Published on March 1, 2014
Notes: Zoology- VI Semester, University of Mumbai, India Comparative Anatomy of Chordates Urogenital System in Vertebrates PART-I: Development of Kidney in Vertebrates Kidney tubules arise from the intermediate mesoderm. This is a ribbon of nephrogenic tissue extending uninterrupted from the level of the heart to the cloaca. It lies just lateral to the segmental/dorsal mesoderm. Almost the entire ribbon produces kidney tubules. The intermediate mesoderm becomes segmented into units termed nephrotomes. The lateral portions of the nephrotomes unite, forming a longitudinal duct on each side of the embryo. The anterior-most tubules are always metameric, since one tubule develops from each mesodermal somite. Farther back, numerous tubules develop in each segment and the metamerism is lost. The longitudinal ducts of the basic patter appear first at the anterior end of the nephrogenic mesoderm as posteriorly directed extensions of the first tubule. Each duct grows caudal until it achieves an opening into the cloaca. At this time, it is known as the pronephric duct. The kidneys of myxinoid cyclostomes closely resemble an archinephros. Excretory organs in protochordates are very different from the higher vertebrates. Balanoglossus (Hemichordata) has a glomerulus in the proboscis region to excrete nitrogenous wastes from the blood. Herdmania (Urochordata) has a neural organ near the solid nerve ganglion located in between the two siphons. Amphioxus (Cephalochordata) possesses protonephridia that carry hundreds of flame cell-like solenocytes that excrete wastes in the atrial cavity and to the outside. A. Archinephros: It is believed that kidneys in all modern vertebrates evolved from a hypothetical kidney known as Archinephros or Holonephros, which extended from anterior to Prof. S. D. Rathod, B. N. Bandodkar College of Science, Thane -400605
Notes: Zoology- VI Semester, University of Mumbai, India the posterior end of the body, with segmentally arranged glomeruli and nephrostomes. Archinephros was found in primitive vertebrate probably extended the entire length of the body cavity & had external glomeruli that drained the coelomic fluid. Each segment of the body contained one pair of Uriniferous tubules with peritoneal funnels. Malphigian bodies remained hanging in the coelom collecting the waste matter from it and pouring in the common archinephric duct. The archinephric duct eventually opened into cloaca. Archinephros is found in larval Myxine and some apodan amphibians. Most vertebrates have embryonic transitory archinephros commonly known as pronephros. B. Pronephric duct: The development of the pronephric duct is a part of the development of the urinary and reproductive organs. The initial kidney tissue of all vertebrates is positioned in the outer part of the intermediate mesoderm, immediately under the ectoderm (in the region from the V cervical segment to III thoracic segment) a series of short evaginations called nephrotomes, from each segment grows dorsally and extends posteriorly, fusing successively forming the a duct. This duct is known as the pronephric duct, mesonephric duct or Wolffian duct. This primodium continues to grow posteriorly until it opens into the ventral part of the cloaca. In the early stages of many vertebrate kidneys these first pronephric tubules or duct function for a brief period as the pronephric kidneys (pronephros). Beyond the pronephros it is termed the Wolffian duct now working as ureter. Thus, the Wolffian duct is what remains of the pronephric duct after the atrophy of the pronephros. C. Pornephros: Pronephros is the most basic of the three excretory organs that develop in vertebrates, corresponding to the first stage of kidney development. Functional pronephros is found in cyclostomes and as embryonic kidney in anamniotes. It is succeeded by the mesonephros, which in fish and amphibians remains as the adult kidney. The pronephros develops from the intermediate mesoderm, as do the later kidneys. It is a paired organ, consisting of a single giant nephron that processes blood filtrate produced from glomeruli or glomera- large embryonic glomeruli. The filtrate is deposited into the coelom. It then passes through thin ciliated Prof. S. D. Rathod, B. N. Bandodkar College of Science, Thane -400605
