A. Lobes - 10 to 18 per kidney arranged so that the apex of each lobe points towards the hilus. A distinct, easily removed connective tissue capsule surrounds the whole kidney. It is easily removed because there is little connective tissue between the nephrons. However fibroblasts are present and in inflammation there is increased connective tissue so that the capsule becomes adherent to the surface.

B. Hilus - a cleft-like opening into the renal sinus which is a deep indentation in the concave aspect of the kidney containing arteries, veins, nerves, fat, and renal pelvis.

C. Renal pelvis - the expanded upper end of the ureter into which a half dozen broad tubes (major calyces) empty. Each major calyx in turn receives 2 to 3 funnel-shaped minor calyces. The broad end of each minor calyx cups over the apex (papilla) of one lobe - the epithelium lining the minor calyx is continuous with the epithelium covering the apex of the papilla.

D. Vessels - the renal artery branches several times in the hilus and sinus. The branches pass around the major and minor calyces to enter the kidney parenchyma between lobes (interlobar arteries).

II. ORGANIZATION OF THE RENAL LOBE - each renal lobe is divided into cortex and medulla.

A. Cortex - caps over the medulla and extends between the lobes to form the renal columns of Bertin. It is subdivided into two parts.

1. Convoluted (pars convoluta) - consists of convoluted tubules (proximal and distal), renal corpuscles, interlobular arteries and veins, intralobular arteries and veins, and afferent and efferent arterioles. The pars convoluta surrounds medullary rays.

2. Straight (pars radiata) - consists of many bundles of straight tubules projecting from the base of the medullary pyramid toward the capsule. Each of these bundles (medullary rays) contains ascending and descending limbs of loops of Henle and branched collecting ducts.

B. Medulla (pyramid) - the medulla is shaped like a cone with its apex (papilla) directed toward the hilus and its base toward the capsule. The medulla consists of loops of Henle, vascular bundles arranged for countercurrent exchange (vasa recta), and collecting ducts; all relatively straight tubes. It is divided into two zones:

1. Outer zone - contains: 1) the lower parts of the loops of Henle (thick and thin) of nephrons arising in subcapsular and middle cortical areas; 2) thick descending and ascending and a portion of the thin segment of juxtamedullary nephrons; 3) vasa recta; and 4) collecting ducts. The straight segment of the proximal tubule is limited to the outer stripe of the outer zone while the straight segment of the distal tubule (thick ascending segment of the renal loop) extends through the entire outer zone - both the inner and outer stripes.

2. Inner zone (renal papilla) - contains only the thin segments of the long loops of Henle of juxtamedullary nephrons, capillaries from the vasa recta, collecting (papillary) ducts and interstitial cells oriented perpendicular to the tubules. The papillary ducts open onto the apex, forming the area cribrosa.

C. Vessels (veins follow the arteries and are named accordingly).

1. Interlobar arteries - pass between lobes in the substance of the renal columns of Bertin to branch at the level of the base of the pyramids. The branches enter the lobes between the cortex and medulla, forming the intralobar or arcuate arteries.

III. ORGANIZATION OF LOBULE - one medullary ray with its associated pars convoluta and medulla, composes a lobule.

A. Development - the branched collecting tubule of the medullary ray develops as a branching out-growth of the mesonephric duct. Nephrons arise around the collecting duct by organization of nephrogenic mesenchyme. These eventually fuse with the duct and its branches. The loops of Henle develop near the collecting ducts to form the medullary ray.

B. Vasculature

1. Interlobular arteries - arise from the arcuate (intralobar) arteries and follow a straight course toward the capsule in the pars convoluta about equidistant from adjacent medullary rays. They supply adjacent lobules and terminal branches to the capsule.

2. Intralobular arteries - afferent arterioles arise directly from the interlobular arteries or from short intralobular arteries. The afferent arterioles supply the glomerulus of the renal corpuscles.

3. Efferent arterioles - smaller than afferent arteriole: In the middle and subcapsular cortex the efferent arterioles supply the fenestrated peritubular capillaries of the pars convoluta. Efferent arterioles arising from juxtamedullary glomeruli turn down into the medulla to form the arteriole rectae, the arterial component of the vasa recta.

IV. NEPHRON AND COLLECTING TUBULE - STRUCTURE AND FUNCTION

A. General description - the nephron consists of an epithelial tube that has an expanded closed end (renal corpuscle) into which a tuft of capillaries (glomerulus) projects. The tube is further described in three sections: 1) the proximal convoluted tubule; 2) the loop of Henle composed of the thick descending segment, the thin segment, and the thick ascending segment; and 3) the distal convoluted tubule. The thick ascending segment of Henle's loop returns to the vascular pole of its renal corpuscle where it participates in the formation of the juxtaglomerular apparatus and then continues as the distal convoluted tubule. Nephrons empty into the collecting ducts by way of arched collecting tubules. These and the collecting ducts differentiate from the mesonephric duct while the nephron itself arises from metanephrogenic mesenchyme. These different origins can be correlated with their different functions and justifies limiting the term nephron to that derived from the metanephrogenic mesenchyme. Uriniferous (conveying urine) tubule can be used for the entire passageway.

