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 system.
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 lost

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 duct).
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 paramesonephric.

Reproductive system
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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