Notes: Zoology- VI Semester, University of Mumbai, India tubules into the pronephric nephron where it is processed for solute recovery. The organ is active in adult forms of some primitive fish, like lampreys or hagfish (cyclostomes). It is present at the embryo of more advanced fish and at the larval stage of amphibians (anamniotes) where it plays an essential role in osmoregulation. In others it degenerate but the pronephric duct persists. In human, it is rudimentary, appears at the end of the third week (day 20) and replaced by mesonephros after 3.5 weeks. Despite this transient appearance in mammals, the pronephros is essential for the development of the adult kidneys. D. Opisthonephros: The opisthonephros is the functional adult kidney in lampreys (cyclostomes), most fishes, and amphibians. It is formed from the extended mesonephros along with tubules from the posterior nephric ridge. In 1949, Hyman wrote the opisthonephros “has used up the mesomere tissue from which in amniotes both mesonephros and metanephros come.” The mesonephros and metanephros of amniotes are derived from different parts of the anamniots opisthonephros. The mesonephros is derived from anterior part of the opisthonephros whereas the metanephros is derived from the posterior part of the opisthonephros. In amniotes, most of the mesonephros became the epididymis, and the archinephric duct became the vas deferens. The mesonephros act as embryonic kidney and a more complex metanephros acts as the adult kidney in amniotes. Some accounts call opisthonephros the ‘mesonephros’, but the opisthonephros in anamniotes (lampreys, fish, and amphibians) differ considerably than the mesonephros in amniotes (reptiles, birds, and mammals). Thus, the term mesonephros is usually reserved for the embryonic kidney of amniotes. Prof. S. D. Rathod, B. N. Bandodkar College of Science, Thane -400605
Notes: Zoology- VI Semester, University of Mumbai, India E. Mesonephros: The mesonephros is one of three excretory organs that develop in vertebrates. It serves as the main excretory organ of aquatic vertebrates and as a temporary kidney in reptiles, birds, and mammals. A long portion of the intermediate mesoderm posterior to the pronephric tissue develops to form the more advanced mesonephric kidney. The development of the pronephric duct proceeds in a cranialto-caudal direction. As it elongates posteriorly, the pronephric duct induces nearby intermediate mesoderm in the thoraco-lumbar area to become epithelial tubules called mesonephric tubules. Each mesonephric tubule receives a blood supply from a branch of the aorta, ending in a capillary tuft analogous to the glomerulus of the definitive nephron. The mesonephric tubule forms a capsule around the capillary tuft, allowing for filtration of blood. This filtrate flows through the mesonephric tubule and is drained into the continuation of the pronephric duct, now called the mesonephric duct or Wolffian duct. The nephrotomes of the pronephros degenerate while the mesonephric duct extends towards the most caudal end of the embryo, ultimately attaching to the cloaca. The mammalian mesonephros is similar to the kidneys of aquatic amphibians and fishes. Once the more complex mesonephros forms the pronephros undergoes apoptosis (degeneration) in amphibians. In fishes the nephron degenerates but the organ remains and becomes a component of the immune system. Jawed fishes & amphibians - among males, some anterior tubules of mesonephros conduct sperm from testis to mesonephric duct called as sexual kidney while the rest is the uriniferous kidney. Amniote embryos mesonephros functions for a short time after hatching or birth during a new kidney called the metanephros is developing. The gonads, ovary or testis, also develop in the intermediate mesoderm. They originally form as swellings that lie just ventral to the anterior mesonephric kidney. A mullarian duct also develops in the intermediate mesoderm near the mesonephric duct. Prof. S. D. Rathod, B. N. Bandodkar College of Science, Thane -400605
Notes: Zoology- VI Semester, University of Mumbai, India F. Metanephros: Reptiles, birds, and mammals develop a metanephric kidney and lose the mesonephric kidney in the process of early development. The metanephric kidney begins as a small outgrowth from the more posterior end of the mesonephric duct. This is initially called a metanephric bud. Later, as the nephric tissue develops around this new duct a new kidney structure forms. The metanephric duct becomes the ureter. In both males and females the ureter eventually becomes separated from the mesonephric duct. As males develop the mesonephric duct makes connection with the testis as the primary sperm conducting duct, and the mullerian duct is lost. In some fish species the testis secondarily grows a separate sperm duct as the embryo develops. In females the mullerian duct becomes the passage for eggs. The females of fish and amphibians retain the mesonephric duct as a urinary duct. In reptiles, birds, and mammals (amniotes) the metanephric kidney replaces the mesonephric kidney. The duct of the mesonephros forms the Wolffian duct and ureter of the adult kidney. Subsequently, the mesonephric kidneys become small vestigial remnants. Prof. S. D. Rathod, B. N. Bandodkar College of Science, Thane -400605
