| The reproductive system arises in close association with
the urinary system, and the two associated systems are often called the
urogenital system. The urogenital system arises from the
intermediate mesoderm. The urinary system arises before
gonadal development begins. The early development of the genital system
is the same in both sexes and is called the undifferentiated stage.
Kidneys and Internal Genital Organs
Develoment of the urinary system begins on about day 22, when 5 to 7
paired cervical segments give rise to nephric vesicles or nephrotomes.
In humans these are vestigial and never differentiate into a functional
state; in some lower vertebrates they would develop into functional
embryonic pronephroi (Greek for ''first kidneys''). The
nephrotomes disappear by day 24 or 25.
Early in the fourth week, nephric tubules begin to develop within a
pair of elongated swellings - the mesonephric ridges - from the upper
thoracic to the third lumbar level. About 40 mesonephric tubules
are produced in craniocaudal succession. The cranial ones regress
as the more caudal ones form, so there are never more than 30
pairs. By the fifth week, there is a massive regression of the
cranial ones, leaving about 20 pairs in the first 3 lumbar
levels.
These mesonephric tubules will develop into excretory units
(renal corpuscle plus tubule) that look like an abbreviated version of
the adult (metanephric) kidney tubules. Their duct, the bilateral mesonephric
or Wolffian duct, arises on about day 24 as a pair of solid
longitudinal rods. The rods grow caudally towards and fuse with
the ventrolateral surface of the cloaca (about day 26). The region
of fusion will become part of the posterior wall of the future bladder.
As the rods fuse with the cloaca, they begin to canalize (develop a
lumen) from their caudal toward their cranial end.
The mesonephric tubules fuse with the mesonephric duct which provides
a passage to the cloaca. The mesonephric excretory units function
from about weeks 6 to 10, producing a small amount of urine. After week
10, they stop functioning and regress.
NB: In the male, a few modified mesonephric tubules and the
mesonephric duct will persist. The former will become the ductuli
efferentes (efferent ductules) of the testis and the latter will
giver rise to the epidydimis and vas deferens. In the
female, these structures will regress.
The formation of the definitive kidneys (metanephric kidneys)
begins on about day 28. Ureteric buds (which give rise to the future
ureters and collecting duct system) sprout from the distal part of the mesonephric
ducts. The buds induce the intermediate mesoderm of the sacral region to
form the metanephric blastema (future nephric tubules). There are
reciprocal inductions between the branching ureteric bud and the
metanephrogenic mesoderm in the development of the kidneys. (The
development of the kidney will not be discussed further.) Probably due
to differential growth of the lumbar and sacral regions, the kidneys
ascend to just below the adrenal glands between the sixth and ninth
week.
Between the fourth and sixth week, the cloaca septates to give rise
to 2 structures: the rectum posteriorly and the urogenital sinus
anteriorly. The urogenital sinus gives rise to: 1) the
bladder in both sexes; 2) the membranous (= entire) urethra in females,
and the membranous and prostatic urethra in males; 3) the vestibule of
the vagina in females and the penile urethra in males.
While the urogenital sinus is forming, the mouths of the mesonephric
ducts expand and flatten and blend into the wall of the part that will
become the bladder. The ureteric buds (future ureters) arising from the
mesonephric ducts intercalate into the future bladder's posterior
wall. When the mesonephric ducts regress in females, the ureters
persist.
During the sixth week, a new pair of ducts, the paramesonephric or
Müllerian ducts, begin to form just lateral to the mesonephric
ducts. They arise by a craniocaudal invagination of a ribbon of
thickened coelomic epithelium from the third thoracic segment to the
posterior wall of the urogenital sinus. Their caudal tips grow to
connect with the urogenital sinus just medial to the openings of the two
mesonephric ducts. The tips adhere to each other just before contacting
the urogenital sinus.
In female embryos, as the Müllerian ducts contact the urogenital
sinus, they begin to fuse from their caudal tips cranially
(between third and fifth month), forming a tube with a single lumen, the
uterovaginal canal (future uterus and upper vagina). The unfused
portions of the Müllerian ducts give rise to the Fallopian tubes.
The posterior urogenital sinus contacted by the Müllerian ducts thickens to form
a pair of swellings called sinovaginal bulbs, which will give rise to
the lower fifth of the vagina.
NB: In males, the paramesonephric ducts will regress under the
influence of Müllerian inhibiting hormone (MIH) produced by pre-Sertoli
cells of the developing testis, which normally begins to differentiate
at the beginning of the seventh week.
