Development of the Urinary System PDF

Development of the urinary system Urinary system and Genital system Both develop from a common mesodermal ridge (intermediate mesoderm) Objectives 1.Describe the three sets of excretory organs. a.The pronephron. b.The mesonephros and its duct. c.The metanephros, the permanent kidney. 2.Desc...

Development of the urinary system Urinary system and Genital system Both develop from a common mesodermal ridge (intermediate mesoderm) Objectives 1.Describe the three sets of excretory organs. a.The pronephron. b.The mesonephros and its duct. c.The metanephros, the permanent kidney. 2.Describe the positional changes of the kidneys and understand how congenital anomalies of position and renal vessels occur. 3.Describe the formation of the urinary bladder and urethra. 4.Explain the embryological basis for common congenital disorders of the urinary tract, e.g. horseshoe kidney, ectopic urethral orifices, exstrophy of the bladder and urachal anomalies Kidney Systems 1.The pronephros ,the first of these systems is rudimentary and nonfunctional. 2.Mesonephros, the second may function for a short time during the early fetal period. 3.Metanephros, the third forms the permanent kidney.  Pronephros 1.At the beginning of the fourth week, the pronephros is represented by 7 to 10 solid cell groups in the cervical region. 2.These groups form vestigial excretory units, nephrotomes, that regress before more caudal ones are formed. 3.The internal and external glomeruli and nephric tubule are formed and the last two open into the intraembryonic cavity. 4.By the end of the fourth week, all indications of the pronephric system have disappeared. ! The mesonephros which forms in the thoracic and lumbar regions, is large and is characterized by excretory units (nephrons) and its own collecting duct, the mesonephric or wolffian duct. In the human, it may function briefly.In the male, a few of the caudal tubules and the mesonephric duct persist and participate in formation of the genital system, but they disappear in the female. ! Metanephros: The Definitive Kidney ! The third urinary organ, the metanephros, or permanent kidney, ! appears in the fifth week. ! Its excretory units develop from metanephric mesoderm ! Collecting System ! develop from the ureteric bud, an outgrowth of the mesonephric duct close to its entrance to the cloaca The bud penetrates the metanephric tissue,Subsequently, the bud dilates, forming the primitive renal pelvis, and splits into cranial and caudal portions, the future major calyces ! Each calyx forms two new buds while penetrating the metanephric tissue. These uretric buds continue to subdivide until 12 or more generations of tubules have formed ! Meanwhile, at the periphery, more tubules form until the end of the fifth month. ! The tubules of the second order enlarge and absorb those of the third and fourth generations, forming the minor calyces of the renal pelvis. ! During further development,collecting tubules of the fifth and successive generations elongate considerably and converge on the minor calyx, forming the renal pyramid. The ureteric bud gives rise to the ureter, the renal pelvis, the major and minor calyces, and approximately 1 to 3 million collecting tubules. Excretory System Each newly formed collecting tubule is covered by a metanephric tissue cap cells of the tissue cap form small vesicles, the renal vesicles, which in turn give rise to small S-shaped tubules. ! Capillaries grow into the pocket at one end of the S and differentiate into glomeruli. These tubules, together with their glomeruli, form nephrons, or excretory units. ! The proximal end of each nephron forms Bowman's capsule, which is deeply indented by a glomerulus The distal end forms an open connection with one of the collecting tubules, establishing a passageway from Bowman's capsule to the collecting unit. Continuous lengthening of the excretory tubule results in formation of the proximal convoluted tubule, loop of Henle, and distal convoluted tubule so the kidney develops from two sources: (a) metanephric mesoderm, which provides excretory units and (b) the ureteric bud, which gives rise to the collecting system. ! Nephrons are formed until birth, at which time there are approximately 1 million in each kidney. ! Urine production begins early in gestation, soon after differentiation of the glomerular capillaries, which start to form by the 10th week. ! At birth, the kidneys have a lobulate appearance, ! but the lobulation disappears during infancy as a result of further growth of the nephrons, although there is no increase in their number. Renal dysplasias and agenesis are a spectrum of severe malformations that represent the primary diseases requiring dialysis and transplantation in the first years of life. Multicystic dysplastic kidney is one example of this group of abnormalities in which numerous ducts are surrounded by undifferentiated cells. 1. Nephrons fail to develop 2. Ureteric bud fails to branch, 3. The collecting ducts never form. In some cases, these defects cause involution of the kidneys and renal agenesis ! Renal agenesis, may be unilateral (incidence ~ 1/1000 births, can be asymptomatic). ! or bilateral ! which occurs in 1/10,000 births, results in renal failure. The baby presents with Potter sequence, ! characterized by 1. anuria, 2. oligohydramnios (decreased volume of amniotic fluid), 3. and hypoplastic lungs secondary to the oligohydramnios. 