Unlike the situation with gonads and external genitalia, the embryo lays down the rudiments of two complete sets of internal reproductive structures, one male and one female. In each sex, hormones stimulate one set to grow, while the other set is suppressed and degenerates.
As the fetus matures, and its various organs are put into place, the reproductive structures are initially established in such a way as to allow them to follow the male or female pathways of development, depending on the genetic and hormonal signals received. In the case of the gonads, this is achieved by first making ambiguous or “bipotential” precursor tissues, the genital ridges, which have the ability to form either testes or ovaries. Similarly, the genital tubercle and labioscrotal folds can develop either into the penis or scrotum of a typical male, or the clitoris and labia of a typical female. The internal sex organs—that is, the sex-specific reproductive glands, ducts and organs other than the gonads—develop in a different way altogether. For these organs, two complete sets of precursor tissues, male-type and female-type, are initially laid down; the growth of one of these is promoted, while the other is encouraged to disintegrate, depending on which chemical signals and hormones are produced by the developing gonads.
The embryonic structures that give rise to the internal sex organs in males and females are called the Wolffian and Müllerian ducts, respectively. In the sexually indifferent fetus, both sets of ducts are attached at one end to the developing gonads, which are positioned next to the kidneys, and converge at the other end near the base of the genital tubercle, making a V-shape.
(Image from http://web.uvic.ca/~lalonde/psyc335/notes/lecture03.html)
In males, the Müllerian ducts regress under the influence of a chemical signal called anti-Müllerian hormone (AMH) or Müllerian inhibiting substance (MIS), produced by the Sertoli cells of the developing testes. By about the 12th week of gestation, testosterone produced by the testes promotes the development of the Wolffian ducts into spermatic ducts that connect the testes to the urethra so that sperm can be ejaculated. Later, the seminal vesicle and the prostate gland also form along these ducts.
In females, it is the Wolffian ducts that wither and disappear, while growth of the Müllerian ducts is promoted, ultimately resulting in the formation of the oviducts, Fallopian tubes, uterus, cervix, and upper vagina.
Because testes are initially formed inside the abdomen of the embryo, they need to gradually migrate downwards into the scrotum, usually before birth—a process known as testicular descent.
Testicular descent is thought to occur in two phases.
In the first (11-15 weeks), the testes migrate down the abdomen under the influence of hormones Insulin-like factor 3 (INSL3) and, possibly, anti-Müllerian hormone (also called AMH or Müllerian inhibiting substance, MIS), produced by the Leydig cells and Sertoli cells of the testes, respectively.
The second phase, starting at 25 weeks, involves the testes exiting the abdominal cavity via the inguinal canal, a ring at the groin through which they enter the scrotum. This phase depends largely on a number of chemical signals including testosterone, produced by Leydig cells of the testes.
Testicular descent is usually complete by the time of birth, but in some boys may not be finalized until several months later.
A final note is that each testis must descend independently of the other, and so it is possible that one testis may find the scrotum while the other remains undescended.
Hughes, IA and Acerini, CL. Factors controlling testis descent. European Journal of Endocrinology 159 Suppl 1: S75-82, 2008 (open access).
Last updated: 16 July 2015 PK
Edit history: Author P. Koopman 9/12; revised PK 5/13, 10/13