Comparative Account of Urinogenital Systems of Vertebrate

Urinary system of vertebrates includes kidneys and their ducts, while reproductive system includes male and female gonads and their ducts. Kidneys excrete harmful metabolic nitrogenous wastes and regulate the composition of body fluids, while reproductive organs perpetuate the species. Thus, kidneys and gonads remain functionally unrelated. However, the two systems are intimately related morphologically in vertebrates because the male urinary ducts are also used for discharging gametes. For this reason, it is more convenient to treat and describe the two systems together as the urogenital or urinogenital system.

Vertebrate Kidneys and Ducts

Basic structure and origin

Vertebrate kidneys are a pair of compact organs, lying dorsal to coelom in trunk region, one on either side of dorsal aorta. They are all built in accordance with a basic pattern. Each kidney is composed of a large number of units called uriniferous tubules or nephrons. Their number, complexity and arrangement differ in different groups of vertebrates. Kidney tubules arise in the embryo in a linear series from a special part of mesoderm called mesomere or nephrotome. It is the ribbon-like intermediate mesoderm, running between segmental mesoderm (epimere) and lateral plate mesoderm (hypomere) on either side along the entire trunk from heart to cloaca. A uriniferous tubule is differentiated into three parts : peritoneal funnel, tubule and Malpighian body.

• Peritoneal funnel :- Near the free end of a uriniferous tubule is a funnel-like ciliated structure called peritoneal funnel. It opens into coelom (splanchnocoel) by a wide aperture, the coelomostome or nephrostome, for draining wastes from coelomic fluid. Nephrostomes are usually confined to embryos and larvae and considered vestiges of a hypothetical primitive kidney.
• Malpighian body :- A tubule begins as a blind, cup-like, hollow, double-walled Bowman’s capsule. It encloses a tuft of blood capillaries, called glomerulus. It is supplied blood by a branch of renal artery, called afferent glomercular arteriole. An efferent glomerular arteriole emerges out of glomerulus to join the capillary network surrounding the tubule. Bowman’s capsule and enclosed glomerulus together form a renal corpuscle or Malpighian body. Encapsulated glomeruli are termed internal glomeruli which are common. Those without a capsule and suspended freely in coelomic cavity are called external glomeruli (embryos and larvae). Capsules without glomeruli are termed aglomerular, such as found in embryos, larvae and some fishes.
• Tubule :- Malpighian bodies filter water, salts and other substances from blood. During passage through tubules more substances are secreted into filtrate, while some are reabsorbed. All the tubules of embryonic kidney are convoluted ductules that conduct the final filtrate to a longitudinal duct which opens behind into embryonic cloaca.

Archinephros

Archinephros is the name given to the .hypothetical primitive kidney of ancestral vertebrates. It may be regarded as a complete kidney or holonephros as it extended the entire length of coelom. Its tubules were segmentally arranged, one nephron for each body segment. Each tubule opened by a peritoneal funnel or nephrostome into coelom. Near each nephrostome was suspended in coelom an external glomerulus (without capsule). All the tubules were drained by a common longitudinal Wolffian or archinephric duct opening behind into cloaca. Such a hypothetical archinephros is found today in the larVae of certain cyclostomes (Myxine), but not in any adult vertebrate. It is supposed to have given rise to all the kidneys of later vertebrates during the course of evolution. Modern vertebrates exhibit three different kinds of adult kidneys : pronephros, mesonephros and metanephros. It is supposed that these represent the sequence or three successive stages of development of the ancestral archinephros, and all the three are never functional at the same time.

