Before it is possible to discuss reproduction in kangaroos it is necessary to look at some background information regarding marsupials in general.
In the embryos of amniote vertebrates (reptiles, birds and mammals) there are three pairs of ducts which are concerned with excretion and reproduction. These are the Wollfian ducts, the ureters and the Mullerian ducts or oviducts. The Wollfian ducts are initially involved with excretion but during development they become sperm ducts or the vasa deferentia of the adult. The Mullerian ducts provide the fallopian tubes, uterus and vagina of the female tract. During the development of marsupials the ureters pass to the bladder between the Mullerian ducts, while in placental mammals the ureters enter the bladder by passing laterally to, or outside, the Mullerian ducts.
The independent reorganisation of the urogenital ducts in early marsupials and placentals was most probably related to a separation of the urogenital tract and the gut and not primarily related to reproduction. This reorganisation has allowed the ureters to open directly into the bladder instead of the lower gut. If the urine is held in an impermeable bladder, as distinct from the permeable lower gut, it can be concentrated, thus enabling a considerable saving of water when excreting waste products from the kidney.
Previously, it had been suggested that placentals had a selective change in embryogenesis which resulted in the alternative path for the ureters. This change, it was suggested, allowed for the fusion of the Mullerian duct derivatives to form a large uterus and vagina and thus permit the production of a lager advanced young. Such a simple explanation for the differences between the reproductive systems of placentals and marsupials is not supportable. Fusion of the Mullerian ducts is not necessary to achieve large offspring. Some placentals, even ungulates which give birth to advance young, have completely separate uterine horns and marsupials also show some fusion and development of the reproductive tract.
In marsupials there are two lateral vaginae up which the sperm travels on insemination. In all ancestral mammalian forms birth or egg laying presumably occurred via these lateral vaginae, the homologues of the midline vagina of placentals. In modern marsupials, such as kangaroos, birth occurs through a midline pseudovaginal canal. This short cut to the outside forms from the cul-de-sac formed where each lateral vagina loops around a ureter at the base of the uteri. A more ancestral vaginal condition is still found in small dasyurids and some didelphids, where there is still a septum separating the right and left vaginal culs-de-sac. In most marsupials the pseudovaginal canals opens and closes with each birth.
The urogenital tract of kangaroos is basically similar to that of other marsupials. The ovary, which produces the eggs, is enclosed by the delicate membranous fimbria. The fimbria is an extension of the funnel or infundibulum of the oviduct into which the eggs are shed. These pass into an expanded region, the ampulla, where fertilisation most likely occurs. The ampulla leads into a very convoluted section of the oviduct, the isthmus, which in turn leads into the uterus. At the junction of the oviduct and uterus there is a constriction or sphincter which allows the passage of the egg but normally prevents backflow of fluid from the uterus.
There are two discrete uteri. They have two basic layers: the internal glandular endometrium, which can produce copious secretions, and the outer myometrium. The myometrium is made up of layers of muscles whose function is to expel the young at the appropriate time. The reproductive cycle results in major variations of the form and function of the tissues in the uteri. With kangaroos the uteri have separate openings into a combined vaginal cul-de-sac. The lateral and the median vaginae join at the beginning of the urogenital sinus, into which the urethra from the bladder also empties. Among the kangaroos epithelia and of the large vaginal cul-de-sac and urogenital sinus become continuous at the time of the first birth. A permanent median vagina is thus formed, except in the Eastern grey kangaroo. In view of this condition it is difficult accept that simple anatomical constraints would have limited the size of marsupial young if there had been strong adaptive pressures for the birth of large young.
Posted by Madfish Willie at December 26, 2003 12:12 AM | TrackBack