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Chapter 5 ESSENTIALS OF HUMAN HISTOLOGY URINARY SYSTEM Urinary system is concerned with: ❖ Formation of urine ❖ Removal of waste products ❖ Production of some hormones ❖ Regulation of fluid and electrolytes balance of the bod...

Chapter 5 ESSENTIALS OF HUMAN HISTOLOGY URINARY SYSTEM Urinary system is concerned with: ❖ Formation of urine ❖ Removal of waste products ❖ Production of some hormones ❖ Regulation of fluid and electrolytes balance of the body It is formed of: 1- Two kidneys 2- Two ureters 3- Urinary bladder 4- Urethra KIDNEY - The main excretory organ in the body. - Each kidney is bean shaped with 11cm long, 5cm wide and 2cm thick. - Each kidney is surrounded with tough CT capsule that can be easily stripped off. - Outside the capsule there is a mass of peri-renal fat, which supports kidney. - Each kidney has a concave medial border ( hilum) that leads to a concavity in the kidney substance called the renal sinus. - Within the sinus lie renal vessels, lymphatics, sympathetic nerves, pelvis of ureter and adipose tissue. - Renal pelvis is divided into 2-3 major calyces, each of which is subdivided into 2-3 minor calyces (Figure 5-1). 102 Chapter 5 ESSENTIALS OF HUMAN HISTOLOGY - Longitudinal section of kidney reveals differentiation of the substance into outer dark cortex and inner pale medulla (Figures 5-1&2). ▪ Cortex: granular due to convolution of proximal and distal convoluted tubules ▪ Medulla: appears radially striated due to presence of loops of Henle and collecting tubules. ▪ Medulla: formed of conical masses known as triangular medullary pyramids with their bases directed towards cortex and their apices directed towards the hilum forming renal papillae. ▪ Medullary projections extending from the bases of the pyramids (medullary rays) into the cortex. ▪ Each kidney is divided into lobes and lobules. - Renal lobe: consists of one medullary pyramid and the corresponding cortical tissues. - Renal lobule: formed of one medullary ray and the associated cortical tissues. - Connective tissue between renal lobes and lobules is scanty. - Human kidney is multilobular formed of 8-18 lobes. 103 Chapter 5 ESSENTIALS OF HUMAN HISTOLOGY Histological structure 1-Stroma: - Kidney is covered with fibrous connective tissue capsule and is surrounded by peri-renal fat. - Very little C.T. around blood vessels and a delicate reticular tissue is present between the parenchyma. 2-Parenchyma: - Compound tubular gland 104 Chapter 5 ESSENTIALS OF HUMAN HISTOLOGY - Formed of urinfrous tubules - highly vascular Urinefrous tubules consist of: a- Nephrons produce urine. b- Collecting tubules concentrate urine. NEPHRON - Structural and functional unite of the kidney. - Each kidney is formed of 1-3 million nephrons. - Two or three nephrons can be drained by one collecting tubule that join others to form duct of Bellini. - Each nephron consists of (Figure 5-3): 1- Malpighian corpuscle (Renal corpuscle) 2- Proximal convoluted tubule 3- Loop of Henle 4- Distal convoluted tubule 105 Chapter 5 ESSENTIALS OF HUMAN HISTOLOGY 1- Malpighian corpuscle (Renal corpuscle) - Present in the cortex. - Formed of: a) Glomerulus (tortuous tuft of capillaries) b) Bowman's capsule (epithelial capsule covering the glomerulus) - Has two poles; vascular and urinary. - Vascular pole: the site where afferent arteriole enters and efferent arteriole leaves. - Urinary pole: the site where the proximal convoluted tubule begins. 106 Chapter 5 ESSENTIALS OF HUMAN HISTOLOGY a) Glomerulus: ▪ a tuft of anastomosing capillaries formed from afferent arteriole that enters Malpigian corpuscle at its vascular pole and gives capillary loops of the glomerulus and unite to form efferent arteriole. ▪ Glomerular capillaries: lined by fenestrated endothelium that has no diaphragm and rests on thick basement membrane 300nm thick. - Basement membrane is formed of three layers; the middle is the lamina densa formed of collagen type IV. the outer layer (less electron dense), called lamina rara externa. the inner layer ( less electron dense), called lamina rara interna. ▪ In between loops of capillaries there are cells called intraglomerular mesangial cells that give support to capillaries in the places where the 107 Chapter 5 ESSENTIALS OF HUMAN HISTOLOGY visceral layer of Bowman’s capsule (podocytes) does not come in contact with the capillaries. b) Bowman's capsule: - Formed of two