Notes: Zoology- VI Semester, University of Mumbai, India PART-II: Structure of nephron, urogenital ducts, Urinary bladder in vertebrates Vertebrate kidneys consist of glomeruli, tubules surrounded by peritubular capillaries, & longitudinal ducts. Variations in kidney structure among vertebrates are primarily in the number & arrangement of the glomeruli & tubules. Structure of nephron: In pronephros: 1st embryonic tubules in all vertebrates; called pronephric tubules because they are the 1st to develop & are anteriorly located. Number - never very many (e.g., 3 in frogs, 7 in human embryos, & 12 in chicken embryos). The duct that drains the pronephros is called the pronephric duct. The Prof. S. D. Rathod, B. N. Bandodkar College of Science, Thane -400605
Notes: Zoology- VI Semester, University of Mumbai, India pronephros is temporary & functional only until mesonephric glomeruli & tubules further back become functional. Glomeruli of some pronephros, lack Bowman’s capsule and peritoneal funnel and hence known as external glomeruli. In certain examples the glomeruli unite to form a complex glomus into the large cavities known as pronephric chambers. The uriniferous tubules open into a common pronephric duct which runs posterior to join cloaca. Structure of nephron: In mesonephros: In mesonephric kidney nephrons are formed by corpuscles & tubules that develop caudal to pronephric region; form connections with existing pronephric duct (which is now called the mesonephric duct). Paired segmental uriniferous tubules, each with peritoneal funnel open into coelomic cavity. The glomerulus in mesonephros is enclosed in bowman’s capsules (internal glomerulus). The mesonephric tubules later undergo secondary division and form numerous tubules and thus the segmental arrangement is lost. The secondary mesonephric tubules are lack of peritoneal funnel. In some fishes the kidney lacks the malpighian bodies and thus called as aglomerular kidney. It is the functional adult kidney in fish & amphibians (& sometimes called the opisthonephros) and embryonic kidney in reptiles, birds, & mammals. Structure of nephron: In Metanephros: It is an adult amniote kidney. It is formed from the posterior most nephrotomes behind the embryonic mesonephros. A tubular outgrowth arises near the cloaca which gradually grows into a new metanephric duct finally working as ureter. The distal part swells and develops nephrotome of metanephros giving out thousands of uriniferous tubules. The intermediate mesoderm undergoes a series of branching to form the collecting duct system of the kidney. It also forms the major and minor calyces and the renal pelvis. Mammalian kidneys are divided into the cortex (5), medulla (6), & pelvis (4): Cortex - contains renal corpuscles & lots of capillaries. Medulla - contains collecting ducts and loops of Henle; divided into pyramids (7) & columns (2). Pelvis - hollow; receives the urine (which exits the kidney via the ureter (3). Prof. S. D. Rathod, B. N. Bandodkar College of Science, Thane -400605
Notes: Zoology- VI Semester, University of Mumbai, India The metanephric tubules become long and much coiled structures. Each kidney tubule of mammals is composed of the following parts: Proximal convoluted tubule Loop of Henle with ascending and descending portions Distal convoluted tubule. These have bowman’s capsules enclosing glomeruli. Metanephric uriniferous tubules are lack of peritoneal funnels and thus have no connection with coelomic cavity. The number of corpuscles is large; up to about 4.5 million is some species. It is drained by a duct called the metanephric duct or ureter. Tubules of mammalian kidney have U-shaped Loops of Henle (avian kidney has very short loops & reptilian kidney has no loops). Metanephros indicates an isolation of excretory organ from reproductive system and hence exhibits the advancement in the evolution of amniotes. Structure of Male Urogenital Ducts: Some fishes (e.g., gar & sturgeon) & amphibians - mesonephric duct transmits sperm & urine. Sharks - mesonephric duct is used primarily for sperm transport; accessory urinary duct develops. Some amphibians - mesonephric duct transports only sperm; new accessory urinary duct drains the kidney. Teleosts - mesonephric duct drains kidney; separate sperm duct develops. Amniotes - embryonic mesonephric ducts transport sperm in adults. Prof. S. D. Rathod, B. N. Bandodkar College of Science, Thane -400605
