BIOL 432 HO Urinary System

8/13/2019 BIOL 432 HO Urinary System

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HISTOLOGY OF THE HUMAN URINARY SYSTEM

I. Introduction

A. Function

The human urinary system is partially responsible for regulation of the chemical compositionof blood and tissue fluids. It has two major functions.

1. Removal of nitrogenous waste products (urea, uric acid) from the blood

2. Maintenance of normal fluid (water) and salt balance in blood and in the body as a whole

B. Anatomy of the urinary system

1. The urinary system consists of urine forming structures and urine conducting(transporting) structures.

2. The kidneys contain urine forming and urine conducting structures. The kidneys formurine by "filtration" of blood followed by selective reabsorption of water and usabledissolved solutes. The fluid remaining in the kidney tubules after this process is

completed is called urine and is carried out of the kidney by conducting structures.

3. Tubular excretory passageways either store urine temporarily and/or conduct urine outof the body. Tubular excretory passageways include

a. Ureters

b. Bladder

c. Urethra

II. Microanatomy of the Organs of the Urinary System

A. Kidneys

1. Organization of the kidney (Figure 20.1)

a. Parenchyma

Parenchyma of the kidney includes the epithelia which line the nephrons andcollecting ducts. The vascular elements which are closely associated with thenephron should also be considered part of the kidney parenchyma since they areessential to the functions of the kidney.

The parenchymal area of the kidney is commonly divided into the outer cortex andthe inner medulla.


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(1) The cortex is defined as the area containing renal corpuscles, but also containsproximal and distal convoluted tubules, descending and ascending thick limbsof the loops of Henle, and the initial parts of the collecting ducts.

(2) The medulla is defined as the region lacking renal corpuscles. The medullacontains most of the length of the loops of Henle and most of the length of thecollecting ducts (collecting tubules).

b. Stroma

(1) Interstitial stroma

The loose FECT located between the nephron components consists of finecollagen fibers, scattered fibroblasts, and occasional lymphoid areas and ishighly vascularized. Myofibroblasts are numerous in interstitial connectivetissue in the medulla. Loose FECT around large blood vessels and in the wallof the renal pelvis is more typical loose FECT, and may be rich in adipose cellsaround the renal pelvis.

(2) Capsule (Figure 20.2)

The outer thin capsule of dense FECT contains relatively large collage fibersarranged in a two dimensional array. The outer capsule is covered bymesothelium on surfaces exposed to the peritoneal cavity. A layer ofmyofibroblasts lies between the dense FECT outer capsule and theparenchyma of the cortex in the human kidney but may be absent in smallermammals. The myofibroblast layer is sometimes called the inner layer of thecapsule and the outer dense FECT is called the outer layer of the capsule.

c. Renal pelvis(Figure 20.1, 20.4)

The renal pelvis connects the nephrons to the ureter draining each kidney. A set ofmajor calyces drain into the renal pelvis, and a set of minor calyces drain into each

major calyx. A human kidney may contain between 8 and 18 lobes, with each lobedraining into a separate minor calyx. The collecting ducts in a lobe empty into theminor calyx on a renal papilla. The renal papilla may have small areas of simplecolumnar epithelium around the attachment of a collecting duct, but the epitheliumrapidly changes to the transitional epithelium typical of the ureters and bladder. Theepithelium of the renal papilla lies on the sparse stromal connective tissue of themedulla of the kidney while larger arrays of loose to occasionally moderately denseFECT lie under the epithelium deeper in the calyces and around the renal pelvisitself. The deeper connective tissue is the dense regular FECT of the capsulewhere the pelvis crosses the capsule.


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2. Structure of nephrons (Figure 20.3, 20.6)

a. Renal (or Malpighian) corpuscle(Figure 20.7, 20.10 - 20.13, Plate 72)

(1) Glomerulus

(a) The glomerulus consists of a tuft of fenestrated (type II) capillaries whichare located between an afferent arteriole and an efferent arteriole.

(b) The arterioles are attached to the glomerulus at the vascular pole of therenal corpuscle.

(2) Bowman's capsule

(a) Bowman's capsule consists of a double walled epithelial cup-shapedstructure with the walls separated by a slit-like capsular space calledBowman's space.

(b) The outer or parietal layer is a simple squamous epithelium which forms asphere around the glomerulus.

(c) The inner or visceral layer is tightly associated with the individual capillariesof the glomerulus. The cells (podocytes) of the visceral layer have longinterdigitating processes which wrap around the glomerular capillaries. Adistinct basal lamina is formed between the glomerular capillary endothelialcells and the podocytes which surround them.

(d) The urinary pole of the glomerulus is the attachment between the outerlayer of Bowman's capsule and the simple cuboidal epithelium of theproximal convoluted tubule.

(3) Mesangial cells

(a) Mesangial cells are located in the center of the tuft of glomerular capillaries(between capillaries) and between the glomerulus and the juxtaglomerularapparatus. Mesangial cell nuclei are usually more heterochromatic thanendothelial cell and podocyte nuclei.

