Renal – Histo

• The renal calyces, renal pelvis, ureters, and urinary bladder are all lined by transitional epithelium, also called urothelium.

• This specialized epithelium:

  • Is stratified, but the surface cells (umbrella cells) can change shape depending on bladder fullness.

  • Provides a watertight barrier and resists urine toxicity.

  • Allows distension without tearing or losing integrity.

❌ Why the Other Options Are Wrong:

Simple columnar:

• Found in gastrointestinal tract, not urinary passages.

Simple cuboidal:

• Lines renal tubules (PCT, DCT), not the collecting ducts or calyces.

Pseudostratified columnar:

• Common in respiratory tract and male reproductive tract, not in urinary pathways.

Stratified columnar:

• Rare; occurs in some ducts (e.g., salivary glands), not in the kidney.

The visceral layer of Bowman’s capsule is formed by podocytes — specialized epithelial cells with a stellate (star-shaped) body. Each podocyte gives off primary processes that branch into numerous secondary foot processes (pedicels), which interdigitate with neighboring cells to form filtration slits. These slits are bridged by a thin slit diaphragm, a key part of the glomerular filtration barrier.

Podocytes play a vital role in maintaining selective filtration and preventing protein loss in urine. Injury to podocytes (e.g., in minimal change disease or diabetic nephropathy) leads to proteinuria.

❌ Why the Other Options Are Wrong

• Brushed cells:
  Found in the proximal convoluted tubule, not in Bowman’s capsule. Their brush border (microvilli) increases surface area for reabsorption.

• Endothelium:
  Lines the glomerular capillaries, not Bowman’s capsule. It is fenestrated, forming part of the filtration barrier but not the visceral layer.

• Mesangial cells:
  Located between glomerular capillaries; they provide structural support, secrete matrix and cytokines, and have phagocytic properties — but they don’t form the capsule lining.

• Simple cuboidal cells:
  These line tubular structures like the PCT and DCT, not the visceral layer of Bowman’s capsule.

The renal filtration barrier is a highly specialized tri-layered structure designed to selectively filter blood plasma while retaining cells and large proteins. It ensures that essential molecules like glucose and amino acids can pass through while keeping albumin and blood cells inside the circulation.

It consists of three key components (from inside to outside):
1️⃣ Fenestrated endothelium of glomerular capillaries — allows plasma through but blocks blood cells.
2️⃣ Glomerular basement membrane (basal lamina) — acts as a size and charge barrier.
3️⃣ Podocyte foot processes with slit diaphragms — fine-tune filtration selectivity at the outermost layer.

There is no continuous epithelium in this barrier — the endothelium is fenestrated specifically to allow ultrafiltration.

❌ Why the Other Options Are Wrong

• Podocytes:
  These form the visceral layer of Bowman’s capsule and create slit diaphragms between foot processes — a crucial part of the filtration barrier.

• Fenestrated endothelium of glomerular capillaries:
  The first filtration layer. Its pores (~70–100 nm) allow water and solutes but not cells to pass through.

• Basal lamina (glomerular basement membrane):
  The shared basement membrane between endothelium and podocytes, rich in type IV collagen and heparan sulfate, providing both mechanical strength and charge selectivity.

• Endothelial cells:
  They line the glomerular capillaries and contribute to the fenestrations forming part of the barrier.

The vasa recta are straight capillary vessels that run parallel to the loops of Henle in juxtamedullary nephrons.
They arise from the efferent arterioles of these nephrons and play a key role in maintaining the countercurrent exchange mechanism, which is essential for urine concentration and water reabsorption.

❌ Why Others Are Wrong:

• Peritubular capillaries:
  Surround proximal and distal convoluted tubules in cortical nephrons, not the loops of Henle deep in the medulla.

• Interlobular arteries:
  Small branches of the arcuate arteries supplying afferent arterioles — not associated with the loops directly.

• Interlobar arteries:
  Larger vessels between renal pyramids — far away from nephron structures.

• Efferent arterioles:
  Drain the glomerulus but give rise to the vasa recta — they don’t directly surround the loops.

✅ Proximal convoluted tubule

Why Other Options Are Wrong
❌ Glomerulus – Composed of fenestrated capillaries and specialized podocytes, not cuboidal brush-bordered epithelium.

❌ Distal convoluted tubule – Lined by cuboidal cells without a brush border; fewer mitochondria and less prominent reabsorptive specialization.

❌ Collecting duct – Lined by simple cuboidal to columnar cells, but lacks brush border and has clearer cytoplasm for water balance rather than massive reabsorption.