layers. ▪ The outer parietal layer: lined with simple squamous epithelium. ▪ The inner visceral layer: lined with modified cells called podocytes which are adherent to glomerular capillaries. - The space between parietal and visceral layers is called capsular space: receives glomerular filtrate and is continuous with the lumen of proximal convoluted tubule. Podocyte: - L/M: ▪ Star shaped with multiple processes. ▪ Basement membrane: well developed and can be demonstrated as PAS +ve line (formed of glycoprotein). - E/M (Figure 5-4): ▪ A large cell that consists of body, primary processes (major) and secondary processes (minor). ▪ The cell body consists of cytoplasm and central nucleus with extended chromatin. ▪ The cytoplasm contains mitochondria, Golgi body, rough endoplasmic reticulum, microtubules and microfilaments. ▪ The cell body gives rise to several primary processes that extend parallel to long axis of blood capillaries. ▪ Each primary (major) process gives rise to numerous secondary (minor) processes called pedicles that end in foot like structures implanted on the 108 Chapter 5 ESSENTIALS OF HUMAN HISTOLOGY basement membrane of glomerular blood capillaries. ▪ Pedicles (minor processes) from one podocyte embrace more than one capillary, i.e. the pedicles of two podocytes alternate in position on a single capillary. ▪ In between the foot process, there are filtration slits that are covered with diaphragm. ▪ Primary and secondary processes contain only microtubules & microfilaments. ▪ Podocytes are separated from glomerular capillaries by subpodocytic space. 109 Chapter 5 ESSENTIALS OF HUMAN HISTOLOGY - Function of Podocyte: ❖ Has an important role in the blood renal barrier. ❖ Regeneration of basement membrane. - Blood-renal barrier is formed of (Figure 5-4): 1. Fenestrated endothelium of blood capillaries. 2. Thick basement membrane: the only continuous layer of the filtration barrier. 3. Filtration slits (60-100nm) with their covering diaphragms. - Function of blood-renal barrier: Formation of the glomerular filtrate. A. When blood is filtered through the glomerular capillaries, plasma passes through the fenestrae holding back red blood corpuscles, white blood cells and blood platelets. B. Protein with high molecular weight (> 68,000 D) cannot pass through the thick basement membrane but protein with small molecular weight, sugar, amino acids can pass with the filtrate. C. Filtration slits and the overlying diaphragms prevent the passage of molecules according to their electrostatic charge. 2- Proximal convoluted tubule ❖ begins in the cortex at the urinary pole of renal corpuscle. ❖ at first highly convoluted then straightens to become continuous with descending thick segment of loop of Henle in the medulla. ❖ Each tubule is 60um in diameter and 14mm long with small lumen. ❖ lined with a single layer of pyramidal cells that rest on basement membrane. ❖ By L/M (Figure 5-6): cross section of proximal convoluted tubule shows: ▪ Lumen: narrow. ▪ formed of 4-5 pyramidal cells. 110  Chapter 5 ESSENTIALS OF HUMAN HISTOLOGY ▪ cells are strongly acidophilic ▪ nuclei are rounded and central. ▪ apical surface shows brush border. ▪ basal part has acidophilic striation. ▪ cell boundaries are indistinct. ❖ E/M shows that (Figure 5-7): apical surface has numerous long microvilli. basal part has basal infoldings (basal striation). Numerous mitochondria in between basal infoldings (which is the cause of acidophilia). Lateral cell membrane interdigitations (indistinct border by LM). N.B. These are the characters of ion transporting cells. Function of proximal convoluted tubules: 1. Reabsorption of 85% of sodium by active process and 85% of water by passive process through the luminal micorvilli. 2. Reabsorption of all glucose and all amino acids. 3. Reabsorption of low molecular weight proteins by pinocytosis and then 111 Chapter 5 ESSENTIALS OF HUMAN HISTOLOGY act on it by lysosomes to transfer it to amino acids. 4. Excretion of certain metabolites, dyes and drugs. 5. Getting ride of waste products like urea and uric acids in urine. - The end result is isotonic solution. 3- Loop of Henle: - U shaped tube present mainly in medulla. - Consists of four parts: 1. Thick descending part: ▪ Starts in the cortex and extends to the medulla. ▪ Similar to proximal convoluted tubule in structure and function. 