Gonads
Introduction
In the chapter on renal
development (see Chapter 36) we talked about the development of the gonadal
ridge from intermediate mesoderm, an important source of cells for
the reproductive system and the location for the beginning of the development
of the gonads.
Gonads are formed from three
sources of cells: the intermediate mesoderm, the mesodermal
epithelium that lines the developing urogenital ridge and germ cells.
Germ cells originate in
the extra‐embryonic endoderm of the yolk
sac near the allantois and migrate along the dorsal mesentery of the hindgut to
reach the gonadal ridge at the beginning of week 5 (Figure 38.1). By the sixth
week they invade the gonadal ridge (see Figure 36.2). Also at this time
the epithelium overlying the mesoderm begins to proliferate, penetrating the
mesoderm and forming cords that are continuous with the surface
epithelium (Figure 38.2).
This indifferent gonad has a
discernible external cortex and internal medulla. If the
migrating germ cells fail to arrive the gonads will not develop because of the
absence of reciprocal interactions between germ cells and surrounding
epithelia.
Female
In the early female gonad the
cortex develops and the medulla regresses. The primitive sex cords dissociate
and form irregular cell clusters containing germ cells (Figure 38.3). These
cords and clusters disappear and are replaced with
blood vessels and connective tissue.
Surface epithelia continue to
proliferate and produce a second wave of sex cords that remain close to the
surface. In the fourth month of development these also dissociate and form cell
clusters surrounding one or more germ cells. This is the primitive follicle and
the sur-rounding epithelial cells develop into follicular cells (see Figure
9.1). Each primitive germ cell becomes an oogonium. Oogonia divide
significantly before birth but there is no division postnatally.
A part of peritoneum attached to
the gonad develops into the gubernaculum. This structure passes through
the abdominal wall (the future inguinal canal) and attaches to the internal surface
of the labioscrotal swellings (see Figure 37.8). The ovaries descend into the
pelvis, and the gubernaculum becomes attached to the uterus. In the adult the
gubernaculum remains as the round ligament (passing through the inguinal
canal) of the uterus and the ovarian ligament.
Male
The cortex regresses and the
medulla develops (Figure 38.4). Testes develop quicker than ovaries, and the
primitive sex cords do not degenerate but continue to grow into the medulla.
Testosterone producing cells,
called Leydig cells, develop from mesoderm
of the gonadal ridge and are located between the developing sex cords. They
produce testosterone by week 8.
The primitive sex cords break up
and form two networks of tubes: the rete testis and the seminiferous
tubules. The tunica albuginea (thick fibrous connective tissue)
develops to separate the networks from the
surface epithelia. The rete testes are the connection between the seminiferous
tubules and the efferent ducts of the testes (see Figure 8.1), which are
derived from the mesonephric tubules (see Chapter 36).
In the fourth month the
seminiferous tubules contain two important cell types: primitive germ cells
that form spermatogonia, and Sertoli cells that have support roles for
the cells passing through spermatogenesis. The male gubernaculum runs
from the inferior pole of the testis to the labioscrotal folds (see Figure
37.9) and guides the testis into the scrotum, along
with the ductus deferens and its blood vessels, as the foetus becomes longer
and the pelvis becomes larger. The inguinal canal normally closes behind the
testis, but failure of this process increases the risk of an indirect inguinal
hernia.
The gonads develop in the abdomen
and hence receive their blood supply directly from the abdominal aorta. The
male arteries are called the testicular arteries and the female arteries
the ovarian arteries. During the descent of the testes their blood vessels
are pulled behind them as they pass through the inguinal canal and into the
scrotum. The lymphatic system of both gonads also follows these pathways.
Clinical relevance
Undescended testes (cryptorchidism)
describes the failure of the testes to descend
normally into the scrotum by birth. This may occur bilaterally or unilaterally,
and is more common in premature males. The
testes may remain in the abdominal cavity, at a point along their normal route
of descent or within the inguinal canal (Figure 38.5). Often, the testes will
have descended to the scrotum by the end of the first year, but testes that
remain undescended are likely to cause fertility problems. Undescended testes,
even if they later descend, are linked to an increased risk of testicular
cancer.
Hormonal imbalances can result in
a varied range of developmental abnormalities to the reproductive system.
Chromosomal defects are also responsible for many genital abnormalities, often
presenting with other congenital defects. Those with gonadal dysgenesis have
male chromosomes but no testes. Patients can have female external genitalia and
underdeveloped female internal genitalia
or ambiguous external genitalia and a mixture of both sexes internally, but are
often raised as girls.
Ovarian and testicular
cancers are relatively common forms of cancer. If testicular cancer is
suspected it is often from a lump
found in one testis and diagnosed through an ultrasound scan. It is important to
remember that lymph drainage is to the retroperitoneal para‐aortic lymph nodes
rather than pelvic nodes, and these are involved in the staging of testicular
cancer. Affected nodes must also be removed surgically together with the
testis. The prognosis for testicular cancers is generally good. Ovarian
cancer symptoms are often absent and if present, unspecific. An increase in
abdominal size and urinary problems are possible. Surgical treatment is often
required but because of the lack of e diagnosis the prognosis is generally poor.
