Ducts of kidneys and the ducts of gonads show variations in vertebrates so that sometimes they are common and sometimes exchange their normal functions. The function of kidney ducts is to transport urine and ducts of gonads carry either the sperms or ova. In male and female vertebrates their modifications are different because of different roles they play in reproduction.
MALE URINOGENITAL DUCTS
In the embryonic basic pattern the vasa efferentia originate by the modification of uriniferous tubules and even vas deferens is considered a modification of archinephric duct and sometimes mesonephric duct performs both the function.
In the lungfish Neoceratodus, garpike and frog, mesonephros is the functional kidney in embryonic as well as adult stage and mesonephric duct functions as both urinary and genital duct. Tubules of the anterior region of kidney modify as vasa efferentia and epididymis and get connected to testis to collect spermatozoa, while the posterior region remains as functional kidney and transports urinary wastes to the exterior. In frog there is a longitudinal bidder’scanal that connects the genital and urinary parts of kidney.
Mesonephros is the functional kidney in sharks but the mesonephric duct leaves the function of urinary transport and gets attached with testis to produce vasaefferentia, epididymis and vasdeferens. Mesonephros in the adult elasmobranchs develops a new accessory urinary duct to transport urinary wastes. Both the urinary as well as the genital ducts open into a common chamber called cloaca. In sharks mesonephros is long and narrow and extends up to the posterior end of body and is therefore called opisthonephros. Pronephric region becomes lymphoid, called head kidney and the anterior genital part is called Leydiggland.
Modification of urinogenital ducts in bony fishes is opposite of elasmobranchs in the sense that mesonephric duct retains its original function of transport of urine and does not modify. Therefore, testis has to develop its own vasa efferentia, epididymis and vas deferens to transport sperms. Some amphibians also show this condition while primitive bony fishes, such as Polypterus, exhibit an intermediate condition in which vas deferens also has connection with the kidney.
Amphibian male urinogenital system is either close to the basic pattern as in frog and apoda or is comparable with teleost system as in some urodeles.
AMNIOTES (Reptiles, Birds and Mammals)
In amniotes mesonephros is the functional kidney in embryos and metanephros is the functional kidney in adults and mesonephros disappears during embryonic development. As metanephros develops its own duct called metanephricduct or ureter, mesonephric duct owing to disappearance of mesonephros becomes free to transform into functional vas deference. In fact during early embryonic development as the mesonephros atrophies, mesonephric duct splits into a Mullerianduct and a Wolffianduct. In the case of male amniotes Mullerian duct disappears and Wolffian duct produces a functional vasdeferens, while in females wolffian duct disappears and Mullerian duct produces functional oviduct and uterus. Rudiments of mesonephric tissue and tubules can be seen as rudiments in adult amniotes.
Cyclostomes have no ducts and the sperms and ova are released into coelom from where they are transported to outside through the genital opening.
Cartilaginous fishes display a pair of solid ovaries which are wrapped around the oviducts. Both oviducts fuse on the anterior side to form a common ostiumtubae that captures ova which are released into the coelom. Oviducts are modifies differently in the oviparous and viviparous sharks. In egg laying sharks there is a shellgland in the middle part of the oviduct that secretes a leathery shell around the yolk laden egg. These shelled eggs can be temporarily stored in uteri which open into cloacal chamber. Fertilization is always internal in elasmobranchs for which males possess a pair of claspers (copulatory organs) which are actually modified pelvic fins. In viviparous sharks a yolk sac placenta develops in uterus for the nourishment and development of the embryo.
Bony fishes have saccular ovaries and ova are released into the lumen of ovary from where they are transported out via oviducts which open to the outside by a single genital opening. There is no ostium tubae in teleosts and genital and urinary openings are separate.
Amphibians also possess saccular ovaries but ova are not shed inside the lumen but are released outside into the coelom from where ostium collects them and transports to outside via the oviduct. Uterus secretes a jelly-like material that envelops the ova when they are released into water. Jelly helps the ova to float on the surface of water and also provides a medium for sperms to swim and search for ova. Ovary also carries a Bidder’sorgan which is the dormant part of ovary and is capable of developing into a functional ovary should the original ovary be damaged or destroyed.
Reptiles lay shelled and cleioid eggs that can develop on land. Therefore, the oviduct is modified to provide nourishment and shell to the ovum. Snakes and lizards have saccular ovaries while crocodiles and turtles possess solid ovaries. Albumen glands are located in the walls of oviduct in the anterior region and shell glands on the posterior in walls of uterus. Snakes and lizards lack albumen glands and hence their eggs are smaller in size as compared to the eggs of turtles and crocodiles.
Birds lay a large and cleioid egg as compared to their body size. The egg is protected by a calcareous shell and is nourished by yolk and albumen. Birds possess only the left ovary and oviduct and the right one is missing but in many
birds of prey both ovaries and oviducts are present and functional. The modification is apparently to reduce weight while they are airborne. Ovaries are solid and produce a large ovum coated with yolk, for which the ostium tubae is specialized with a fimbriated infundibulum that grabs the egg and virtually sucks it into the oviduct. Fertilization takes place in the anteriormost part of the oviduct. Albumen secreting area of the oviduct is called magnum and the middle portion is known as isthmus where a shell membrane is secreted over the albumen. Uterus has shell glands and the egg stays in uterus till it is laid in a nest for incubation.
Except for monotremes all mammals are viviparous and they develop their embryos in uterus. Monotremes have two distinct uteri which open into a common urinogenital sinus that opens into a cloaca. Oviducts of monotremes are modified in a similar way as in reptiles for laying shelled eggs. Some placental mammals such as mouse, hare, elephant and some bats also have two uteri but a single vagina.As the embryo has to develop in uterus for considerable period, uteri in mammals have modified in four different ways as described below:
This type of uterus is found in monotremes, marsupials, and some placentals like mice, lagomorphs, elephants and some bats. Almost the entire oviduct is modified into uterus and embryo can be implanted in any of the uteri or in both. Marsupials develop chorio-vitelline placenta while placentals develop chorio-allantoic type for complete development of the embryo.
Bipartite uterus develops on the posterior end of the oviduct while the anterior portions remain as a pair of cornua or horns of uterus. The lumen of the uterus is divided by a septum into two chambers and embryo can be implanted in any of the chambers. This type of placenta is found in pigs, cattle, other ruminants, some bats and some carnivores.
Bicornuate placenta is similar to the bipartite type except that there is no partition in the lumen of the uterus. So there is a single uterus with two horns or cornua, opening into a vagina on the posterior end. This uterus is found in many ungulates, cetaceans, insectivores and some carnivores.
Simplex uterus is found in primates, some bats and edentates. There are no horns in the uterus and hence the oviducts, which are called fallopiantubes, connect to the large triangular uterus directly. On the posterior end the uterus opens into vagina by an extension called cervix.
Urinary bladder is used for temporary storage of urine till it is excreted out. It is absent in cyclostomes, elasmobranchs, some lizards, snakes, crocodiles and most of the birds. Majority of fishes possess a tubal bladder which is formed by the enlargement of mesonephric duct. Amphibians, lung fishes and monotremes have a cloacal bladder. An allantoic bladder is found in majority of mammals, tortoises and turtles and some lizards.