2. Thin descending part: ▪ Present in the medulla. ▪ Lined with simple squamous epithelium ▪ Cross section of the thin limb is similar to blood capillary wall but their lumens contain no blood cells and their nuclei protrude slightly to the lumen (Figures 5-6 and 5-8). 112  Chapter 5 ESSENTIALS OF HUMAN HISTOLOGY ▪ The descending limb is highly permeable to water and impermeable to sodium, so urine becomes hypertonic. 3. Thin ascending part: ▪ In the medulla. ▪ lined with simple squamous epithelium. 4. Thick ascending part: ▪ Starts in the medulla and extends to the cortex. ▪ Similar to the distal convoluted tubule in structure and function. ▪ The thick ascending limb is permeable to sodium and impermeable to water, so urine becomes hypotonic N.B. the filtrate passes to distal convoluted tubules is hypotonic. 4- Distal convoluted tubules: - It is the continuation of thick ascending loop of Henle in the corticomedullary zone and then joins collecting tubules in the cortex. - By L/M (Figure 5-6) cross section of distal convoluted tubule shows: ▪ Lumen: wide. ▪ It is formed of 5-8 simple cubical small cells. ▪ Cells are acidophilic but less than PCT. 113  Chapter 5 ESSENTIALS OF HUMAN HISTOLOGY ▪ nuclei are rounded nearly apical. ▪ apical surface has no brush border. ▪ basal part has acidophilic striation. ▪ cell boundaries are more distinct than PCT. - E/M shows that: ▪ apical part has few short microvilli. ▪ basal part contains mitochondria inbetween basal infoldings. - Function of distal tubules: 1. Reabsorb 15% of sodium (active pump mechanism) under the control of aldosterone hormone of adrenal cortex and 15% of water in the convoluted part under the control of antidiuretic hormone (ADH). 2. Excrete hydrogen, ammonium and potassium ions. 3. Help in maintaining acid-base balance of the body. Juxtaglomerular apparatus Composed of (Figure 5-9): 1- Juxtaglomerular cells. 2- Macula densa. 3- Polar cushion or lacis cells or extraglomerular mesangial cells. 1- Juxtaglomerular cells: Modified smooth muscle cells of the tunica media of afferent arteriole. Large cubical with rounded nuclei Cytoplasm contains many secretory granules which are (PAS) +ve. By EM, it shows rER, Golgi and mitochondria. Internal elastic lamina is absent, so juxtaglomerular cells are in contact in one side with blood and the endothelium and with cells of macula densa on the 114 Chapter 5 ESSENTIALS OF HUMAN HISTOLOGY other side due to absence of basement membrane of macula densa. Function of Juxtaglomerular cells: i. Secretion of renin which converts plasma angiotensinogen into angiotensin I (mild vaso constrictor). Angiotensin I is then converted into angiotensin II (potent vaso constrictor) by the effect of converting enzyme in the lung. Angiotensin II stimulates production of aldosterone by adrenal cortex that acts directly on the distal convoluted tubules causing water and sodium retention and subsequently increasing the blood pressure. ii. Secretion of erythropoietin stimulates formation of RBC.s in bone marrow. 115 Chapter 5 ESSENTIALS OF HUMAN HISTOLOGY 2- Macula densa: ▪ Is the part of distal convoluted tubule in the concavity between afferent and efferent arterioles of the same nephron. ▪ Cells become columnar and their nuclei are packed. ▪ By E/M, cells have numerous microvilli and infranulear Golgi apparatus. ▪ Absence of basement membrane. ▪ Functions: It is sensitive to the chloride ion content of tubular fluid, producing signals that promote constriction of glomerular afferent arteriole, so it regulates the rate of glomerular filtrate. 3- Extraglomerular mesangial cells, (Polar cushion or Lacis cells): occupy the space between afferent and efferent arterioles and macula densa. With pale staining. Function: May be phagocytic COLLECTING TUBULES (Figures 5-3 and 5-6)  not a part of the nephron.  has different embryonic origin.  starts at the medullary rays in the cortex by union of two or three distal convoluted tubules.  The main collecting tubule enters the medullary pyramid.  Union of several medullary collecting tubules form straight papillary duct (duct of Bellini) that opens on the apex of renal papillae into a minor calyx.  Two to four minor calyces join to form a major calyx that opens in renal pelvis.  By L/M: o The tubule is lined with simple cubical epithelium (in small tubules) or simple columnar epithelium (in large tubules). 116 Chapter 5 ESSENTIALS OF HUMAN HISTOLOGY o Cytoplasm is pale acidophilic. o Cell borders are distinct. o Lumen is wide. o Nuclei are dark and central.  