4. In 85% of cases, other severe defects, including absence or abnormalities of the vagina and uterus, vas deferens, and seminal vesicles, accompany this condition. ! congenital polycystic kidney disease ! numerous cysts form. It may be inherited as an A. autosomal recessive B. autosomal dominant disorder C. or may be caused by other factors. Autosomal recessive polycystic kidney disease, which occurs in 1/5,000 births, is a progressive disorder in which cysts form from collecting ducts. The kidneys become very large, and renal failure occurs in infancy or childhood. In autosomal dominant polycystic kidney disease, A. cysts form from all segments of the nephron and B. usually do not cause renal failure until adulthood. C. The autosomal dominant disease is more common (1/500 to 1/1,000 births D. but less progressive than the autosomal recessive disease. ! Duplication of the ureter results from early splitting of the ureteric bud.Splitting may be partial or complete, and metanephric tissue may be divided into two parts, each with its own renal pelvis and ureter. ! In rare cases, one ureter opens into the bladder, and the other is ectopic, entering the 1-vagina,2-urethra, 3-or vestibule Position of the Kidney initially in the pelvic region, later shifts to in the abdomen. Migration includes (i) cranial shift from L4 to L1/T12, (ii) lateral displacement (meeting up with the adrenal glands ) and (iii) a 90° rotation so the renal pelvis faces the midline. This ascent of the kidney is caused by 1) diminution of body curvature 2) and by growth of the body in the lumbar and sacral regions in the pelvis, the metanephros receives its arterial supply from a pelvic branch of the aorta. During its ascent to the abdominal level, it is vascularized by arteries that originate from the aorta at continuously higher levels. The lower vessels usually degenerate, but some may remain Clinical Correlates Abnormal Location of the Kidneys During their ascent, the kidneys pass through the arterial fork formed by the umbilical arteries, but occasionally, one of them fails to do so. Remaining in the pelvis close to the common iliac artery, it is known as a pelvic kidney Sometimes, the kidneys are pushed so close together during their passage through the arterial fork, that the lower poles fuse, forming a horseshoe kidney The horseshoe kidney is usually at the level of the lower lumbar vertebrae, since its ascent is prevented by the root of the inferior mesenteric artery ! Horseshoe kidney is found in 1/600 people. ! Accessory renal arteries are common; ! These arteries usually arise from the aorta and enter the superior or inferior poles of the kidneys. ! Function of the Kidney ! The definitive kidney formed from the metanephros becomes functional near the 12th week. ! Urine is passed into the amniotic cavity and mixes with the amniotic fluid. ! The fluid is swallowed by the fetus and recycles through the kidneys. ! During fetal life, the kidneys are not responsible for excretion of waste products, since the placenta serves this function Bladder and Urethra During the 4th-7th weeks of development, the cloaca divides into the 1) urogenital sinus anteriorly 2) and the anal canal posteriorly The urorectal septum is a layer of mesoderm between the primitive anal canal and the urogenital sinus. The tip of the septum will form the perineal body Three portions of the urogenital sinus can be distinguished: 1st The upper and largest part is the urinary bladder Initially, the bladder is continuous with the allantois, but when the allantois is obliterated, a thick fibrous cord, the urachus, remains and connects the apex of the bladder with the umbilicus). In the adult, it forms the median umbilical ligament 2nd part ,the pelvic part of the urogenital sinus, which in the male gives rise to the prostatic and membranous parts of the urethra. 3rd part, the last part is the phallic part of the urogenital sinus. It is flattened first and as the genital tubercle grows, it will be pulled ventrally). It differs greatly between the two sexes.) The ureters - 1.The caudal portions of the mesonephric ducts are absorbed into the wall of the urinary bladder Consequently, the ureters, initially outgrowths from the mesonephric ducts, enter the bladder separately. - 2.As a result of ascent of the kidneys, the orifices of the ureters move farther cranially; those of the mesonephric ducts move close together to enter the prostatic urethra and in the male become the ejaculatory ducts. - 3.With time, the mesodermal lining of the trigone is replaced by endodermal epithelium, so that finally, the inside of the bladder is completely lined with endodermal epithelium. ! Urethra !The epithelium of the urethra in both sexes originates in the endoderm; ! the surrounding connective and smooth muscle tissue is derived from visceral mesoderm. ! At the end of the3rd month, ! In male: epithelium of the prostatic urethra begins to proliferate and forms prostate gland ! In the female, ! the cranial part of the urethra gives rise to the urethral and paraurethral glands.  Clinical Correlates Bladder Defects ! Urachal fistula.When the lumen of the intraembryonic portion of the allantois persists, cause urine to drain from the umbilicus. ! Urachal cyst.If only a local area of the allantois persists, secretory activity of its lining results in a cystic dilation. ! Urachal sinus. When the lumen in the upper part persists, it forms a urachal sinus. This sinus is usually continuous with the urinary bladder. ! Exstrophy of the bladder is a ventral body wall defect in which the bladder mucosa is exposed. Exstrophy of the bladder is probably due to failure of the lateral body wall folds to close in the midline in the pelvic region ! This anomaly is rare, occurring in 2/100,000 live births. Thank you