The Reproductive System
Diﬀerentiation of accessory structures and the external
genitalia into male or female structures depends on whether
testosterone is present. When testes form, they quickly begin
to release testosterone, which continues until four to ﬁve days
aFer birth and which causes the development of male accessory
ducts and external genitalia. In the absence of testosterone, the
female ducts and external genitalia develop.
Any interference with the normal pattern of sex hormone pro-
duction in the embryo results in abnormalities. ±or example:
If the embryonic testes do not produce testosterone, a ge-
netic male develops female accessory structures and external
If a genetic female is exposed to testosterone (as might hap-
pen if the mother has an androgen-producing tumor of her
adrenal gland), the embryo has ovaries but develops the male
ducts and glands, as well as a penis and an empty scrotum.
It appears that the female pattern of reproductive structures has
an intrinsic ability to develop (it is the default condition). In the
absence of testosterone it proceeds to do so, regardless of the
embryo’s genetic makeup.
Individuals with external genitalia that do not “match” their
gonads are called
who possess both ovarian and
testicular tissue are rare.) Many pseudohermaphrodites have
sought sex-change operations to match their outer selves (ex-
ternal genitalia) with their inner selves (gonads).
Descent of the Gonads
About two months before birth, the testes begin their descent
toward the scrotum, dragging their supplying blood vessels and
nerves along behind them. ²ey ﬁnally exit from the pelvic cav-
ity via the inguinal canals and enter the scrotum. Testosterone
made by the fetus’s testes stimulates this migration. ²e migra-
tion is mechanically guided by a strong ﬁbrous cord called the
(“governor”), which extends from the testis to
the ﬂoor of the scrotal sac.
Initially the gubernaculum is a column of soF connective tis-
sue, but it becomes increasingly ﬁbrous as it continues to grow.
By the seventh month of fetal development, it stops growing
and its inferior part ﬁlls the inguinal canal. ²e gubernaculum’s
cessation of growth, coupled with the rapid growth of the fetal
body, helps to pull the testes into the scrotum.
covering of the testis is derived from
a ﬁngerlike outpocketing of the parietal peritoneum, the
. ²e accompanying blood vessels, nerves, and fascial
layers form part of the
, which helps suspend the
testis within the scrotum.
Like the testes, the ovaries descend during fetal development,
but in this case only to the pelvic brim, where the tentlike broad
ligament stops their progress. Each ovary is guided in its descent
by a gubernaculum (anchored in the labium majus) that later
divides, becoming the ovarian and round ligaments that help
support the internal genitalia in the pelvis.
Aﬀected individuals usually have a single Y chromosome, two
or more X chromosomes, and are sterile males. Although XXY
males are normal (or only slightly below normal) intellectually,
the incidence of mental retardation increases as the number of
X chromosomes rises. Males with no X chromosome (YO) die
during embryonic development.
Sexual Differentiation of the Reproductive System
²e gonads of both males and females begin their develop-
ment during week 5 of gestation as masses of mesoderm called
. ²ese ridges bulge from
the dorsal abdominal wall just medial to the mesonephros (a
transient kidney system, see p. 983). ²e
(future female ducts) develop lateral to the
mesonephric (Wolﬃan) ducts
(future male ducts), and both
sets of ducts empty into a common chamber called the
At this stage of development, the embryo is said to be in the
sexually indiﬀerent stage
, because the gonadal ridge tissue can
develop into either male or female gonads and both duct sys-
tems are present.
Shortly aFer the gonadal ridges appear,
migrate to them from a diﬀerent region of the embryo, the
hindgut, presumably guided by a gradient of chemical signals
(chemokines). ²ere they seed the developing gonads with stem
cells destined to become spermatogonia or oogonia. Once these
cells are in residence, the gonadal ridges form testes or ovaries,
depending on the genetic makeup of the embryo.
²e process of forming the testes begins in week 7 in male
embryos. Seminiferous tubules form in the internal part of the
gonadal ridges and join the mesonephric duct via the eﬀerent
ductules. ±urther development of the mesonephric duct pro-
duces the duct system of the male. ²e paramesonephric ducts
In female embryos the process of gonad formation begins
about a week later. ²e outer, or cortical, part of each immature
ovary forms follicles, and the paramesonephric ducts diﬀerenti-
ate into the structures of the female duct system (the uterine
tubes and uterus). ²e mesonephric ducts degenerate.
Like the gonads, the external genitalia arise from the same
structures in both sexes
. During the sexually
indiﬀerent stage, all embryos exhibit a small external projection
. ²e urogenital sinus, which devel-
ops from subdivision of the cloaca (future urethra and blad-
der), lies deep to the tubercle. ²e
, the external
opening of the urogenital sinus, is on the tubercle’s inferior sur-
face and is ﬂanked laterally by the
and then the
During week 8, the external genitalia begin to develop rap-
idly. In males, the genital tubercle enlarges, forming the penis.
²e urethral folds fuse in the midline, forming the spongy ure-
thra in the penis. Only the tips of the folds remain unfused to
at the tip of the penis. ²e labioscrotal
swellings also fuse medially to form the scrotum.
In females, the genital tubercle gives rise to the clitoris and
the urethral groove persists as the vestibule. ²e unfused ure-
thral folds become the labia minora, and the unfused labioscro-
tal folds become the labia majora.