By E/M : Cytoplasm has few organelles. Interdigitations between the cells are not marked. Few microvilli and basal infoldings.  Function of collecting tubules: Water is reabsorbed under the control of antidiuretic hormone and so they concentrate the urine and conduct it to the calyces and the ureter. N.B: 117 Chapter 5 ESSENTIALS OF HUMAN HISTOLOGY ▪ Efferent arteriole connects two capillary beds ▪ The first arterial capillary plexus is glomerular capillary. ▪ The second arteriovenous capillary plexus is peritubular capillary. So this can be considered as arterial portal circulation. ▪ Function of: - the glomerular capillary is filtration of blood - the peritubular capillary is nutrition of the renal tubules. Excretory passages: Formed of minor calyces, major calyces, renal pelvis, ureter, urinary bladder and urethra. Urine is collected from ducts of Bellini to minor calyces that join major calyces and drain in renal pelvis. URETER The ureter starts from renal pelvis and ends in urinary bladder. It has thin wall and its lumen is stellate or star shaped. Histological structure (Figure 5-11): 1-Mucosa: is thrown into longitudinal folds and consists of: a- Epithelium: transitional b- Lamina propria: formed of dense CT, blood vessels and lymphatic nodules. 2- Muscle layer: formed of smooth muscles arranged as follows: in the upper two thirds of ureter inner longitudinal and outer circular in the lower third there is an additional outer longitudinal layer. 3-Adventitia: formed of fibroelastic CT. 118 Chapter 5 ESSENTIALS OF HUMAN HISTOLOGY URINARY BLADDER - The wall is thick and folded in empty bladder with wide lumen. - The folds disappear on distention. Histological structure (Figure 5-12): 1- Mucosa consists of: a) Epithelium: transitional ▪ a special type of stratified epithelium where the numbers of layers are changeable according to the state of the organ distended or empty. ▪ In empty bladder the epithelium has several layers (6-8 layers) ▪ In distended bladder the epithelium has two or three layers ▪ The superficial epithelial cells are large rounded (dome-shaped) with one 119 Chapter 5 ESSENTIALS OF HUMAN HISTOLOGY or two nuclei and bulge into the lumen. ▪ Superficial cells of the epithelium (Figure 5-13) have a thick membrane of thick plates (plaques) separated by narrow bands of thinner membrane that are responsible for osmotic barrier between urine and tissue fluid.When the bladder contracts the thinner regions are folded and the thicker plates are invaginated to form elliptical cytoplasmic vesicles that act as a reservoir when the bladder is distended. b) Lamina propria: formed of dense, then loose CT. 120 Chapter 5 ESSENTIALS OF HUMAN HISTOLOGY 2. Muscular layer: Thick smooth muscle fibers: arranged into inner and outer longitudinal and middle circular layers. 3. Adventitia: - dense connective tissue. - The upper surface of the bladder is covered by peritoneum. URETHRA Female urethra: - length : from 4-5 cm. - Extends from internal orifice at urinary bladder to external orifice above & anterior to vagina. Histological structure: 1-Mucosa: a) Epithelium: upper part is lined with transitional epithelium lower part is lined with stratified squamous non- keratinized epithelium. 121 Chapter 5 ESSENTIALS OF HUMAN HISTOLOGY b) Lamina propria: fibroelastic along its length. 2-Muscle layer: - in inner longitudinal and outer circular smooth muscle. - As the urethra pierces the uro-genital diaphragm, skeletal muscle forms an external sphincter around the urethra that gives voluntary control of micturition. Male urethra: - long (about 20 cm) tube - conducts urine from urinary bladder and seminal fluid from male genitalia to outside of the body. - Many glands open in the course of male urethra like bulbourethral gland and glands of littre. - Consists of three parts: 1– Prostatic urethra: ▪ present within prostate gland ▪ lined by transitional epithelium ▪ An elevation projects into its interior ▪ Ejaculatory ducts open at the sides of this elevation 2– Membranous urethra: lined by stratified and pseudo stratified columnar epithelium 3- Penile urethra: divided into two portions: a) Bulbous urethra:  passes through the corpus spongiosum  lined by pseudostratified or stratified columnar epithelium b) Pendulous urethra: passes through the glans penis and dilate distally forming fossa navicularis. Pendulus uethra is lined mostly by stratified squamous epithelium 122