Pronephros

In the embryos of all vertebrates, the first kidney tubules appear dorsal to the anterior end of coelom, on either side. These are called pronephros as they are first to appear. Pronephros is also termed head kidney due to its anterior position immediately behind the head. A pronephros consists of 3 to 15 tubules segmentally arranged, one opposite each of the anterior mesodermal somites. There are only 3 pronephric tubules in frog embryo, 7 in human embryo and about a dozen in chick embryo. Each tubule opens into coelom by a funnel or nephrostome. Also projecting into coelom near each tubule and not connected with it is an external or naked glomerulus without capsule. In some cases, glomeruli unite to form a single compound glomerulus, called glomus. Glomus and tubules become surrounded by a large pronephric chamber derived from pericardial or paraperitoneal cavity. Originally each tubule has its individual external aperture, but secondarily, all tubules of a pronephros open into a common pronephric duct, leading posteriorly into the embryonic cloaca. Pronephros is functional, if at all, only in embryonic or larval stage. It is mostly transitory and soon replaced by the next stage or mesonepliros. However, a pronephros is retained throughout life in adult cyclostomes and a few teleost fishes, but it is nonurinary and mostly lymphoidal in function.

Mesonephros

In the embryo, a mesonephros develops from the middle part of intermediate mesoderm, posterior to each pronephros soon after its degeneration. At first, the new mesonephric tubules join the existing pronephric duct and are segmentally disposed. Later on the tubules multiply by budding so that their segmental arrangement is disturbed due to increased number of tubules per segment. Tubules of pronephros and mesonephros develop similarly and are homologous. However, mesonephros is functionally better than pronephros because mesonephric tubules are more numerous, longer and develop internal glomeruli enclosed in capsules forming Malpighian bodies. Thus, they remove liquid wastes directly from glomerular blood rather than indirectly from coelomic fluid as in case of a pronephros. The mesonephros is also termed Wolffian body. With disappearance of pronephros, the old pronephric duct becomes the Wolffian or mesonephric duct. In amniotes (reptiles, birds and mammals), mesonephros is functional only in the embryos, replaced by metanephros in the adults. In fishes and amphibians, mesonephros is functional both in embryos as well as adults. In sharks and caecilians, tubules extend posteriorly throughout the length of coelom. Such a kidney is sometimes called a posterior kidney or opisthonephros. Whereas in adult anurans, urodeles and embryonic amniotes, the mesonephros does not extend posteriorly. Mesonephric kidney is not metamerie, but in myxinoids it is segmental and sometimes called a holonephros. Nephrostomes are generally lacking in mesonephros of embryonic amniotes.

Metanephros

The functional kidney of higher vertebrates or amniotes is a metanephros. It is formed from the posterior end of the nephrogenic mesoderm which is displaced somewhat anteriorly and laterally. When metanephric tubules develop, all the mesonephric tubules disappear except those associated with the testis in male and forming vasa efferentia. The adult kidney (metanephros) of amniotes differs from that of anamniotes (mesonephros or opisthonephros) chiefly in :-

• Its origin from only caudal end of nephrogenic mesoderm.
• In greater multiplication and posterior concentration of nephrons or tubules. They are particularly very large in number and highly convoluted in birds and mammals, hence the large size of kidney. It is estimated that each kidney of man is composed of about 1 million nephrons. The high rate of metabolism yields a large amount of wastes to be excreted.
• In developing a new urinary duct, called metanephric duct or ureter. It is budded off from the base ‘of the Wolffian duct (mesonephric duct). It grows anteriorly and dorsally, and eventually the metanephric tubules open into it. Its dilated distal tip forms pelvis which forks several times to become the collecting tubules. Its proximal portion becomes the metanephric duct or ureter that empties into cloaca or urinary bladder in mammals.
• The mammalian metanephros shows greatest organization of all, with several additional features. A thin, U-shaped loop of Henle forms between proximal and distal convolutions of a metanephric tubule. Such loops are absent in reptiles and rudimentary in birds. Kidney shows an outer cortex with concentration of renal corpuscles, and an inner medulla having collecting tubules and loops of Henle, which are aggregated into one or several pyramids tapering into pelvis. Mammalian kidneys do not receive afferent venous blood supply as there is no renal portal system.