(b) Mesangial cells may be modified smooth muscle cells, or at least comefrom the same embryonic cell populations as smooth muscle. Thepositioning of mesangial cells around capillaries resembles pericytes.

(c) Mesangial cells are phagocytic and have as a major function the cleaningof the basement membranes which serve as filtration surfaces in theglomerulus. They may also support parts of glomerular capillaries and be

involved in regulation of glomerular blood flow.


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(4) Function of renal corpuscles

As blood flows through the glomerulus, blood plasma components smaller thanproteins cross the wall of the type II (fenestrated) capillaries and pass throughthe basal lamina and between the "foot" processes of the podocytes intocapsular space of Bowman's capsule, forming the filtrate. The basal laminaappears to act as the actual filter.

b. Uriniferous tubules

(1) Proximal convoluted tubule (PCT)(Figure 20.14, 20.15, Plate 72)

(a) The PCT is lined by simple cuboidal epithelium which is usually morestrongly acidophilic than the epithelium of DCTs or collecting tubules.

(b) The lining cells have numerous long apical microvilli, basal infoldings, andlateral canaliculi. The microvilli reduce the apparent diameter of the lumenin LM preparations.

(c) Numerous mitochondria are associated with the basal and lateral surfacesof the cells.

(d) The PCT is the site of most of the reabsorption of useful solutes and waterfrom the filtrate. Approximately 80% of solutes and water in the initialfiltrate are resorbed by the PCT under most conditions in humans.

(2) Loop of Henle(Figure 20.17, Plate 73)

(a) Descending thick limb

The descending thick limb is lined by simple cuboidal epithelium whosecells closely resemble PCT cells and apparently function similarly.

(b) Thin limb

The thin limb is lined by simple squamous epithelium which is permeable towater and small solutes such as salt and urea.

(c) Ascending thick limb

The ascending thick limb is lined by simple cuboidal epithelium whichresembles DCT epithelium. The plasma membranes of the cells arerelatively impermeable to diffusion of water and small solutes and the cellsactively transport salt (sodium and chloride ions) from their apical to their

basal surfaces.

(d) Overall function

The loop of Henle establishes and maintains a high salt concentration inthe interstitial fluid of the deep medulla.


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(3) Distal convoluted tubule(DCT) (Figure 20.18, Plate 72)

(a) The DCT is lined by simple cuboidal epithelium with basal infoldings, lateralcanaliculi, and numerous basally located mitochondria. The cells do notstain as darkly as PCT cells in LM preparations and the lumen usuallyappears larger than PCT lumens. The lateral margins of DCT cells areusually less distinct than collecting tubule cells.

(b) The DCT is the location of hormonally regulated (by aldosterone) Na+for K+

exchange.

(4) Collecting tubules and collecting ducts(Ducts of Bellini) (Plate 73)

(a) Collecting tubules and ducts are lined by simple epithelia which graduallychange from flattened simple cuboidal epithelium (initial collecting tubules)to simple columnar epithelium (collecting ducts) as the duct system passesfrom the cortex to the renal papilla.

(b) The most abundant epithelial cells are lightly stained by hematoxylin andeosin and have extensive lateral junctions which cause the lateral cell

borders to appear very distinct in light microscopy. Occasional darkerstaining cells with short apical microvilli and apical microfolds also occur.

(c) The epithelia of the collecting tubules and ducts in the medulla increasetheir water permeability in response to vasopressin (antidiuretic hormone).This allows water to move out into the high salt concentration in theinterstitial fluid of the medulla, resulting in water reabsorption from theurine. The abundant lighter cells have aquaporin transmembrane channels(aquaporin-2) regulated by antidiuretic hormone (ADH). The less abundantdarker cells actively transport hydrogen ions or bicarbonate ions into thelumen to adjust the pH of blood.

c. Peritubular capillaries(Figure 20.21)

(1) A capillary network supplied by the efferent arteriole is located around the PCTand DCT of the nephron.

(2) These capillaries allow absorption (back into the blood) of the solutes and waterwhich were resorbed back into the tissue fluid (from the filtrate) by the PCT andthe DCT.


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d. Vasa recta

(1) Sets of parallel arterioles and venules supply and drain the capillary plexusaround the loops of Henle and collecting ducts in the medulla.

(2) Because of the parallel arrangement, the high salt levels in the blood in thevenules (absorbed into the blood from the high salt tissue fluid in the medulla)tend to diffuse into the arterioles (and are carried back into the medulla) insteadof being carried out of the medulla. This process reduces the energy which

must be used to maintain the high salt concentrations in the medulla.

e. Juxtaglomerular apparatus (JG apparatus)(Figure 20.7)

(1) The JG apparatus is located in the region of the nephron in which theascending limb of the loop of Henle (distal convoluted tubule) and the adjacentafferent and efferent arterioles are positioned next to each other.

