Reproductive System: Ducts And Genitalia
Time period: day 35 to postnatal
development
The reproductive systems develop
from a series of epithelial cell‐lined ducts, derived from mesoderm. The
initial stage of genital development is the same for both sexes up to week 7,
and is called the indifferent stage.
The indifferent stage involves
the mesonephric ducts (or Wolffian ducts) from the developing urinary
system and the paramesonephric ducts (or Müllerian ducts), named because
of their location lateral to the mesonephric ducts (Figures 37.1 and 37.2). The
paramesonephric ducts form from longitudinal invaginations of the surface
epithelium of the gondal ridge.
Female
The paramesonephric ducts
descend, meet in the midline and fuse in the pelvic region to form the uterovaginal
primordium (Figure 37.3). This bulges into the dorsal wall of the
developing urogenital sinus (see Chapters 33 and 36) but does not break the wall.
The bulge forms the paramesonephric tubercle (or sinus tubercle, or
Müller tubercle).
The paramesonephric ducts open
into the peritoneal cavity, and the free unfused cranial ends become the uterine
tubes. The uterus forms from the midline uterovaginal primordium.
The paramesonephric tubercle
induces the urogenital sinus to form 2 outgrowths of cells within its lumen.
These outgrowths proliferate and form the sinovaginal bulbs, which fuse
and form the vaginal plate (Figure 37.4). This will canalise to form a
hollow core, which is completed by the fifth month.
The inferior part of the vagina
probably forms from the vaginal plate, and the superior part from uterovaginal
primordium. The vagina is separated from the urogenital sinus by the hymen.
The female reproductive system
(Figure 37.5) is likely to grow from 2 tissue origins: the lining of the lower
portion of the vagina is endodermal and the upper portion, fornices and uterus
are meso- dermal. The muscle and connective tissues of the vagina and uterus
are derived from the surrounding mesenchyme.
The mesonephric ducts degenerate,
although remnants may remain.
Male
Mesonephric ducts become
the efferent ductules and epididymis of the testes, the ductus deferens (or vas
deferens) and the ejaculatory duct (Figures 37.6 and 37.7).
The seminal vesicles form as an
outgrowth from the ductus deferens, whereas the prostate gland arises from
numerous out-growths from the urethra. The endodermal cells of the urethra
differentiate to become the glandular tissue of the prostate gland, and the
surrounding mesenchyme forms the smooth muscle and connective tissue.
Paramesonephric ducts degenerate
(although remnants can remain).
Until the ninth week of development
the external genitals appear the same for both sexes (Figures 37.8 and 37.9).
You cannot see the difference in the sex of a developing embryo until around 11 weeks’ gestation. To prevent mistakes made
in ultrasound identification, if the sex of the foetus is required it is
identified at the 20‐week scan.
During the indifferent stage, the
cloacal membrane is surrounded by mesenchymal folds called urogenital
(cloacal) folds that fuse ventrally into a genital tubercle. Around
week 7, the urogenital septum splits the cloacal membrane into a ventral
urogenital membrane and a dorsal anal membrane.
Another pair of folds develop lateral
to the urogenital folds, called the labioscrotal swellings. The urogenital
membrane degenerates leaving the urogenital sinus in direct communication with the
amniotic cavity. The genital tubercle elongates and forms the primordial phallus.
Female
Induced by oestrogens secreted
from the placenta and foetal ovaries, the genital turbercle develops into the clitoris
(Figure 37.8). During the third and fourth months the clitoris is larger
than its male counterpart. The urogenital groove remains open and develops into
the vestibule which contains the openings of the vagina and urethra. The
urogenital folds remain largely unfused (the two sides only meet posteriorly)
and become the labia minora. The labioscrotal swellings become the labia
majora.
Male
Induced by androgens secreted
from the developing testes, the primordial phallus grows to form the penis (Figure
37.9). The urogenital sinus forms a groove bound laterally by the urogenital
folds, and endodermal cells divide and line the groove which is now termed the urethral
plate. The urethral folds eventually fuse on the underside (penile raphe)
surrounding a tube (the spongy part of the urethra).
The urethra temporarily ends
blindly in the anterior part of the penis. In the fourth month the terminal
part of the urethra is formed when cells from the glans grow internally
producing an epithelial cord. A lumen then forms and creates the external urethral
meatus. The lateral genital swellings form the scrotum and the visible
line of fusion is the scrotal raphe.
The SRY gene (sex‐determining
region of the Y chromosome) encodes for a transcription factor that is
expressed in the gonad during the indifferent stage, triggering male
development. If this transcription factor is absent female development occurs.
Hypospadias is caused by
incomplete fusion of the urethral folds in the male, and the urethra opens onto
the ventral surface of the penis. Epispadias results from the genital
tubercle developing in the area of the urorectal septum, causing the urethra to
open on the dorsal surface of the penis. Epispadias usually occurs in males but
can occur in females and results in a split clitoris and an abnormally
positioned urethral opening.
Congenital adrenal hyperplasia is an enzyme deficiency causing
the adrenal glands to fail to produce sufficient cortisol and aldosterone, but the body produces excess
androgens. This can result in ambiguous
genitalia development in females but will not
affect males. Further developmental problems occur, such as precocious puberty.
