Urinary System PDF

Urinary system Learning objectives • Understand the three stages in kidney development: pronephros, mesonephros and metanephros. • Describe the origin of the structures responsible for urine production and collecting ducts. • Consider briefly some abnormalities in the development of the urinary...

Urinary system Learning objectives • Understand the three stages in kidney development: pronephros, mesonephros and metanephros. • Describe the origin of the structures responsible for urine production and collecting ducts. • Consider briefly some abnormalities in the development of the urinary system. Origin of the urinary system The urinary system is considered a derivative of the intermediate mesoderm which is situated between the paraxial mesoderm (somites) and the lateral mesoderm (parietal and visceral mesoderm bordering the coelom). The intermediate mesoderm forms a urogenital ridge, consisting of a laterally-positioned nephrogenic cord (it will become the kidneys and ureters) and a mediallypositioned gonadal ridge (for gonads and genital tract formation). Because of their common embryological origin, the urinary and genital organs share some common structures and pathways, and together can be referred to as the urogenital system. From each bilateral nephrogenic cord, three generations of kidneys develop chronologically, in cranial-caudal sequence. These excretory structures are referred to as the pronephros, mesonephros, and metanephros and they exhibit increasing complexity and refinement in their components and functions. The pronephros is the most basic of the three excretory organs; it is limited to the embryonic stage of fish and amphibians; in mammals, pronephric structures are vestigial and not functional, except in sheep. The mesonephros becomes the adult kidney in fish and amphibians; it is also the functional kidney in most embryos of reptiles, birds and mammals, but at a certain stage of development the mesonephros is gradually replaced by the metanephros; it forms the definitive kidney in reptiles, birds and mammals. https://sway.office.com/oTBh1KcHz7k39KaQ#content=33K6mdDQ8QJ59y 1 - Development of the urinary system Although the functions of the urinary and the genital systems are unrelated, their development is intimately interwoven and often use common ducts. They develop from the intermediate mesoderm, which causes a longitudinal swelling that is referred to as the urogenital ridge. Kidney development proceeds through three successive phases of nephrogenic primordia: pronephros, mesonephros and metanephros. Although kidneys are functional in-utero, they may not be essential in certain mammalian embryos because the placenta is able to remove toxic agents from foetal blood. In such cases, the role of the mesonephros and metanephros can be supplementary for the foetus. Basically, the development of the kidneys requires the formation of two components: • • Filtration system. It is made of nephric functional units capable of selective plasma filtration. As evolution progressed, these functional units developed from simple structures to highly complex and efficient units called nephrons. These nephric units share two basic parts: ◦ Glomerulus (glomeruli in plural). It is a network of capillaries for increasing the surface area for plasma filtration. ◦ Nephric tubules. They receive the plasma filtrate from the glomerulus in the glomerular capsules. The nephrics tubules return desirable constituents to the blood while they convey the undesirable molecules towards the collecting system. Collecting system. It is referred to a set of structures for collecting urine from the nephric tubules and transporting it out of the body. 2 - Nephrons are composed of glomeruli that filter the blood to the Bowman's glomerular capsule; the primary urine is convey by tubular structures referred to as the nephric tubules that reabsorb and concentrate primary urine. The collecting ducts recieve the urine from the nephric ducts and transfers it into the bladder. The pronephros and the mesonephros share a similar developmental process that can be briefly summarized as follow: — The mesoderm of the nephrogenic cord undergoes segmentation. — Each nephrogenic segment forms a nephric tubule. One end of each tubule forms a cuplike outgrowth, called the glomerular capsule, into which a knot of capillaries known as glomerulus is pushed. The other end fuses with the final part of adjacent tubules to give rise to a common nephric duct which grows caudally toward the cloaca. Pronephros The pronephros represents the first stage of kidney development. In most domestic species, pronephros only develop briefly as immature structures that fail reach a functional stage. The mesonephros consists of 7-8 rudimentary pronephric tubules adjacent to a pronephric duct that grows caudally to the cloaca. Only in sheep, pronephric tubules become quite well develoed and connect to the mesonephric duct. The pronephric tubules soon degenerate, except for the pronephric duct, which is used by the mesonephros to become the functional mesonephric duct. https://sway.office.com/oTBh1KcHz7k39KaQ#content=TgNZlTOrXxBFwu 3 - The pronephros is the most basic of the three generations of renal primordia. It develops briefly at the level of cervical region where the intermediate mesoderm forms 7-8 pairs of pronephric tubules. However, in most species, these structures do not become functional and soon atrophy, which coincides with the development of the mesonephros. Mesonephros The mesonephros represents the second stage of kidney development and becomes the functional adult kidney in most fish and amphibians. It is made up of an increased number of functional units called mesonephric nephrons. It consists of dozens to hundreds (usually 70-80) mesonephric tubules which drain to a common mesonephric duct (former pronephric duct) that open into the cloaca. One end of each tubule forms the glomerular capsules that surround a tuft of capillaries called the glomerulus that is supplied by lateral branches of the dorsal aorta. The other end of the tubule communicates with the mesonephric duct. Eventually, the mesonephros degenerates and it is replaced by the metanephros, but some mesonephric remnants will be integrated into the development of the adrenal gland and male genital system. https://sway.office.com/oTBh1KcHz7k39KaQ#content=mD2fSolLEilUPC 4 - Development