Urogenital of Vertebrates

• Vertebrate kidneys are a pair of compact organs, lying dorsal to coelom in trunk region, one on either side of dorsal aorta.
• When uterine horns are absent and both uteri fuse completely with a single internal cavity, it is termed simplex uterus (Primates, some bats, armadillos).
• Each kidney is composed of a large number of units called uriniferous tubule s or nephrons.
• The number, complexity and arrangement of nephrons 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.
• 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, is called mesomere or nephrotome.
• A uriniferous tubule is differentiated into three parts: peritoneal funnel, tubule and Malpighian body.
• Near the free end of a uriniferous tubule is a funnel-like ciliated structure called peritoneal funnel, which opens into coelom (splanchnocoel) by a wide aperture, the coelomostome or nephrostome, for draining wastes from coelomic fluid.
• A tubule begins as a blind, cup-like, hollow, double-walled Bowman’s capsule, enclosing a tuft of blood capillaries, called glomerulus.
• In adult males, Mullerian duct becomes vestigial and functionless.
• When uteri partially fuse so as to form two horns and two separate lumens inside, it is called bipartite uterus (hamster, rabbit).
• 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.
• 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) which process is termed ovulation.
• Snakes and lizards have a pair of retractile, grooved and sac-like hemipenis which can be everted through cloaca.
• In female anamniotes, ovaries are large, occupying much of the body cavity and produce thousands of eggs as fertilization is external.
• Turtles, crocodilians, some birds (drakes, ganders, ostriches) and prototherian mammals have an unpaired, grooved and erectile penis formed as a thickening of cloacal floor.
• The retraction of hemipenis in snakes and lizards is controlled by modified body wall musculature.
• In adult females, Mullerian duct grows larger and becomes the female genital duct or oviduct.
• Only higher mammals have a true external, erectile penis with a tubular groove continuous with a spongy urethra.
• 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.
• In amniotes, ovaries produce fewer eggs because fertilization is internal.
• Mammalian oviducts are relatively short, narrow and convoluted and called the fallopian tubes.
• Ovaries are not attached to kidneys like testes in the males.
• Histologically, an ovary is a mass of connective tissue with an outer layer of germinal epithelium showing ova in various stages of development.
• When there are two horns but a single internal cavity it is termed bicornuate uterus (ungulates).
• A series of accessory sex glands associated with penis secrete a fluid in which sperm are carried.
• Only the right ovary is functional in many elasmobranchs, where as only the left ovary becomes mature in birds and some primitive mammals (e.g Ornithorhynchus).
• Mammalian eggs contain very little yolk so that their ovaries also remain quite small.
• Condition of uteri varies in different mammals.
• Ovaries are generally paired structures, but only a single median ovary occurs in cyclostomes, as also in some teleosts (e.g perch).
• Ovaries of reptiles and birds are still large and the eggs produced contain much yolk.
• When uteri remain double without fusion, it is called duplex uterus (marsupials).
• Bowman’s capsule and enclosed glomerulus together form a renal corpuscle or Malpighian body.
• Encapsulated glomeruli are termed internal glomeruli which are common.
• In male frog, the anterior portion disappears and the posterior portion becomes the adult functional testis.
• The anterior genital part of kidney along with the part of mesonephric duct forms an epididymis.
• Mesonephric or Wolffian duct becomes solely a genital duct or vas deferens.
• Testes of vertebrates are paired organs of moderate size, usually found attached to kidneys.
• Some mammals such as monotremes, insectivores, elephants, whales, etc., lack scrotal sacs so that their testes remain permanently intra-abdominal like ovaries.
• Passage between abdominal cavity and scrotal sac, through which testis descends, is called inguinal canal.