Chapter 27
The Reproductive System
1055
27
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.
Homeostatic Imbalance
27.10
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
genitalia.
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
pseudohermaphrodites
(soo-do-her-maf
9
ro-
dīts). (True
hermaphrodites
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
gubernaculum
(“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.
²e
tunica vaginalis
covering of the testis is derived from
a fingerlike outpocketing of the parietal peritoneum, the
vagi-
nal process
. ²e accompanying blood vessels, nerves, and fascial
layers form part of the
spermatic cord
, 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
the
gonadal ridges
(Figure 27.23)
. ²ese ridges bulge from
the dorsal abdominal wall just medial to the mesonephros (a
transient kidney system, see p. 983). ²e
paramesonephric
, or
Müllerian, ducts
(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
cloaca
.
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,
primordial germ
cells
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
degenerate.
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
(Figure 27.24)
. During the sexually
indifferent stage, all embryos exhibit a small external projection
called the
genital tubercle
. ²e urogenital sinus, which devel-
ops from subdivision of the cloaca (future urethra and blad-
der), lies deep to the tubercle. ²e
urethral groove
, the external
opening of the urogenital sinus, is on the tubercle’s inferior sur-
face and is flanked laterally by the
urethral folds
and then the
labioscrotal swellings
.
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
form the
urethral orifice
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.
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