The Development of the Gonads
The initial stage of gonadal development occurs during the fifth week
when a thickened area of mesodermal epithelium develops on the medial
side of the mesonephros. Proliferation of the epithelium and the
underlying mesenchyme produces a bulge called the gonadal or genital
ridge. Fingerlike epithelial cords, called primary
or medullary sex cords, grow into the ridge.
The genital ridge is also invaded by primordial germ cells
(future spermatogonia or oogonia) from the yolk sac. During folding of
the embryo, the dorsal part of the yolk sac is incorporated into the
embryo. The primordial germ cells migrate along the dorsal
mesentary of the hindgut to the gonadal ridges. During the sixth
week, they enter the underlying mesenchume and begin to be incorporated
into the sex cords.
Up to the beginning of the seventh week, male and female embryos
appear morphologically identical. In both sexes, germ cells and sex
cords are present in the cortical and medullary regions of the
undifferentiated gonad, and complete mesonephric and paramesonephric
ducts lie side by side.
At the beginning of the seventh week, the sex cord cells of
male embryos begin to produce testis determining factor (TDF).
The gene for TDF is located on the short arm of the Y chromosome.
This region is called the sex-determining region of the Y chromosome (SRY).
Under the influence of TDF, cells in the medullary region of the
primitive sex cords differentiate into (pre-)Sertoli cells, while those
in the cortical region degenerate. The differentiating pre-Sertoli
cells organize to form testis cords. At puberty, the testis cords
will canalize to form the seminiferous tubules. Testis cords that are
distal to the germ cell region will give rise to the rete testis.
During the seventh week, the testis also rounds up, reducing its contact
with the mesonephros, which exerts a feminizing influence. The
cell-to-cell contact between pre-Sertoli cells and the primordial germ
cells appears to play a role in the development of male gametes.
Pre-Sertoli cells secrete Müllerian inhibiting hormone (MIH) (also
called Müllerian inhibiting substance or anti-Müllerian
hormone). MIH (which closely resembles transforming growth factor
Beta) causes the paramesonephric ducts to regress. It is also the
probable inducer for the differentiation of the androgen-producing
Leydig cells in the mesenchyme of the genital ridges. The
andorgens are necessary for the mesonephric duct to give rise to the
male genital ducts. Pre-Sertoli cells also inhibit germ cell development
before meiosis begins.
In female embryos, TDF is not produced as no Y chromosome is
present. Development is slowed. The primordial germ cells remain
concentrated in the outer cortical region or near the cortico-medullary
boundary. Pre-Sertoli cells do not arise from the primary sex cords,
which form a rete in the medulla and degenerate (tenth to twelfth week).
Secondary (or medullary) sex cords invaginate from
the germinal epithelium and incorporate the primordial germ
cells.
The follicle cells of the ovary arise from the secondary sex cords,
or possibly from both the secondary and some primary sex cord cells. The
female germ cells differentiate into oogonia (apparently with some
influence from the mesonephros) and enter meiosis to become primary
oocytes, before further differentiation is inhibited by the follicle
cells. Individual oocytes resume gametogenesis at puberty.
The number of primordial follicles is maximum at 5 months (about 7
million). At birth, there are about 2 million, and by puberty only
40,000 are left. About 400 will become secondary oocytes.
The Development of the External Genitalia
The external genitalia develop from the same primordia in both
sexes. Early in the fourth week, proliferating mesenchyme produces
a genital tubercle at the cranial end of the cloacal
membrane. The genital tubercle soon elongates to become a phallus.
At the same time, labioscrotal folds (or genital folds)
and urogenital (or urethral) folds develop on each
side of the cloacal membrane.
While the development of male internal genital ducts depends on
testosterone, the development of male external genitalia depends on the
metabolite of testosterone, dihydrotestosterone (DHT). DHT causes the
phallus to enlarge and elongate and form a penis. As this occurs,
the urogenital folds develop and form the lateral walls of the urethral
groove of the ventral surface of the penis. The labioscrotal folds
fuse to form the scrotum.
In females, the pattern of the external genitalia more closely
resembles the undifferentiated stage. The phallus regresses to
become the clitoris, the urogenital folds become the labia minora, and
the labioscrotal folds become the labia majora. The urogenital sinus
remains open as the vestibule, into which both the urethra and the
vagina open.
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Development & Homeostasis| Immunology | Cardiovascular | Respiratory 