Urinary Bladders

Most vertebrates have a urinary bladder to store urine before it is discharged. However, it is lacking in cyclostomes, elasmobranchs, some lizards, snakes, crocodilians and most birds. In most fishes it is simply a terminal enlargement of mesonephric ducts and called a tubal bladder. In Dipnoi, it evaginates from dorsal wall of cloaca and is probably homologous to the rectal gland of elasmobranchs. In tetrapods, it evaginates from the ventral wall of cloaca. In amphibians, it is termed a cloacai bladder. In amniotes, the adult bladder is derived from the proximal part of embryonic allantois, hence called an allantoic bladder. Kidney ducts or ureters generally open dorsally into cloaca. But in mammals, except monotremes, the ureters lead directly into the urinary bladder which opens to outside through a short tube, the urethra. Mammals lack a cloaca as the dorsal part of embryonic cloaca forms the rectum and the ventral part becomes the urethra.

Gonads and their Ducts

Reproduction is sexual in vertebrates, and the sexes are separate (dioecious) with the exception of hagfishes and a few bony fishes having a hermaphrodite gonad. Reproductive glands or gonads of males are called testes which produce the male gametes called sperm. Female gonads are called ovaries which produce ova. In the embryo, gonads originate as a pair of thick elevated folds or genital ridges of coelomic epithelium from the roof of coelom, one on either side of the dorsal mesentery. Genital ridges are much longer than the functional adult gonads, suggesting that in the ancestral vertebrates the gonads extended the whole length of the pleuroperitoneal cavity. The functional adult gonad is derived from the middle or gonal part of genital ridge, while its anterior progonal and posterior epigonal parts remain sterile. Gonads remain suspended in coelom from dorsal bodywall by a fold of dorsal mesentery, called mesorchium in males and mesovarium in females. Generally, one pair of gonads is present. But, some vertebrates have a single gonad only because of either fusion of both embryonic genital ridges (most cyclostomes, perch and some other fishes), or degeneration of one juvenile gonad (hagfishes, some elasmobranchs and lizards, alligators and most birds). Associated with the gonads are special gonoducts or genital ducts, vasa deferentia in males and oviducts in females, to transport gametes to cloaca or to outside body. However, cyclostomes and a few elasmobranchs lack genital ducts. Their eggs and sperm escape body cavity via abdominal pores.

Testes and male genital ducts

Testes of vertebrates are paired organs of moderate size, usually found attached to kidneys. Each testis is a compact gland, covered by coelomic epithelium and composed of numerous highly coiled seminiferous tubules embedded in connective tissue. Tubules are lined by germinal epithelium which gives rise to billions of sperm. On maturity the sperm are set free in the lumen of tubules and move towards the genital ducts.

Some Cyclostomes have a single median testis without a genital duct. Sperms are released in the coelom from where they pass through abdominal pore, located at posterior part of coelom. In dogfish, the two testes are elongated bodies. In most anamniotes, the opisthonephros (or mesonephros) is differentiated into anterior genital and posterior renal portions. In the anterior genital portion in males, some uriniferous tubules lose excretory function, form slender vasa efferentia, and become continuous with seminiferous tubules of the adjacent testis. They serve to convey sperm of testis to the mesonephric duct of kidney. Thus, in male anamniotes, mesonephric or wolffian duct forms a urinogenital duct, serving both as a vas deferens for sperm as well as a ureter for urine. However, in many elasmobranchs (e.g. dogfish), accessory urinary ducts drain urine from kidney to cloaca so that the mesonephric duct serves entirely or mainly as a vas deferens. The anterior genital part of kidney along with the part of mesonephric duct forms an epididymis.

In the embryos of Anura, each testis is made of two portions. In male frog, the anterior portion disappear and the posterior portion becomes the adult functional testis. In adult male toad, the anterior portion also persists as the Bidder’s organ, containing large cells similar to immature ova.

In male amniotes, a metanephros develops as the adult functional kidney with its own urinary duct or ureter to transport urine. Thus, mesonephric or Wolffian duct becomes solely a genital duct or vas deferens. The remnants of embryonic mesonephros and a coiled portion of mesonephric duct become the epididymis of the adult kidney. From each testis sperms pass first through epididymis, then through vas deferens to reach urethra.

In most mammals testes descend permanently into extra-abdominal skin bags called scrotal sacs. In rabbits, bats and rodents, they are lowered into sacs and retracted at will. Passage between abdominal cavity and scrotal sac, through which testis descends, is called inguinal canal. However some mammals such as monotremes, insectivores, elephants, whales, etc., lack scrotal sacs so that their testes remain permanently intra-abdominal like ovaries.