Notes: Zoology- VI Semester, University of Mumbai, India Intromittent organs: Useful when fertilization is internal; introduce sperm into female reproductive tract. Found in some fish, some birds, reptiles, & mammals. Cartilaginous fish - appendages of pelvic fins called claspers direct sperm into female reproductive tract. Structure of Female Urogenital Ducts: Typically consists of a pair of oviducts that extend from ostia to the cloaca. Different segments of oviducts perform special functions including internal fertilization. Cartilaginous fish - 2 ostia fuse to form single ostium (or osteum); shell gland secretes albumen & a shell; uterus holds eggs until laying. Teleosts - ducts are continuous with cavity of the ovary. Lungfish & amphibians - oviducts long & convoluted; lining secretes jelly-like material around each egg. Crocodilians, some lizards, & nearly all birds- 1 coiled oviduct lined with glands that add albumen, shells, &, sometimes, pigment. Prof. S. D. Rathod, B. N. Bandodkar College of Science, Thane -400605
Notes: Zoology- VI Semester, University of Mumbai, India Monotremes - tract is reptilian; caudal end secretes a shell before egg passes into the cloaca. Placental mammals - embryonic ducts give rise to oviducts, uteri, & vaginas. Adult tract is paired anteriorly & unpaired posteriorly (typically terminating as an unpaired vagina). Oviducts (fallopian tubes) are relatively short, small in diameter, convoluted, & lined with cilia; begin at ostium bordered with fimbria or cilia. Uterus and Vagina: Marsupials - no fusion of embryonic ducts so there are 2 tracts (duplex uterus) Other placental mammals - varying degrees of fusion: Bipartite uterus - 2 uterine horns, a uterine body (with 2 lumens), & a single vagina Bicornuate uterus - 2 uterine horns, a uterine body (with a single lumen), & a single vagina Simplex uterus - no uterine horns & oviducts open directly into body of uterus Vagina - fused terminal portion of oviducts that opens either into urogenital sinus or to the exterior; receives male intromittent organ. Prof. S. D. Rathod, B. N. Bandodkar College of Science, Thane -400605
Notes: Zoology- VI Semester, University of Mumbai, India Prof. S. D. Rathod, B. N. Bandodkar College of Science, Thane -400605
Notes: Zoology- VI Semester, University of Mumbai, India Structure of Urinary bladders & Cloaca: Urinary bladders are found in all vertebrates except agnathans, snakes, crocodilians, some lizards, & birds (except ostriches). Fish - bladders are terminal enlargements of the mesonephric ducts called tubal bladders. Tetrapod urinary bladder: Amphibians through Mammals - bladders arise as evaginations of ventral wall of the cloaca. The ureters attach to the urinary bladder, which is a derivative of the allantois in mammals, not an outgrowth from the cloacal wall. There is no cloaca in therian mammals. The urinary bladder drains into the urethra to release urine when desired rather than continuously as it is formed. Uses of urine: Reproduction (e.g., providing males with information concerning the reproductive status of a female) Behavioral (e.g., marking territories) Moisten soil (some freshwater turtles use urine to soften the ground and make it easier to dig holes for egg-laying). Cloaca: In vertebrates, common chamber and outlet into which the intestinal, urinary, and genital tracts open. It is present in amphibians, reptiles, birds, some fishes (e.g., sharks), and monotreme mammals but is absent in placental mammals and most bony fishes. Certain animals (e.g., many reptiles and some birds, including ducks) have an accessory organ (penis) within the cloaca that is used to direct the sperm into the female's cloaca. Most birds mate by joining their cloacas in a “cloacal kiss”; muscular contractions transfer the sperm from the male to the female. Receives digestive, reproductive, and urinary products and tracts No cloaca Fish – may have three separate openings Lost in mammals above monotremes Prof. S. D. Rathod, B. N. Bandodkar College of Science, Thane -400605
Notes: Zoology- VI Semester, University of Mumbai, India Cloaca subdivisions: Coprodaeum : Receives alimentary canal Urodaeum : Receives urinary and reproductive products Proctodaeum : Associated with excretory Figure: Subdivisions of cloaca shown in a lizard. Figure: Subdivisions of cloaca shown in a Bird. Prepared by Mr. S. D. Rathod Associate Professor Department of Zoology B. N. Bandodkar College of Science, Thane Prof. S. D. Rathod, B. N. Bandodkar College of Science, Thane -400605
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