(2) The macula densaconsists of cells of the ascending thick limb of the loop ofHenle (DCT type cells) which are next to the efferent arteriole and are muchtaller than adjacent DCT cells. These cells apparently monitor the osmoticconcentration in the tubule contents and cause increased release of renin by

the JG cells when the osmolarity of the tubular fluid is too high.

(3) Juxtaglomerular cells(JG cells) are smooth muscle cells in the tunica mediaof the afferent arteriole (and sometimes efferent arteriole as well) which havebecome modified into secretory epithelial-like cells. JG cells secrete reninwhich converts angiotensinogen in blood plasma into angiotensin I which is inturn converted into angiotensin II by converting enzyme in the lungs.

(4) The JG complex apparently "fine tunes" blood pressure within the glomerulus toallow for an appropriate filtration rate. When filtration occurs too rapidly or tooslowly, fluid moving through the tubule will have an altered osmolarity whenreaching the macula densa. The macula densa releases locally acting

hormones which alter release of renin by the JG cells. JG cells may also altertheir renin release in response to changes in stretching resulting from changesin blood pressure in the afferent arteriole. Renin release ultimately results inangiotensin II formation which causes constriction of arterioles throughout thebody, increasing blood pressure. This in turn increases filtration in theglomerulus.


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3. A summary of critical features in identifying parts of nephrons

Structure Location Features

Renal corpuscle cortex clump of capillaries surrounded

by a sphere of simple squamous

epithelium. The podocytes and

mesangial cells around the capillaries

are complex

PCT cortex circular or oval wavy profiles

lined by simple "cuboidal"

epithelium with long microvilli

more numerous and darker staining

than DCTs

Descending thick outer same cell type as PCT, but located

limb, loop of medulla in the medulla

Henle tubules are straight (not wavy)

apical microvilli may be visible

Thin limb, loop middle simple squamous epithelium

of Henle and deep contains no blood cells

medulla nuclei frequently bulge into lumen

Ascending thick middle same cell type as DCT, but located

limb, loop of medulla in the medulla

Henle tubules are straight (not wavy)

cell boundaries are less distinct

than in collecting ducts

DCT cortex circular or oval wavy profiles

lined by simple "cuboidal"epithelium which lacks long

microvilli

less numerous than PCTs

JG cells cortex rounded cells in the smooth muscle

layer of an afferent arteriole next

to a macula densa in the DCT at

the vascular pole of a renal

corpuscle

Macula densa cortex columnar cells in the wall of a DCT

where it passes next to an afferent

arteriole at the vascular pole of a

renal corpuscle

Collecting ducts cortex distinct lateral cell boundaries

and light cytoplasm

medulla cells are cuboidal or columnar


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C. Excretory passageways

1. Ureters(Figure 20.22, Plate 74)

a. The mucosaconsists of a transitional epithelium underlain by a lamina propria (+submucosa) of loose to moderately dense FECT.

b. The muscularis externaconsists of two or three layers of smooth muscle. The

inner layer contains approximately longitudinal smooth muscle cells. The middlelayer contains approximately circular smooth muscle cells. An outer layer ofapproximately longitudinal smooth muscle is present only in the lower 1/3 of theureter.

c. The adventitia/serosaconsists of loose to moderately dense FECT +/-mesothelium.

2. Bladder(Figure 20.22, Plate 75)

a. The mucosa consists of a transitional epithelium underlain by a lamina propria (+

submucosa) which is mostly moderately dense FECT but contains loose FECTdirectly under the epithelium and near smooth muscle bundles.

b. The muscularis externa consists of three layers of smooth muscle bundles. Theinner layer consists of approximately longitudinal smooth muscle bundles. Themiddle layer consists of approximately circular smooth muscle bundles. The outerlayer consists of approximately longitudinal smooth muscle bundles.

c. The adventitia/serosacontains loose to moderately dense FECT +/- mesothelium.

3. Urethra

a. The mucosacontains an epithelium which varies with location. The initial shortsegment of the urethra is lined by transitional epithelium. The middle segment(which may be absent) is lined by a mixture of stratified and pseudostratifiedcolumnar epithelium. The outer (longest) segment is lined by stratified squamousepithelium. The region of stratified/pseudostratified columnar epithelium is muchlonger in males than in females, and may be absent in some females. Theepithelium is underlain by a lamina propria (+ submucosa) of loose FECT in bothsexes. In males the lamina propria is erectile tissue, and the homologous layercontains numerous thin-walled veins in females.

b. The muscularis externadiffers in males and females. In females the muscularis

consists of an inner sparse layer of smooth muscle bundles and a thicker outercircular layer of smooth muscle (surrounded by skeletal muscle at the sphincter). Inmales the "muscularis externa" is occupied by cavernous tissue of the corpusspongiosum of the penis.

c. The adventitiais loose FECT in females and is replaced by tissues of the penis inmales.

BIOL 432 HO Urinary System

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