Histology of Urinary System Tissues

Histology of Major Tissues of the Urinary System

• The urinary system's major tissues include the capsule, parenchyma, kidney, nephrons, urinary bladder, urethra, and ureter.
• The capsule is composed of collagen fibers, smooth muscle fibers, and blood capillaries.
• The parenchyma consists of millions of nephrons, branches of renal arteries, veins, lymphatics, and nerves.
• The kidney is made up of billions of nephrons, which are small tubes.
• At the start of each nephron, a network of capillaries releases water and other molecules into the nephron.
• Urine is collected in the urinary bladder and expelled through the urethra.

Kidney's Internal Structure

• The cortex is the outer layer of the kidney.
• The medulla is the inner portion of the kidney, divided into triangular pyramids.
• The pyramids have a narrow, innermost tip called the papilla.
• The renal pelvis is an extension of the ureter, collecting urine from the kidney.
• The calyces are divisions of the renal pelvis that collect urine from the pyramids and deliver it to the renal pelvis.

Nephron

• The nephron is the basic structural and functional unit of the kidney.
• Its primary function is regulating water and soluble substances in the blood, reabsorbing needed substances, and excreting the rest as urine.
• Nephrons eliminate wastes, regulate blood volume and blood pressure, control electrolytes and metabolites, and regulate blood pH.
• Nephron function relies on hormones like antidiuretic hormone, aldosterone, and parathyroid hormone.

Nephron Structure

• Renal corpuscle consists of a capillary ball (glomerulus) surrounded by a podocyte-lined capsule (Bowman's capsule).
• Proximal convoluted tubule: Lined with simple cuboidal epithelium and a prominent brush border.
• Loop of Henle: A loop with thick and thin ascending and descending portions, often extending into the medulla.
• Distal convoluted tubule: Lined with simple cuboidal epithelium with sparse brush border, the cytoplasm is paler than the proximal convoluted tubule.

Nephron Classification

• Nephrons can be classified by glomerulus location: superficial (near the capsule), midcortical (near the medulla), and juxtamedullary.
• Nephrons can also be classified by the length of their loop of Henle: short loops are typically superficial or midcortical, and long loops are juxtamedullary.

Renal Corpuscle

• The glomerulus is a network of capillaries surrounded by Bowman's capsule.
• Bowman's capsule is a cup-shaped structure which encloses the glomerulus.
• The glomerulus and Bowman's capsule function together to filter blood.

Juxtaglomerular Apparatus

• The Juxtaglomerular apparatus is a microscopic structure in the kidney regulating nephron function.
• It has three components: macula densa, extraglomerular mesangial cells, and juxtaglomerular (granular) cells.

Renal Tubule

• Proximal convoluted tubule: The initial segment of the renal tubule, originating from Bowman's capsule.
• Loop of Henle: The loop connects proximal and distal convoluted tubules, and consists of a descending limb (downward), a loop, and an ascending limb (upward).
• Distal convoluted tubule: Connects the ascending limb of Henle to the collecting duct.
• Collecting tubule: A straight tube formed by merging distal tubules from several nephrons; it's where urine is collected.

Histology of Bowman's Capsule

• The parietal layer (outer layer) of Bowman's capsule is composed of simple squamous epithelium.
• The visceral layer (inner layer), contains podocytes with foot-like processes that wrap around capillaries in the glomerulus.
• Filtration slits between podocyte processes, and the basement membrane are crucial in filtering blood.

Histology of Proximal, Loop of Henle, and Distal Convoluted Tubules

• Proximal tubule walls are low columnar epithelium, and cells exhibit a prominent brush border and endocytosis.
• Loop of Henle has a "U" shape, with descending and ascending segments; its descending segment is permeable to water but not solutes.
• Distal convoluted tubule is made of low cuboidal cells without a brush border.

Ureter

• The ureter wall is made of three layers: mucosa, muscularis, and adventitia.
• The mucosa contains transitional epithelium and lamina propria.
• The muscularis has longitudinal and circular smooth muscle layers, aiding urine transport.

Urethra

• The female urethra is relatively short and exits the body near the vagina.
• The male urethra is longer and has three sections (prostatic, membranous, and spongy).
• The histology of the urethra in both males and females starts out with transitional cells, but it transforms into stratified squamous cells.