B. Cytology and function

1. Renal corpuscle and juxtaglomerular apparatus. Learn the names and relationships of the different epithelia.

a. Relationships

b. The filter - consists of 1)endothelium with open fenestrations that retains formed elements. 2)a basement membrane that is formed by both endothelium and podocytes. It is a thick, negatively charged(heparin sulfate) basal lamina that deflects negatively charged proteins and 3)the filtration slit membrane consisting of pedicel cell coat containing the protein nephrin. Together these elements of the filter exclude from the glomerular filtrate almost all components of the blood having a M.W. greater than 70,000 (albumin). A very small amount of albumin does get through the filter. A mutation of nephrin causes severe nephrotic syndrome (albumin in the urine and generalized edema). Severe crushing injury of muscle that releases large amounts of myoglobin(40,000 M.W.)into the circulation can cause kidney failure because myoglobin precipitates in the nephron. The basement membrane is dynamic - synthesis and breakdown involves the endothelium, podocytes and mesangial cells.

c. Mesangium - The mesangium consists of cells and extracellular matrix similar to basal lamina between the glomerular capillaries. They maintain the matrix, contribute to glomerular basement membrane turnover and have the potential to contract and modify the flow of blood through the glomerulus. Injury by various mechanisms, including diabetes, causes increased deposition of matrix and glomerulosclerosis results.

d. Lacis cells - form cushion around vascular pole between mesangium and JG cells - similar to, and may be a reservoir of, mesangial cells.

e. Juxtaglomerular (JG) apparatus - consists of modified smooth muscle cells(JG cells) mostly around the afferent arteriole, the macula densa of the distal tubule and associated extraglomerular mesangial cells. The JG cells have features of smooth muscle but in addition they have round nuclei and many secretion granules containing stored renin. The JG cells secrete renin into the arteriole (it lacks an elastic lamina) as a result of sympathetic stimulation and when macula densa cells detect that the volume and sodium concentration in the tubular fluid are reduced. Renin is a peptidase that converts circulating angiotensinogen to the inactive angiotensin I that in turn is converted to the octapeptide, angiotensin II, by the endothelial cells of the lung. Angiotensin II causes vasoconstriction and acting on the kidneys through the adrenal and pituitary glands increases blood pressure and plasma volume.

f. Macula densa - clump of specialized cells in the distal tubule. They are polarized toward JG cells and there is no basement membrane between them. They stimulate secretion of renin when salt concentration and the fluid volume in the distal tubule are reduced. Elevated angiotensin 2 inhibits. (Science 237:1618-1620, 1987)

2. Proximal tubule - convoluted and straight segments - convoluted segments have the greatest diameter and are the longest tubules in the pars convoluta. Their profiles in sections are therefore the largest and most numerous.

a. Function - 80% of glomerular filtrate reabsorbed, including most nutrients: glucose, amino acids, proteins, peptides, salt and water. Some substances (toxins, drugs and creatinine) are actively secreted into the lumen reducing their concentration in the filtered blood.

b. Structure - large cuboidal cells highly specialized for absorption, digestion, and active transport. Their brush border microvilli contain hydrolases and sodium dependent cotransporters of nutrients. Apical tubules and vacuoles participate in receptor mediated endocytosis. Lysosomes degrade proteins removed from the glomerular filtrate by endocytosis. There is also a transepithelial network of endoplasmic reticulum. Many mitochondria are associated with basal infoldings and interdigitations for active transport especially of sodium and water out of the cell (sodium, potassium ATPase). The straight segment is noticeably less complex than the convoluted segment. It is the descending thick segment of the loop of Henle. All parts of the proximal tubule are highly permeable to water (aquaporin 1) so that the luminal fluid is always isotonic with the interstitial fluid. This means that it is hypertonic as it enters the thin segment of the loop of Henle at the junction between the outer and inner stripes of the outer medulla..

3. Loop of Henle

a. Structure - The longest loops, from the juxtamedullary nephrons, consist of three segments; 1)the thick descending segment (straight part of proximal tubule that extends to the middle of the outer medulla), 2) the thin segment that has both a descending part and an ascending part, and 3)the thick ascending segment (straight part of the distal tubule that begins at the junction between the inner and outer medulla). The thin segment is not as attenuated as capillaries and is not fenestrated - junctional complexes hold cells together. Cortical nephrons may not have a thin ascending segment.

b. Function - counter current multiplication; thin descending equilibrates passively with interstitium. The ascending limb (especially the thick segment) uses a Na-K-2Cl symporter (blocked by furosemide, Lasix) to pump electrolyte into the interstitium while retaining water. It is moderately permeable to urea. These actions result in a hypertonic interstitium (4 fold at the papilla) and hypotonic luminal fluid as it returns to the cortex.