❌ Loop of Henle – Thin limbs are lined by simple squamous epithelium, not cuboidal with brush border; thick ascending limb has cuboidal cells but no brush border.

✅ Ureter

• Why Other Options Are Wrong
  ❌ Urinary bladder – Although also lined by transitional epithelium, the bladder typically has a thicker wall with irregular, interlacing smooth muscle bundles (detrusor muscle) rather than the organized three-layer pattern.
  ❌ Urethra – The urethra is not entirely lined by transitional epithelium; much of it is lined by stratified columnar or stratified squamous epithelium, and its muscular arrangement differs.
  ❌ Vas deferens – This has pseudostratified columnar epithelium with stereocilia, not transitional epithelium, and its muscle coat is thicker with a different arrangement.
  ❌ Prostate – Composed of glands in a fibromuscular stroma, lined by columnar epithelium, not transitional, and lacks a well-defined three-layer muscular wall.

Step-by-Step Reasoning

1. Transitional epithelium (urothelium)

• Specialized epithelium found in the renal pelvis, ureters, urinary bladder, and proximal urethra.

• Function: stretchable lining that protects underlying tissue from urine.

2. Mucus glands in submucosa

• Found mainly in the submucosa of urethra and sometimes urinary bladder, providing lubrication and protection against urine’s toxicity.

• These glands are not present in the nephron segments like PCT, DCT, or Loop of Henle.

3. Check each option

• Proximal convoluted tubule → simple cuboidal epithelium, no glands. ❌

• Distal convoluted tubule → simple cuboidal epithelium, no glands. ❌

• Urinary bladder → lined by transitional epithelium; submucosal glands (especially near urethra) present → ✅ correct.

• Loop of Henle → thin segment, simple squamous epithelium, no glands. ❌

• None of these → incorrect.

Correct Answer: Urinary bladder ✅

Step-by-Step Reasoning

1. Function of proximal convoluted tubule

• Main site of reabsorption: water, electrolytes, glucose, amino acids.

• Requires a large surface area → microvilli form a brush border.

• Cells are metabolically active → many mitochondria.

2. Epithelium type

• Simple cuboidal epithelium with brush border:

  • Single layer of cube-shaped cells.

  • Microvilli increase surface area for absorption.

  • Found throughout proximal convoluted tubule.

3. Check other options

• Pseudostratified columnar → found in respiratory tract. ❌

• Simple columnar → found in small intestine, gallbladder; taller cells, no prominent brush border like PCT. ❌

• Simple squamous → forms glomerular capsule and thin limbs of Henle. ❌

• Transitional epithelium → lines ureters, bladder, renal pelvis; specialized for stretch. ❌

Correct Answer: Simple cuboidal epithelium ✅

Step-by-Step Reasoning

1. Function of ureter epithelium

   • The ureters carry urine from the renal pelvis to the bladder.

   • Urine is hypertonic and potentially irritating, so the epithelium must stretch and protect underlying tissue.

2. Transitional epithelium (urothelium)

   • Specially adapted for the urinary tract.

   • Features:

     • Stratified appearance, but cells can stretch when the ureter fills with urine.

     • Umbrella cells at the surface resist the toxicity of urine.

   • Found in: renal pelvis, ureters, bladder, proximal urethra.

3. Check other options

• Stratified squamous non-keratinized → lines esophagus, oral cavity; cannot stretch like transitional epithelium. ❌

• Stratified squamous keratinized → epidermis; not in ureter. ❌

• Pseudostratified columnar → respiratory tract. ❌

• Simple squamous → lining of blood vessels or alveoli; too thin for ureter. ❌

Correct Answer: Transitional epithelium ✅

Step-by-Step Reasoning

1. What is the urinary space?

   • Also called Bowman’s space.

   • It’s the space between the visceral layer (podocytes) and the parietal layer of Bowman’s capsule.

   • This is where the glomerular filtrate first collects before flowing into the proximal convoluted tubule.

2. Check each option

   • Proximal convoluted tubule → drains filtrate from Bowman’s space but does not itself contain urinary space.

   • Distal convoluted tubule → part of tubular system, no urinary space.

   • Bowman’s capsule → has urinary (Bowman’s) space → ✅ correct.

   • Collecting duct → carries urine toward renal pelvis, no urinary space.

   • Loop of Henle → responsible for concentration/dilution of urine, not urinary space.

Correct Answer: Bowman’s capsule ✅

1. Glomerular filtration barrier layers

   • Fenestrated endothelium (pores ~70–100 nm).