Copulatory organs

Copulatory organs are absent in anamniotes, since they have usually external fertilization. But, in amniotes, fertilization is internal, and preceded by copulation or mating. Male amniotes usually develop intromittant or copulatory organs for transferring sperm into the genital tract of females, during copulation. They are particularly characteristic of reptiles and mammals.

In elasmobranchs (e.g. dogfish), bases of pelvic fins are modified as intromittant organs called claspers. These are grooved, cylindrical structures that are inserted into the female cloaca to inject sperm. In dog fishes and some allied forms there is blind muscular sac called siphon, located at the base of claspers. This sac gets filled with sea water which is used to force the spermatic fluid into the cloacae of female. In several teleosts, the anal fin is modified as a gonopodium for sperm transport. It is modification of anal fin. Snakes and lizards have a pair of retractile, grooved and sac-like hemipenes which can be everted through cloaca. Their retraction is controlled by modified body wall musculature. Turtles, crocodilians, some birds (drakes, ganders, ostriches) and prototherian mammals have an unpaired, grooved and erectile penis formed as a thickening of cloacal floor. Only higher mammals have a true external, erectile penis with a tubular groove continuous with a spongy urethra. A series of accessory sex glands associated with penis secrete a fluid in which sperm are carried.

Ovaries and female genital ducts

In female anamniotes, ovaries are large, occupying much of the body cavity and produce thousands of eggs as fertilization is external. In amniotes, ovaries produce fewer eggs because fertilization is internal. Ovaries of reptiles and birds are still large and the eggs produced contain much yolk. However, mammalian eggs contain very little yolk so that their ovaries also remain quite small.

Ovaries are generally paired structures, but only a single median ovary occurs in cyclostomes, as also in some teleosts (e.g. perch). They are not attached to kidneys like testes in the males. Only the right ovary is functional in many elasmobranchs, whereas only the left ovary becomes mature in birds and some primitive mammals (e.g. Ornithorhynchus).

Histologically, an ovary is a mass of connective tissue with an outer layer of germinal epithelium showing ova in various stages of development. Ovaries are hollow and saccular in fishes and amphibians but compact in amniotes, especially in mammals, in which each ovum is surrounded by a follicle. Mature eggs are released either internally into the central ovarian cavity (teleosts) which is continuous with the lumen of the oviduct, or extruded externally into the surrounding coelom or body cavity (Tetrapoda). This process is termed ovulation.

In all vertebrate embryos, except cyclostomes, the coelomic epithelium on the outside of mesonephric duct develops a groove which becomes closed to form a tube called Mullerian duct. In adult males, Mullerian duct becomes vestigial and functionless. In adult females, it grows larger and becomes the female genital duct or oviduct. It opens anteriorly into coelom, in the region of degenerating pronephros, by a coelomic funnel or ostium, and terminates posteriorly into cloaca. In female elasmobranchs, the Mullerian duct is formed differently by the longitudinal splitting of the pronephric duct. Thus, in adult female anamniotes, both the Mullerian duct (oviduct) and the Wolffian duct (mesonephric or urinary duct) are present. But, in adult female amniotes, with the development of adult metanephros and its metanephric duct or ureter, mesonephros and its duct (Wolffian duct) degenerate leaving only vestiges known as provarium.

In viviparous mammals, posterior ends of both the Mullerian ducts become fused and are modified into a uterus in which the embryos develop, and a vagina which receives the male intromittant organ during copulation. The remaining anterior parts or oviducts are relatively short, narrow and convoluted and called the fallopian tubes. Condition of uteri varies in different mammals. When uteri remain double without fusion, it is called duplex uterus (marsupials). When uteri partially fuse so as to form two horns and two separate lumens inside, it is called bipartite uterus (hamster, rabbit). When there are two horns but a single internal cavity it is termed bicornuate uterus (ungulates). When uterine horns are absent and both uteri fuse completely with a single internal cavity, it is termed simplex uterus (Primates, some bats, armadillos).

References

• MODERN TEXT BOOK OF ZOOLOGY: VERTEBRATES