous” genitalia, and how the study of one such particular genetic defect led to the creation of a billion dollar blockbuster drug in common use for purposes of shrinking enlarged prostates and growing hair in men with male pattern baldness.
Wishing you the best of health,
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Andrew Siegel MD practices in Maywood, NJ. He is board-certified in both urology and female pelvic medicine/reconstructive surgery and is Assistant Clinical Professor of Surgery at the Rutgers-New Jersey Medical School and attending urologist at Hackensack University Medical Center. He is a Castle Connolly Top Doctor New York Metro area and Top Doctor New Jersey.
Dr. Siegel is the author ofTHE KEGEL FIX: Recharging Female Pelvic, Sexual and Urinary Health (www.TheKegelFix.com) and MALE PELVIC FITNESS: Optimizing Sexual & Urinary Health (www.MalePelvicFitness.com).
Tags: Bartholin's glands, clitoris, Cowper's glands, genital homologues, genitals, labia majora, male vs female, penis, prostate gland, raphe, scrotum, Skene's glands, vulva
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