   • Glomerular basement membrane (negatively charged, prevents large proteins).

   • Slit diaphragm between podocyte foot processes, which contains filtration slit pores.

2. Width of slit pores

   • These pores are much smaller than endothelial fenestrations.

   • Standard physiology texts (e.g., Guyton, Ganong) state slit pores are ~25 nm wide.

3. Significance

   • This narrow width, along with negative charge, restricts albumin and other plasma proteins.

   • Allows water, electrolytes, glucose, and small solutes to pass freely.

Correct Answer: 25 nm ✅

The proximal convoluted tubule (PCT) is lined by simple cuboidal epithelium. These cells are large, with a centrally placed nucleus and a dense brush border of microvilli on the apical surface. The brush border greatly increases surface area for reabsorption of water, electrolytes, glucose, and amino acids — which is why the PCT handles about 65–70% of the filtrate reabsorption. The presence of abundant mitochondria in these cells also supports their high metabolic activity.

Explanation of the Incorrect Options

❌ Pseudostratified columnar epithelium
This type is seen in parts of the respiratory tract (e.g., trachea, bronchi), not in renal tubules.

❌ Simple squamous epithelium
This lines structures specialized for passive diffusion (e.g., alveoli, Bowman’s capsule parietal layer, thin segments of the loop of Henle), not the PCT which requires active reabsorption.

❌ Simple columnar epithelium
This type is seen in absorptive linings like the gastrointestinal tract, not in the PCT.

❌ Transitional epithelium
Also called urothelium, this is found in the urinary bladder, ureters, and renal pelvis, not in the nephron tubules.

The nephron has portions that lie in both the renal cortex and the renal medulla. The proximal convoluted tubule (PCT) lies entirely within the renal cortex, near the glomerulus. This is where most reabsorption of water, electrolytes, glucose, and amino acids occurs. The PCT is a highly coiled segment and histologically lined with cuboidal epithelium with a dense brush border of microvilli, increasing surface area for reabsorption. Its location in the cortex is essential because it is closely associated with the glomerulus for immediate processing of filtrate.

Explanation of the Incorrect Options

Medullary collecting duct
As the name suggests, the medullary collecting ducts lie in the medulla, not in the cortex. They are involved in final concentration of urine under the influence of ADH.

Thin descending limb
This portion of the loop of Henle dips into the renal medulla. It is highly permeable to water, allowing water reabsorption into the hyperosmotic medullary interstitium.

Thin ascending limb
Like the thin descending limb, this segment is also located in the medulla. It is impermeable to water but allows passive transport of ions.

None of these
This would be incorrect because the proximal convoluted tubule clearly lies in the cortex.

Reference :- Laiq Hussain Histology Page 224

Distance between adjacent podocyte cell bodies or foot processes

• This refers to the overall spacing from one podocyte process to the next, including the cytoplasmic extensions and the intervening slit region.

• Width: approximately 40 nm.

• It represents the center-to-center distance between adjacent foot processes — broader than the slit pore itself.

Hence, the filtration slit refers to the overall space between the pedicels, while the filtration slit pores refer to the tiny openings within the slit diaphragm that control molecular passage.

Together with the fenestrated endothelium and glomerular basement membrane (GBM), this structure forms the tripartite glomerular filtration barrier — a selective sieve preventing protein loss while allowing water and small solutes to pass.

⚖️ Summary Comparison

Renal cortex histology:

1. Proximal convoluted tubules (PCT):

   • Lined by simple cuboidal epithelium with a brush border (microvilli)

   • Brush border increases surface area for reabsorption of water, electrolytes, and nutrients

2. Distal convoluted tubules (DCT):

   • Lined by simple cuboidal epithelium without a prominent brush border

3. Glomerulus:

   • Contains capillaries and podocytes, but podocytes are part of the visceral layer of Bowman’s capsule, not an epithelial lining of tubules

Why the Other Options Are Incorrect:

• Cuboidal epithelium lining Bowman’s capsule: Only the parietal layer; not a functional feature like PCT

• Simple columnar epithelium in DCT: DCT is cuboidal, not columnar

• Glomerulus containing podocytes: True, but not considered a tubular cortical histological feature

• None of these: Incorrect

The urinary bladder has a wall composed of:

1. Mucosa: Lined by transitional epithelium (urothelium), which can stretch as the bladder fills.

2. Muscular layer: Smooth muscle arranged in three layers (inner longitudinal, middle circular, outer longitudinal) called the detrusor muscle, responsible for bladder contraction during urination.