of the mesonephros Metanephros In reptiles, birds and mammals the mesonephros is succeeded by the metanephros, which develops as the adult kidney. It consists of a substantially increased number of nephrons, usually from thousands to millions. The metanephros initiates its development in the pelvic region but moves cranially into the abdomen during the growth of the embryo. An ectopic kidney may remain in the pelvis, near the bladder or it may cross over and become fused with the other kidney. Horseshoe kidney is a congenital disorder with renal fusion in the midline. The metanephros originates from two sources: 1. The Ureteric bud, which grows out of the mesonephric duct near the cloaca; the ureteric bud gives rise to the definitive urinary collecting system: ureter, renal pelvis, calyxes and numerous collecting ducts. Hydronephrosis (literally "water inside the kidney") refers to distension and dilation of the collecting system, usually caused by obstruction of the free flow of urine from the kidney. It can be acquired (e.g. kidney stone) or congenital (e.g. ureter atresia). 2. The Metanephric blastema, which is a mass of intermediate mesoderm at the caudal region of the nephrogenic cord. The metanephric blastema is the origin of the nephrons. ◦ Nephron formation. Metanephric mesoderm adjacent to the collecting tubules proliferates to form cellular aggregates. These clusters of mesodermal cells go through a series of stages. Initially, they are spherical solid masses of cells; then they become hollow vesicles, finally, the vesicle elongates forming S-shaped metanephric tubules which open into the collecting tubules. The elaborated differentiation of the metanephric tubules leads to the formation of the functional metanephric units or nephrons. One end of each metanephric tubule expands to form a cup-shaped glomerular capsule (Bowman´s capsule) which surrounds a capillary glomerulus. The capsule and the glomerulus are collectively known as the renal corpuscle. The metanephric tubule continues to elongate forming a U-shaped bend called the loop of Henle. The portion of the tubule adjacent to the capsule becomes coiled and is referred to as the proximal convoluted tubule while the more distal portion of the tubule, which also coils, is known as the distal convoluted tubule. Distal convoluted tubules joint to the collecting ducts to become confluent. Polycystic kidney disease is a congenital genetic disorder in which the renal tubules become structurally abnormal resulting in the development and growing of multiple cysts within the kidneys. The cysts vary in size and, as they accumulate more fluid, they can grow very large. Initially, the ramification of the collecting system within the nephric blastema causes the structure and surface of the kidney to be lobulated. Species-specific variations in the fusion of the foetal lobes result in macroscopic differences in the appearance of the mature kidney. In most mammals (ruminants are the exception), towards the end of the foetal period, the kidney surface is considerably smoothed and the multilobar appearance disappears over postnatal development. Accordingly, the anatomical shape of the adult kidneys ranges from unilobar to distinct multilobar structures, with intermediate gradations of partial and complete lobar fusion. https://sway.office.com/oTBh1KcHz7k39KaQ#content=V4ZUybf4GQKKw2 5 - Development of the metanephros Urogenital sinus and its derivatives The partition of the cloaca led to the formation of the urogenital sinus which receives the openings of the urogenital ducts. Cranially, the urogenital sinus connects with the umbilical portion of the allantois, the allantoic stalk also called the urachus. In this way, urine is initially discharged into the allantoic cavity through the urachus. Caudally, the urogenital sinus is temporarily shut off from the exterior by a urogenital membrane formed by the apposition of the ectoderm and endoderm. At a certain stage of development, the urogenital membrane degenerates leaving the urogenital sinus in direct communication with the amniotic cavity. https://sway.office.com/oTBh1KcHz7k39KaQ#content=dWuTWe0cvpAdah 6 - Development of the bladder and the urethra from the urogenital sinus After birth, the lumen of the urachus usually obliterates; thereby, the urogenital orifice remains as the sole exit for urine in the new-born. If the urachus fails to close at birth, it results in a patent urachus, also called urachal fistula, in which urine continues to flow through the umbilicus. This congenital condition can be also associated with the formation of urine-filled dilations (urachal cysts) that predispose to chronic cystitis. 7 - Diagram illustrating different types of urachal anomalies The urogenital sinus differentiates into the urinary bladder and the urethra. The urinary bladder develops from the cranial end of the urogenital sinus and the adjacent region of the urachus. Differential growth and expansion of the dorsal wall result in separate openings for the mesonephric duct (future male sex duct) and for the ureter. This dorsal part of the urogenital sinus is called the trigone, a smooth trigonal region that anchors the openings of the ureters into the urinary bladder and the sex ducts into the beginning of the urethra. Ectopic ureter is a congenital anomaly in which the ureter opens into the urethra or vagina instead of the bladder; it is a source of congenital incontinence because urine is delivered beyond the vesical urinary sphincter. https://sway.office.com/oTBh1KcHz7k39KaQ#content=XuYUFKlE0g6Snz 8 - Separation of the metanephric duct (ureter) and the mesonephric duct (Wolfian ducts). A pair of paramesonephric duct (Müllerian ducts) complete the set of tubes that open into the urogenital sinus in the early embryo. The urethra develops from the urogenital sinus, caudal to the urinary bladder. Urethra development is gender-specific: • In females, the cranial region of the urogenital sinus becomes the urinary bladder; the midregion of the urogenital sinus becomes the urethra; the caudal region of the urogenital sinus becomes the vestibule and part of the vagina. • In males, the cranial region of the urogenital sinus becomes the urinary bladder; the mid and caudal region of the urogenital sinus becomes the pelvic urethra. The pelvic urethra is prolonged by an additional part, the penile urethra, which is formed inside the penis.