Urinary Bladder

• The urinary bladder is a sac that stores urine.
• It has a mucosa with transitional epithelium.
• Its lamina propria layer contains supportive connective tissue.
• The bladder's muscularis (detrusor muscle) is comprised of interwoven smooth muscle layers
• The bladder's serosa/adventitia is the outer layer; where no serosa exists, it's adventitia.

Urinary Bladder Lining Epithelium

• The lining epithelium of the bladder is transitional epithelium (urothelium).
• In a relaxed bladder, it's five to seven layers thick, but it becomes thinner when stretched.
• This adaptation allows for urine storage without damage to the lining tissue.

Urinary Bladder Lining Epithelium Layers

• Apical layer composed of umbrella cells that are often binucleated and form an impermeable barrier with tight junctions.
• Intermediate layer consists of multiple polygonal cells between the apical and basal layers.
• Basal layer is comprised of small cuboidal cells.

Bladder Lamina Propria

• The lamina propria separates the urothelium and the muscularis propria, consisting of an extracellular matrix (with elastic fibers, capillaries, nerves).
• It contains interstitial cells of Cajal that function as pacemaker cells.

Bladder Muscularis Propria

• It's also known as the detrusor muscle, and contains interwoven longitudinal and circular smooth muscle layers.
• This layer is well-defined in the bladder neck and diffuse in the body of the bladder.

Bladder Serosa and Adventitia

• The serosa is a thin connective tissue layer covering the bladder dome, continuous with the peritoneum.
• The adventitia is a connective tissue layer, acting as the external lining in areas where there is no serosa.

Urinary Bladder Function

• The urothelium's ability to transition from multi-layered to single-layered without damage enables the bladder to accommodate large volumes of urine.
• The bladder's lamina propria plays a role in initiating the micturition reflex.
• The detrusor muscle is under autonomic control, and bladder capacity is dictated by its compliance.

Histology of the Urinary Bladder: Microscopic

• Urothelium, lamina propria, muscularis propria, and serosa/adventitia layers are visible when examining a section of the bladder wall with light microscopy.
• Urothelial cells (umbrella cells) are often multinucleated and dome-shaped.
• The number of layers in the intermediate layer depends on bladder distension.

Urinary Bladder: Electron Microscopy

• Umbrella cells exhibit plaques that are areas of thick, asymmetrical unit membrane, sometimes associated with actin filaments.
• In relaxed bladders, cytoplasmic, membrane-bound actin filaments produce fusiform vesicles.
• During filling, these vesicles unfold and the superficial cells flatten and the plaques become a smooth surface as the bladder stretches

Clinical Correlates

• Cystoscopy is a procedure for examining the urinary bladder mucosa.
• Biopsies are taken from the mucosa of the bladder, or specimens from radical cystectomy procedures, for pathological examination.
• Image-guided percutaneous bladder biopsy may be used in cases where cystoscopy is inadvisable.
• Benign lesions such as von Brunn's nests and cysts may be found.
• Malignant bladder tumors are graded by their depth of invasion.

Urinary Bladder Pathophysiology

• Urge incontinence is characterized by sudden, strong urges to urinate and uncontrolled leakage.
• The causes of urge incontinence can be neurogenic, myogenic, or idiopathic.
• Nerve damage, nerve signaling issues, or bladder cell damage can contribute to this condition.

Histochemistry and Cytochemistry

• In the urinary bladder, the umbrella cells stain positive for CK20; the basal cells stain positive for CD44.
• In carcinoma in situ, all cells display CK20 positivity, p53 positivity, and a high Ki-67 proliferation index.

Tissue Preparation

• Bladder tissue biopsies are obtained by cystoscopy or radical cystectomy for examination.
• Percutaneous bladder biopsy can be done when cystoscopy is contraindicated.
• Samples are preserved in formalin, embedded in paraffin, or processed as frozen sections for standard histological staining.

Explore the histology of major tissues in the urinary system, including the kidney's internal structure. This quiz covers the composition of capsules, parenchyma, nephrons, and urine transport mechanisms. Test your knowledge about the layers and functions of this vital system.