3. Adventitia/serosa: Outer connective tissue layer depending on location.

Key points:

• No skeletal muscle is present in the bladder wall; voluntary control occurs via the external urethral sphincter.

• Pseudostratified columnar epithelium is not present in the bladder.

Why the Other Options Are Incorrect:

• Outer smooth, inner skeletal muscle: Wrong arrangement; bladder has only smooth muscle.

• Skeletal muscle layer lined by transitional epithelium: Skeletal muscle is absent in the bladder wall.

• None of these: Incorrect because one option is true.

• Smooth muscle lined by pseudostratified columnar epithelium: Incorrect epithelial type.

The ureters are lined by transitional epithelium, also called urothelium.

• This specialized epithelium allows the ureters to stretch and recoil as urine passes through.

• It has multiple layers:

  • Basal cells (small and cuboidal)

  • Intermediate cells (columnar or polygonal)

  • Superficial umbrella cells (large, dome-shaped), which can flatten when the ureter is distended.

This property is critical for maintaining a barrier against urine while allowing expansion.

Why the Other Options Are Incorrect

• Simple squamous epithelium – Found in areas like alveoli or endothelium, not in ureters.

• Stratified squamous non-keratinized – Seen in the esophagus or vagina, not the urinary tract.

• Stratified squamous keratinized – Found in the skin (epidermis), not in ureters.

• Pseudostratified columnar epithelium – Seen in respiratory tract, not in urinary system.

The proximal convoluted tubule (PCT) is lined with simple cuboidal epithelium that has a prominent brush border of microvilli on the apical surface. This adaptation increases the surface area for reabsorption, allowing the PCT to reabsorb about 65–70% of the filtrate including glucose, amino acids, sodium, and water.

Incorrect Options:

• Transitional epithelium – Found in structures like the ureter, bladder, and renal pelvis, not in nephron tubules.

• Pseudostratified columnar epithelium – Typically found in the respiratory tract (e.g., trachea), not in the kidney.

• Simple squamous epithelium – Lines Bowman’s capsule (parietal layer) and thin limbs of the loop of Henle, but not the PCT.

• Simple columnar epithelium – Found in parts of the gastrointestinal tract, not in nephron tubules.

• The ureters are part of the urinary tract that transport urine from the kidneys to the bladder.

• They are lined by transitional epithelium (urothelium):

  • Stratified but specialized to stretch without tearing.

  • The superficial cells (umbrella cells) are dome-shaped and can flatten when distended.

  • This epithelium allows the ureter to accommodate fluctuating urine volumes while maintaining a barrier to urine.

Incorrect Options:

• Stratified squamous non-keratinized epithelium ❌ – Found in esophagus and vagina, not urinary tract.

• Pseudostratified columnar epithelium ❌ – Found in respiratory tract.

• Simple squamous epithelium ❌ – Found in blood vessels and Bowman’s capsule, too thin for ureters.

• Stratified squamous keratinized epithelium ❌ – Found in skin epidermis, not urinary tract.

The renal filtration barrier is the specialized structure in the glomerulus that determines which substances can pass from the blood into Bowman’s space to form the filtrate. It consists of three main layers:

1. Fenestrated endothelium of glomerular capillaries
   These are capillary endothelial cells with fenestrations (pores ~70–100 nm) that allow plasma water and solutes to pass but block blood cells.

2. Glomerular basement membrane (basal lamina)
   This is a thick, negatively charged membrane that acts as the main barrier to large and negatively charged molecules such as plasma proteins.

3. Podocytes with filtration slits
   Podocytes are specialized visceral epithelial cells of Bowman’s capsule. Their interdigitating foot processes form filtration slits bridged by a thin diaphragm, further restricting protein passage.

Now, let’s look at each option:

• Continuous epithelium of glomerular capillaries
  ❌ This is not part of the barrier because the glomerular capillaries are fenestrated, not continuous. If the endothelium were continuous, filtration would not occur effectively.

• Podocytes
  ✅ Part of the barrier, as described.

• Basal lamina
  ✅ Integral layer of the barrier.

• Endothelial cells
  ✅ They form the fenestrated capillary lining, part of the barrier.

• Fenestrated endothelium of glomerular capillaries
  ✅ Yes, the correct structure of glomerular endothelial cells.

✅ Correct Answer: Continuous epithelium of glomerular capillaries

✅ Correct Answer: Proximal convoluted tubule

Explanation:

Step 1: Understanding the brush border

• The brush border is formed by microvilli on the apical surface of epithelial cells.

