women's sexual anatomy

by Chris Steidle, MD

The development of human sexual organs is dependant on both a genetic component and exposure of the fetus in its developmental stages to the appropriate hormones. In human embryos, the gonads (ovaries and testes) begin to develop during the second month of fetal life. At approximately 1½ months, the gonads are capable of differentiating, either into ovaries or testes. At this time, the gonads consist of germ cells, which will develop either into oocytes or sperm cells, and other specialized cells that will differentiate into supporting tissues for either the male or female germ cells. There exists also a remnant of an early kidney-type system that forms the initial ductwork for development of the male or female internal sex structures.

Differentiation of the gonadal tissue into testes begins at approximately six weeks of fetal life and requires the presence of the Y chromosome. In the absence of a Y chromosome, the undifferentiated gonads in the female fetus develop into ovaries about two weeks later than testes begin to develop in the male. This refutes the theory that females develop both sexually and emotionally earlier than males do.

The germ cells that will eventually become oocytes proliferate by division and reach a peak of approximately six million by twenty weeks of fetal life. At this stage of development, the oocytes are surrounded by layers of supporting cells where they remain in an arrested state of division until just prior to ovulation. During fetal life, the number of oocytes begins to fall until approximately one to two million are present at birth.

As mentioned earlier, the duct system, including the spermatic cord in males and fallopian tubes in females, develops from remnants of a non-functioning renal system. The mesonephric or wolffian ducts will subsequently give rise to most of the male system, whereas the paramesonephric or mullerian system will develop into the female fallopian tubes, uterus and most of the vagina. Two critical elements that determine which duct structures will develop are antimullerian hormone and testosterone. Antimullerian hormone is synthesized by Sertoli cells after the testes begin to differentiate. In the absence of this hormone, the fetus develops fallopian tubes, a uterus, and the upper portion of the vagina. Testosterone secretion begins approximately two months into fetal life and reaches a maximum at the four-month interval. This increase in testosterone stimulates development of the male duct system, which includes the epididymitis, vas deferens and seminal vesicles. If an ovary is present or if no gonads are present, internal female structures develop.

While there are dramatic changes going on internally in the developing fetus, there are major changes occurring in the external genitalia as well. Prior to differentiation of the gonads into testes or ovaries at one and a half months of fetal life, the external appearance of the male and female fetus is the same. The external genitalia consist of a structure called a genital tubercle with a urogenital sinus or cavity and two lateral labioscrotal swellings. The development of external genitalia is dependent on gonadal steroid hormones. This differs from the development of internal structures where both duct systems develop initially and then either the male or female system regresses. The differentiation of the external genitalia is mainly under the influence of androgens produced by the testes. The genital tubercle forms the penis, the labioscrotal folds will eventually fuse in the midline to produce a scrotum, and the urogenital sinus will eventually become the penile urethra.

The external genitalia in the male are complete at approximately three and half months of fetal life. For the external genitalia tissue to become male-like in appearance, it must be able to process testosterone with the help of an enzyme to one of its derivatives, dihydrotestosterone.

In the absence of a Ychromosome or of adequate testosterone, or if an ovary is present, the external genitalia become female in appearance. The urogenital sinus remains open forming the labia minora and the labia majora form from the laboscrotal folds. The genital tubercle turns into the clitoris, the female equivalent of the penis, and the urogenital sinus develops into a portion of the vagina and the urethra.

An understanding of the developmental process for internal and external sexual organs makes it clear that homologous structures in males and females may respond in similar ways to hormonal or therapeutic influence.

The female external sexual organs are grouped together and called the vulva. The vulvar area includes structures that are visible externally and extends from the pubic bone to the anus. On the front of the body, the mons pubis is a rounded fat-filled cushion that lies over the interior surface of the right and left pubic bones. During puberty the skin of the mons pubis becomes covered with pubic hair that usually grows in a triangular area called the escutcheon. In men, the escutcheon is not as well delineated and hair from the pubic area often extends up to the umbilicus (navel or belly button) as well as downward over the inner aspects of thighs. Genetic and racial differences among women produce a variety of hair patterns, many of which are more like the male in appearance.

The labia majora, the homologue to the male scrotum, are two longitudinal folds of adipose (fat) and fibrous tissue. They form the sides of the vulva and extend from the mons pubis internally to the posterior opening (fourchet) of the vagina. The skin of the labia majora has more pigmentation and is slightly darker than the skin of the surrounding thighs. The size of the labia majora is determined by fat content and responds to various changes as women age. The outer surface of the labia is covered with pubic hair while the inner portion is not. After childbirth, the labia majora become less prominent and later atrophy following menopause.

The labia minora are two small, reddish folds of tissue that are smaller and thinner than the labia majora and are free of pubic hair. At the front end, they come together to form the prepuce (foreskin) of the clitoris; and at the rear end, below the clitoris, they form its supporting membrane or frenulum. The labia minora are endowed with many nerve endings and are extremely sensitive to touch. They comprise one of the few areas of the body that has a large number of sebaceous glands without hair follicles.

The labia majora extend to the midline and fuse to form a small ridge of tissue called the posterior fourchet. This area often becomes obliterated by vaginal deliveries.

The hymen is composed of both elastic and collagen connective tissue and varies widely in structure and shape in adult females. Small tags or nodules of firm fibrous tissue may be left as remnants from the breaking of the hymenal ring. The hymen is so varied in appearance that it is impossible to determine by physical exam whether or not the woman is still a virgin. Occasionally, the hymen is thick and very persistent and must be surgically removed before intercourse can take place.

The clitoris is a central erectile structure at the front of the opening to the vagina that averages three/quarters inch to an inch in length. It consists of two lateral crura or legs that are attached to the pubic symphysis, a body often called the corpus, and a glans or head. The glans has one of the largest concentrations of nerve endings in the female body. The body of the clitoris has two corpus cavernosa in its walls, which are lined by smooth muscle fibers. These structures allow the clitoris to become erect upon stimulation.

January 2000