genitalia

genitalia The genitalia are the organs of reproduction which characterize the male or the female. In common usage, ‘the genitals’ applies only to the external genitalia — the labia and the clitoris, forming the vulva, in the female, and the penis and scrotum in the male. The female internal genitalia are the ovaries, Fallopian tubes, uterus and vagina. In the male, although the testes in fetal life are internal, they become externalized by descent into the scrotum; the vasa deferentia link the external components to the internal — the seminal vesicles and the prostate gland.

In an early fetus there are rudimentary precursors of the future genital organs which are not distinguishable as male or female: sex glands, which will become either ovaries or testes, and two pairs of blind-ended ducts opening into the cloaca, in common with the gut. It is only after about the sixth week that male–female developments diverge. Of the two pairs of ducts, one is destined to mature into the female apparatus, or the other pair into the male, whilst the undeveloped pair in either case normally shrivels away, apart from small anatomical vestiges. The primordial female genital ducts were described by Müller, a distinguished German anatomist and physiologist, in 1825, and hence are known as Müllerian ducts. The innermost parts of these ducts remain separate, and become the Fallopian tubes, or oviducts; the lower parts fuse, and develop into the uterus and vagina. The primordial male genital ducts are known as Wolffian, after the description in 1759 by Wolff, a German anatomist and physiologist (regarded as the founder of the scientific study of embryology). These remain separate on the two sides: the innermost parts develop branches of multiple coiled tubules, which link up with the developing testes, forming the epididymis; the rest of each duct forms a vas deferens (opening into the urethra) and a seminal vesicle. Each testis at first lies in the back of the abdomen but gradually descends to the groin, and usually at about seven months of gestation travels through the inguinal canal — a channel between attachments of abdominal muscles and ligaments — to reach the scrotum.

The rudimentary external genitalia are similarly common to male and female in the early weeks of fetal life. A midline ‘genital tubercle’ appears and enlarges into a primitive phallus, destined to become either a clitoris or a penis; two labioscrotal swellings grow on either side and later fuse into a scrotum in the male, or remain apart as the labia in the female.

A fetus with male chromosomes starts to produce testosterone in its gonads in the second month. It is this which causes the primitive ducts to develop in the male direction. In the absence of such influence, development will be female.

The appearance of the external genitalia at birth is of course the acknowledged diagnostic criterion of sex. But it is occasionally misleading. Some rare individuals who are genetically male may have apparently female genitalia, and vice versa. From the above embryological account it is not difficult to imagine, for example, that a failure at one stage of development could leave the potential scrotum unfused: combined with non-descent of the testes, the infant would appear more female than male.

Sheila Jennett

See urogenital system.See also antenatal development; gender; sex determination; sex hormones.