• Microvilli increase the surface area for reabsorption of water, ions, and nutrients.

Step 2: Location in nephron

• The proximal convoluted tubule (PCT) has:

  • Extensive microvilli (brush border)

  • Many mitochondria to support active transport

• Functions:

  • Reabsorbs ~65–70% of filtered sodium and water

  • Reabsorbs glucose, amino acids, and bicarbonate

Step 3: Clinical relevance

• Loss of brush border, e.g., in ischemic injury, reduces reabsorption efficiency.

❌ Why the other options are incorrect:

Distal convoluted tubule

• ❌ Has few microvilli, no brush border; mainly fine-tunes electrolyte balance.

Loop of Henle

• ❌ Thin limb has simple squamous epithelium, no brush border; thick limb has minimal microvilli.

Pars recta

• ❌ Part of the proximal tubule, but less prominent brush border than the PCT; mainly straight segment.

Glomerulus

• ❌ Composed of capillary loops and podocytes, not epithelial tubules; no brush border.

✅ Correct Answer: Podocytes

Explanation:

Step 1: Components of the juxtaglomerular apparatus
The JGA consists of three main functional elements:

1. Juxtaglomerular (JG) cells – specialized smooth muscle cells in the afferent arteriole that secrete renin.

2. Macula densa – specialized cells of the distal convoluted tubule that sense sodium chloride levels.

3. Extraglomerular mesangial cells (Lacis/Polkissen cells) – supportive cells that transmit signals between the macula densa and JG cells.

Step 2: Podocytes

• Podocytes are part of the glomerular filtration barrier in Bowman’s capsule (visceral layer).

• They are not involved in the structure or function of the JGA.

❌ Why the other options are incorrect:

Juxtaglomerular (JG) cells

• Part of the JGA; secrete renin.

Macula densa

• Part of the JGA; senses NaCl concentration.

Polkissen cells

• Another name for extraglomerular mesangial cells; part of the JGA.

Extraglomerular mesangial cells

• Structural and signaling component of the JGA.

✅ Correct Answer: Lacis cells are the specialized smooth muscle cells

Explanation:

Step 1: Components of the juxtaglomerular apparatus (JGA)

• The JGA consists of three main elements:

  1. Macula densa – specialized cells of the distal convoluted tubule that sense sodium chloride concentration.

  2. Juxtaglomerular (JG) cells – modified smooth muscle cells of the afferent arteriole that secrete renin.

  3. Extraglomerular mesangial cells (Lacis cells) – specialized smooth muscle-like cells that provide structural support and may transmit signals between macula densa and JG cells.

Step 2: Functional notes

• JG cells secrete renin, not macula densa.

• Macula densa is part of the distal tubule, not proximal.

• JG cells are mainly in the afferent arteriole, not equally in afferent and efferent.

❌ Why the other options are incorrect:

The macula densa is part of the proximal tubule

• Incorrect; it is part of the distal convoluted tubule.

The juxtaglomerular (JG) cells are modified epithelial cells

• Incorrect; they are modified smooth muscle cells, not epithelial.

The juxtaglomerular (JG) cells are found equally in both the afferent and efferent arterioles

• Incorrect; primarily located in the afferent arteriole.

Macula densa cells secrete renin

• Incorrect; renin is secreted by JG cells, not the macula densa.

• The bladder is lined by transitional epithelium (urothelium).

• This specialized epithelium allows the bladder wall to stretch and recoil as urine volume changes.

• Transitional epithelium has multiple layers of cells, with dome-shaped “umbrella cells” on the surface that flatten when stretched.

Nephron ❌

• The nephron is lined by simple cuboidal epithelium in most parts (e.g., PCT, DCT, collecting tubule) and simple squamous epithelium in thin limbs of the loop of Henle.

• Not transitional.

Prostate gland ❌

• The prostate is lined by pseudostratified columnar epithelium, sometimes with areas of simple columnar or cuboidal.

• Not transitional.

Collecting duct ❌

• Collecting ducts are lined by simple cuboidal to columnar epithelium depending on segment.

• Not transitional.

Urethra ❌

• The urethra varies:

  • Prostatic urethra → transitional epithelium.

  • Membranous/penile urethra → pseudostratified/stratified columnar.

  • Distal urethra → stratified squamous.

• But when asked in a single best-answer format, the bladder is the classical structure associated with transitional epithelium.

✅ Correct Answer: Stratified squamous non-keratinizing epithelium

Explanation:

Step 1: Anatomical context

• The navicular fossa is the distal dilated part of the male urethra, located within the glans penis.