4. Distal tubule - straight (thick ascending segment of Henle's loop) and convoluted segments have similar structure.

a. Structure - few short irregular microvilli, basal infoldings are extensive and contain long mitochondria.

b. Functions primarily in electrolyte movements. It is impermeable to water and active transport of electrolytes (Na-Cl symporter stimulated by aldosterone) takes place largely through basal plasma membrane. Luminal fluid continues to be hypotonic. Other functions include hydrogen ion secretion into the luminal fluid and calcium homeostasis. Apical calcium channels are under the influence of parathyroid hormone.

5. Terminal segment of distal tubule is formed from nephrogenic mesenchyme not from mesonephric duct, but it is morphologically similar to the collecting duct in that it contains intercalated or dark cells. Five to six of these connect to a single collecting duct.

6. Collecting duct consists of simple cuboidal epithelium becoming columnar in the papilla.

a. Structure and function

1) principal cell is relatively simple, it is light staining and has few organelles. They do have a single apical cilium. Regular interdigitations between cells make distinct cell boundaries visible with light microscope. Sodium reabsorption is regulated by aldosterone and water by antidiuretic hormone. The latter stimulates the delivery of aquaporin 2 to the apical plasma membrane. Aquaporin 1 is expressed constituitively in the basolateral cell membrane. Together these cause increased permeability to water so that it is absorbed into the hyperosmotic interstitium of the medulla and papilla resulting in a hypertonic urine.

2) dark (intercalated) cells are enriched in organelles. These are most abundant in the cortical portion of the collecting duct. Atrial natriuretic hormone has been found bound to these cells. Type A cells acidify the urine by vacuolar H⁺ATPase (same as lysosomal) in their apical plasma membrane and reabsorb potassium. Type B cells secrete bicarbonate and reabsorb chloride.

V. MEDULLARY INTERSTITIUM AND NERVES

A. Vasa recta

1. Structure - descending or arteriolae rectae have endothelium of the continuous non-fenestrated type; ascending or venulae rectae have endothelium of the fenestrated variety but both are highly permeable to water and electrolytes.

2. Function - counter current exchange so that nutrients can be brought to the medulla without washing out the high osmolality of the interstitium

B. Medullary interstitial cell - interposed between capillaries and loops of Henle, their most prominent features are lipid droplets and rough endoplasmic reticulum; changes occur in both with dehydration. Function uncertain, but probably produce prostaglandins.

C. Nerves - primarily sympathetic to afferent and efferent arterioles. Strong stimulation can shut down renal blood flow, especially when augmented by humoral mechanisms such as occur in shock.

URINARY TRACT

I. URETER

A. Parts

1. Major and minor calyces and pelvis

2. Ureter proper

B. Histology

1. Mucosa - transitional epithelium (increasingly it is called urothelium) on a loose to dense connective tissue lamina propria. The surface cells of the epithelium contain many microfilaments and lens-shaped "vesicles" that are really pockets whose lumen opens to the surface.. When the epithelium is stretched, the membrane of the "vesicles" becomes incorporated into the surface membrane. In the ureter the mucosa is thrown into many longitudinal folds. There is no submucosa and no glands.

2. Muscularis - arrangement

a. Calyces - circular to spiral

b. Upper 2/3 of ureter - inner longitudinal, outer circular

c. Lower 1/3 or ureter - a third layer, longitudinally oriented, is added

3. Adventitia - fibroelastic connective tissue

C. Function - peristaltic waves move urine from calyces to bladder. The ureter pierces the bladder wall diagonally, forming a functional check valve as the bladder distends.

II. URINARY BLADDER

A. Histology - similar to that of the ureter except the muscularis is thicker and its arrangement in 3 layers is less easily demonstrated. The adventitia contains a nerve plexus consisting of sympathetic and parasympathetic elements as well as sensory fibers. It is partly covered by mesothelium.

III. URETHRA

A. Male - prostatic, membranous, and cavernous portions.

1. Prostatic - lined by transitional epithelium with a highly vascular lamina propria. Several ducts of the prostate glands open into this portion as well as the paired ejaculatory ducts which enter through the posterior aspect on either side of the colliculus seminalis. Two coats of smooth muscle surround this portion, an inner longitudinal and an outer circular.

2. Membranous - this is the shortest portion. It passes through the urogenital and pelvic diaphragms and is therefore surrounded by skeletal muscle - the external sphincter. The epithelium changes from transitional to stratified and pseudostratified columnar.

3. Cavernous - passes through the bulb, body, and glans of the penis surrounded by erectile tissue (many thin-walled vessels with longitudinal bundles of smooth muscle in their side.) of the corpus spongiosum. Lined by pseudostratified columnar and stratified squamous epithelium. The mucus secreting bulbourethral glands open into the bulb and all along its course, the glands of Littre are found. These open into rather large recesses (lacunae of Morgagni) in which catheters may lodge.

B. Female

Epithelium changes from transitional to pseudostratified columnar and stratified squamous. The lamina propria is thick and contains many veins but lack the smooth muscle bundles seen in the erectile tissue of the male. It contains a few mucus secreting urethral glands. Muscularis consists of smooth muscle surrounded by skeletal muscle of the external sphincter.

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HISTOLOGY OF THE URINARY SYSTEM