Introductory Anatomy: Excretory & Reproductive Systems
Dr D.R.Johnson, Centre for Human Biology
During the process of evolution we quite often find that structures
evolved to do one thing are no longer relevant. The appendix
is a good example: useful in animals feeding on grass it has
no function in man, has become small and may ultimately disappear.
In other cases these relict organs may be taken over for quite
another purpose. This is the case with the excretory and reproductive
systems which are anatomically and evolutionary linked, and where
parts of the excretory system have been taken over by the reproductive
To understand this we must go back to our coelomate ancestor.
Remember this had a gut and a coelom lined with thin mesothelium.
Now a large cavity with a thin wall like this is ideal for dumping
waste, and it seems that the coelom was so used. The result,
of course is that it would gradually fill with ions, salts, water
and what have you. This seems to have been countered by forming
paired lateral ducts, lined with cilia and called nephrotomes.
The cilia control the direction of the fluid, outwards. Interestingly
these excretory ducts probably also carried gametes to the sea,
even at this early stage.
Now the animals we are talking about were segmented, like sliced
bread, and had one coelom and one pair of nephrotomes per segment.
When segmentation was abandoned in favour of an open plan coelom
the nephroptomes became rationalised with paired openings (still)
but only one exit to the sea, towards the rear. The open plan
coelom developed at about the same time as the circulatory system,
and this became involved. The dorsal aorta ran close to the nephrotome
and formed a knot of capillaries, a glomerulus, associated with
each nephrotome. Finally the ciliated duct to the coelom was
This set-up, a glomerulus, a nephric tubule (essentially a
space) and a duct passing posteriorly has formed the basis of
the kidneys ever since. I say kidneys not because we have two
of them but because during vertebrate history there have been
three pairs of kidneys. The most primitive of these was anterior,
the next more posterior and the last most posterior. They are
called pronephros, mesonephros and metanephros.
The pronephros is still seen in some fish and amphibian
larvae: it has glomeruli, ciliated segmented ducts opening into
the coelom and a pronephric duct running posteriorly to the cloaca.
The mesonephros is the kidney of most fish , some adult
amphibia and many mammalian embryos: it has glomeruli, no ciliated
ducts and uses the pronephric duct (which is now called the mesonephric
The metanephros develops behind the mesonephros to form
the definitive kidney in mammals. The important fact here is
that it develops its own duct, the metanephric duct or ureter,
which develops from the cloaca and runs forward to the developing
third generation kidney.
The reproductive system and its ducts.
The gonads develop in the same dorsolateral region as the kidneys.
Gonads are initially equipped with two sets of ducts. Why? Well
remember that nephrotomes, ciliated ducts passing to the outside
originally carried germ cells. When the connection to the coelom
was lost an alternative duct system, the parmesonephric ducts
were developed to provide passage for the germ cells. When, in
turn the mesonephric duct became redundant it was reclaimed by
the reproductive system. As it happens the male reproductive
duct develops from the mesonephric duct and the female from the
The whole embryology of the reproductive system is fascinating.
The sex of an embryo is determined at conception by its chromosome
content. In man 46+XX is female, 46 +XY is male. However the
gonads, when they first form are of an indifferent type, that
is the same in both sexes. Another point of interest is that
the actual germ cells, which are to become eggs and sperm develop
in another part of the embryo entirely from the gonads - in fact
near the heart. They then migrate through the tissue of the embryo
to the gonad. By the time they arrive the gonad has prepared
itself by becoming male or female. In the male changes are under
the influence of the Y chromosome (we know this because of the
few unfortunates who don't have one , or who have an unusual
number of X chromosomes ). Part of the male development is the
formation of interstitial cells, the hormone producing (testosterone)
cells of the testis. The testosterone influences duct development.
With testosterone the mesonephric duct develops to become the
vas deferens and associated structures; without it the mesonephric
duct atrophies and the paramesonephric duct becomes the oviduct
and most of the uterus. In some individuals the tissues are unable
to respond to testosterone. Lets look at the developing male
and female reproductive systems. In the male the indifferent
gonad responds to the effects of the Y chromosome to develop
testis cords which become horseshoe shaped and enclosed within
the thickened tunica albugina of the gonad. The free ends of
the horseshoes are in contact with the redundant mesonephric
duct. The paramesonephric duct develops in response to placental
and maternal hormones, but is unused and later regresses. A little
later we find that the mesonephric duct has continued to develop
and forms the epididymis, the ductus (vas) deferens and the seminal
vesicles. The paramesonephric duct has regressed, to be represented
only by the appendix testis proximally and the utriculus prostaticus,
a small diverticulum in the prostate gland distally.
You will also note that the right hand side of the slide shows
a different relationship between parts, because it covers the
period of the descent of the testes. These were originally situated
on the posterolateral abdominal wall but, as they develop migrate
(by differential growth) to lie behind an outpushing of the anterior
abdominal wall, the vaginal process (vagina here as elsewhere
means scabbard). This is assisted by the gubernaculum (the governor)
a band of contractile connective tissue. The testes ultimately
lie in the scrotum, and the duct system is rearranged to pass
from the scrotum back into the abdominal wall, in the inguinal
canal, before it unites with the terminal duct of the excretory
system, the urethra to enter the penis.
In the female things take a slightly different course. The
medullary cords (the testicular cords of the male) degenerate
because there is no Y chromosome. They are replaced by epithelial
inpushings of the covering of the developing ovary, which break
up to form follicles.
This is where the white mouse comes in. This is a black-eyed
white mouse, not the normal albino which has pink eyes. It is
white, anaemic and sterile because it carries a mutation in a
gene which governs the ability of migrating cells to navigate.
One of these great navigations is that of pigment cells into
the coat, the other is germ cells to the ovary and testis. The
germ cells in these mice set off from the region ahead of the
developing heart and migrate to the gonads - but never get there,
so the mice are sterile. Usually the germ cells arrive in the
testis or ovary and populate the cords or follicles. The paramesonephric
ducts in the female persist and go on to make the Fallopian tubes
or oviducts the uterus and the upper part of the vagina. The
lower part of the vagina is ectodermal and shares its derivation
with the male. Just as the testis and ovary were indeterminate
at first, so are the external genitalia At four weeks we find
a genital tubercle marking the anterior end of a common cloaca,
serving both urinary and genital systems. By six weeks an anal
region has budded off posteriorly. In the female the tissue on
each side of the cloaca becomes the major and minor labia, and
vagina and urethra open between them. The genital tubercle becomes
the relatively small clitoris. In the male the labia unite in
the midline to form the scrotum and part of the penis, the head
of which is formed by the genital tubercle. If this seam remains
incomplete hypospadias results.
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