• It is part of the penile urethra, which transitions from urothelium proximally to stratified squamous epithelium distally.

Step 2: Histological features

• The distal urethra in the glans penis is lined by stratified squamous epithelium.

• Non-keratinizing type → remains moist, allowing passage of urine and semen without abrasion.

• Proximally, the penile urethra may have pseudostratified columnar epithelium, but the navicular fossa is distinctly stratified squamous non-keratinizing.

Step 3: Functional relevance

• Protects against mechanical stress while maintaining moist environment.

• Prevents injury and maintains smooth passage of fluids.

❌ Why the other options are incorrect:

Pseudostratified epithelium

• Found proximal penile urethra, not in the navicular fossa.

Simple squamous epithelium

• Too thin; cannot withstand mechanical stress in distal urethra.

Columnar epithelium

• Proximal urethra may have columnar cells, but navicular fossa does not.

Stratified squamous keratinizing epithelium

• Found on skin surfaces subjected to friction; the navicular fossa is non-keratinized to stay moist.

✅ Correct Answer: Distal tubule cells – brush border

Explanation:

Step 1: Normal histology of nephron segments

• Visceral layer of Bowman’s capsule:

  • Composed of podocytes with foot processes → filtration slits. ✅ Correct

• Thin segment of Henle’s loop:

  • Simple squamous epithelium, permeable to water and some solutes. ✅ Correct

• Proximal tubule cells:

  • Cuboidal epithelium, numerous mitochondria, prominent brush border for reabsorption. ✅ Correct

• Parietal layer of Bowman’s capsule:

  • Simple squamous epithelium, forms the outer layer. ✅ Correct

• Distal tubule cells:

  • Cuboidal epithelium, lacking a brush border; smaller cells than proximal tubule. ❌ Mismatched

Step 2: Key distinguishing feature

• The proximal tubule has a brush border (microvilli) to increase surface area for reabsorption.

• The distal tubule lacks this brush border; it mainly performs selective ion transport.

❌ Why the other options are correct:

• Visceral layer – podocytes → correct filtration barrier.

• Thin Henle loop – simple squamous → allows passive diffusion of water/solutes.

• Proximal tubule – many mitochondria, ion-pumping → supports high reabsorptive activity.

• Parietal layer – simple squamous → forms outer Bowman’s capsule wall.

• Found in the central part of the glomerulus between capillaries.

• Functions:

  • Matrix-forming → they produce mesangial matrix, which supports the glomerular capillary loops.

  • Contractile → contain actin-like filaments; contraction regulates capillary surface area available for filtration.

  • Phagocytic → they clear immune complexes and trapped residues from the glomerular basement membrane (GBM).

  • Secretion → release cytokines, prostaglandins, and growth factors, contributing to glomerular regulation and pathology.

Podocytes ❌

• Specialized epithelial cells of the visceral layer of Bowman’s capsule.

• Have primary and secondary (foot) processes, forming filtration slits.

• Not phagocytic or matrix-forming.

Distal convoluted tubule cells ❌

• Epithelial cells specialized for reabsorption (e.g., Na⁺, Ca²⁺) under hormonal control (aldosterone, PTH).

• Not located among glomerular capillaries, and not phagocytic.

Juxtaglomerular cells ❌

• Modified smooth muscle cells in the afferent arteriole wall.

• They secrete renin, regulating blood pressure via RAAS.

• Not phagocytic, nor matrix-forming inside the glomerulus.

Basal lamina cells ❌

• Refers to the glomerular basement membrane (GBM) structure itself, not discrete cells.

• Provides filtration barrier but does not actively contract or phagocytose.

Bowman’s capsule (Correct) ✅

• The visceral layer of Bowman’s capsule is formed by specialized epithelial cells called podocytes.

• Podocytes have primary processes that give rise to secondary processes (foot processes or pedicels), which wrap around the glomerular capillaries.

• These processes interdigitate, leaving filtration slits that are spanned by slit diaphragms (containing nephrin protein).

• This structure is essential for selective filtration of plasma into the urinary space.

Distal convoluted tubule ❌

• The DCT contains cuboidal epithelial cells with few microvilli (smooth lumen) but no podocytes and no primary/secondary processes.

Proximal convoluted tubule ❌

• PCT cells are also cuboidal, with a brush border (dense microvilli) to increase surface area for absorption.

• These microvilli are not the same as podocyte processes — they are apical modifications, not specialized interdigitating foot processes.

Macula densa ❌

• This is a specialized region of the DCT near the glomerulus.

