Embryology and Anatomy

Embryology and Anatomy

Knowledge of the embryology and anatomy of the female genital system is helpful in understandingboth normal anatomy and the congenital anom-alies that occur. Embryology may be useful in many areas of gynecologic and obstetric practice. For example, in gynecologic oncology, embryology can assist clinicians in predicting the growth and routes of spread of gynecologic cancers; in urogynecology and pelvic reconstructive surgery, it can enhance a surgeon’s comprehension of the compo-nents of pelvic support and possible defects. It can also play a key role in understanding and diagnosing various aspects of sexual dysfunction.

The ovaries, fallopian tubes, uterus, and upper portion of the vagina are derived from the intermediate mesoderm, while the external genitalia develop from genital swellings in the pelvic region. Beginning in the 4th week (postfertil-ization) of development, the intermediate mesoderm forms the urogenital ridges along the posterior body wall. As their name implies, these ridges contribute to the forma-tion of the urinary and genital systems (Fig. 4.1).

The gonads, genital ducts, and external genitalia all pass through an indifferent (undifferentiated) stage in which it is not possible to determine sex based on the appearance of these struc-tures. The genetic sex of an embryo is determined by the sexchromosome (X or Y) carried by the sperm that fertilizes the oocyte. The Y chromosome contains a gene called SRY (sex-determining region on Y) that encodes a protein called testis-determining factor (TDF). When this pro-tein is present, the embryo develops male sex characteris-tics. The ovary-determining gene is WNT4; when this gene is present and SRY is absent, the embryo develops female characteristics. Gonads become structurally male or female bythe 7th week of development, and external genitalia become dif-ferentiated by the 12th week. The influence of androgens iscrucial in the normal development of the external geni-talia. Any condition that increases the level of androgen production in a female embryo will cause developmen-tal anomalies. For example, the genetic disease congen-ital adrenal hyperplasia (CAH) causes a decreasedproduction of cortisol that results in a compensatory increase in androgens. The genitalia of female fetuses with CAH are ambiguous, that is, neither normal female nor normal male.