• Cells here sense sodium chloride concentration and regulate renin release via juxtaglomerular cells.

• They are tightly packed columnar cells, but they do not have primary or secondary processes.

Collecting duct ❌

• The collecting duct contains principal cells (water and Na⁺ regulation) and intercalated cells (acid–base regulation).

• No podocytes or specialized foot processes are present here.

The proximal convoluted tubule (PCT) is histologically distinct:

• Epithelium: lined by simple cuboidal/low columnar cells.

• Brush border: dense microvilli on the luminal surface → forms a well-developed brush border, key for reabsorption.

• Cytoplasm: intensely eosinophilic due to abundant mitochondria (supports active transport).

• Basal striations: infoldings of plasma membrane with mitochondria → give a striated appearance.

👉 Therefore, the statement “they do not have a typical brush border as compared to the distal convoluted tubule” is false.

• In reality, PCT has a very prominent brush border, while the DCT has little or no brush border.

Why the Other Options Are Correct (Specific to PCT)

They have striated borders due to basal infoldings of plasma membrane ✅

• True: supports ion transport.

They are lined by simple cuboidal/low columnar epithelium ✅

• Characteristic of PCT histology.

They have microvilli on their luminal surfaces forming a brush border ✅

• True: hallmark of PCT, allows maximum reabsorption.

They are more eosinophilic due to numerous mitochondria ✅

• True: mitochondria provide energy for active reabsorption.

Why “Ureter” is Correct:

The question provides two key histological clues:

1. Lined by Transitional Epithelium (Urothelium): This is a specialized epithelium designed to stretch and provide a barrier against urine. It is a hallmark of the urinary system. It lines the renal pelvis, ureters, urinary bladder, and proximal urethra.
2. Three Layers of Smooth Muscle in its Wall: This is the most specific clue.
   • The muscularis layer of the ureter has a unique and characteristic arrangement: inner longitudinal, middle circular, and outer longitudinal layers.
   • This specific three-layer pattern is a key identifying feature of the ureter in histology.
3. Narrow Lumen: This description fits the ureter perfectly, as it is a tube with a star-shaped or narrow lumen, especially when not distended by urine.

Therefore, the combination of transitional epithelium + three smooth muscle layers + narrow lumen is diagnostic for the ureter.

Why the Other Options Are Incorrect:

1. Vas Deferens: Incorrect.
   • Lining: It is lined by a pseudostratified columnar epithelium with stereocilia, not transitional epithelium.
   • Muscle: It has a very thick, powerful muscular wall with three layers (inner longitudinal, middle circular, outer longitudinal), but the epithelial lining is the definitive reason to rule it out.
2. Urethra: Incorrect.
   • Lining: The epithelial lining varies along its length: transitional (in the proximal/prostatic part), stratified columnar, and finally stratified squamous at the distal end. It is not exclusively transitional.
   • Muscle: Its muscularis is not consistently organized into three distinct layers like the ureter’s. It often has inner longitudinal and outer circular layers mixed with elastic tissue and skeletal muscle (in the membranous urethra).
3. Prostate: Incorrect.
   • This is not a hollow tube with a “lumen” in the same sense. The prostate is a glandular organ. While it has ducts lined by simple columnar epithelium, the organ itself is not “lined” by an epithelium. Its stroma contains smooth muscle, but it is not arranged in three distinct layers.
4. Urinary Bladder: Incorrect.
   • Lining: It is correctly lined by transitional epithelium.
   • Muscle: This is the key differentiator. The detrusor muscle of the bladder is very thick, but its three layers (inner longitudinal, middle circular, outer longitudinal) are intermingled and not nearly as distinct as the well-organized, separate layers seen in the ureter. The lumen of the bladder is also large and expansive, not “narrow.”

A renal lobule is defined as:

• A central medullary ray in the cortex, surrounded by nephrons (cortical tissue) that drain into it.

• Each lobule drains into a single collecting duct within the medullary ray.

👉 Thus, the medullary ray lies at the center of a renal lobule.

Why the Other Options Are Wrong

Interlobar artery ❌

• Found between renal pyramids, running in the renal columns.

• Not in the center of a lobule.

Cortical column ❌

• Extension of cortex between renal pyramids.

• Helps separate renal lobes, not lobules.

Arcuate artery ❌

• Located at the corticomedullary junction, arching along the base of the pyramids.

• Not central to a lobule.

Renal hilum ❌

• The entry/exit point of vessels, nerves, and ureter in the kidney.

• Not part of lobular organization.

🔎 Step 1: Key Histological Features

The slide shows:

1. Small-caliber tubular structure

2. Lined by transitional epithelium (urothelium)

3. Three layers of smooth muscle in the wall

🔎 Step 2: Matching Features to Structures

• Vas deferens

  • Lined by pseudostratified columnar epithelium, not transitional

  • Thick muscular wall, but usually 2–3 layers, not classic “urothelium”

• Urethra

  • Lined by transitional epithelium proximally, stratified/pseudostratified columnar or squamous distally

  • Smooth muscle layers not always three distinct layers

• Ureter

  • Lined by transitional epithelium throughout

  • Three layers of smooth muscle (inner longitudinal, middle circular, outer longitudinal)

  • Small-caliber tubular structure

• Prostate

  • Glandular, not tubular

  • Lined by simple or pseudostratified columnar epithelium, not transitional

  • Smooth muscle surrounds glands but not in three distinct layers

• Urinary bladder

  • Lined by transitional epithelium

  • Wall has three layers of smooth muscle (detrusor muscle)

  • Typically a larger organ, not “small-caliber tubular”

✅ Correct Answer: Ureter

• Small-caliber tubular structure

• Transitional epithelium

• Three distinct layers of smooth muscle

❌ Why the Others Are Incorrect

• Vas deferens → Pseudostratified columnar epithelium, muscular wall but not classic three smooth muscle layers

• Urethra → Transitional only proximally; variable epithelium and less distinct smooth muscle layering

• Prostate → Glandular, not tubular

• Urinary bladder → Large organ, not small-caliber tubular

https://medcell.org/histology/urinary_system_lab/collecting_ducts.php

https://histology.siu.edu/crr/RN009b.htm

1. Histological Appearance (Cuboidal with Distinct Boundaries)

• Cell Shape: The collecting duct is lined by simple cuboidal epithelium (which can become columnar in the deeper medullary regions).

• Tight Junctions (Distinct Borders): Under light microscopy, the cells of the collecting duct (specifically the Principal Cells) are often described as having distinct cell boundaries. This is a key differentiator from the convoluted tubules (PCT and DCT), where the extensive interdigitations of the lateral membranes make the cell borders appear indistinct or absent. These distinct boundaries correspond to the tight junctions that are crucial for the duct’s regulated permeability.

• Cytoplasm: The cytoplasm is typically pale-staining (clear) compared to the eosinophilic (pink) cytoplasm of the proximal and distal tubules.

2. Physiological Function (Urine Concentration)

• The question specifies a “region responsible for urine concentration.” While the Loop of Henle creates the hyperosmotic gradient, the Collecting Duct is the site where the final concentration of urine actually occurs.

• Under the influence of ADH (Vasopressin), aquaporins are inserted into the apical membranes of these cells, allowing water to be reabsorbed into the hypertonic medullary interstitium, thus concentrating the urine.

Why the other options are unlikely

• Proximal Convoluted Tubule (PCT):

  • Histology: While cuboidal, these cells have a prominent brush border (microvilli) which fills the lumen, and the cell boundaries are indistinct due to basolateral interdigitations.

  • Junctions: The tight junctions here are actually “leaky” to allow paracellular transport of water and ions.

  • Function: Responsible for bulk reabsorption (isomotic), not the final concentration of urine.

• Loop of Henle (Thick Ascending Limb):

  • Histology: Cuboidal cells with indistinct borders (similar to DCT).

  • Function: This is known as the “Diluting Segment.” Although it creates the medullary gradient necessary for concentration, the fluid leaving this segment is actually hypotonic (dilute) because it actively pumps out salts while being impermeable to water.

  • Junctions: It does have very tight junctions (to maintain water impermeability), but the “diluting” function and indistinct borders usually make it a less likely answer than the CD for this specific description.

• Distal Convoluted Tubule (DCT):

  • Location: Located in the cortex, not the medullary region primarily associated with the concentration gradient.

  • Histology: Cuboidal cells with no brush border, but cytoplasm is eosinophilic and borders are less distinct than in the collecting duct.

• Bowman’s Capsule:

  • Histology: The parietal layer is simple squamous epithelium, not cuboidal. The visceral layer consists of podocytes.

Summary Table

https://www.histology.leeds.ac.uk/urinary/bladder.php

An organ with transitional epithelium (urothelium) + a thick muscle coat + highly folded lumen is characteristic of the urinary bladder, especially when empty.

The folds (rugae) allow distension as it fills.

✔ Why This Option Is Correct

Urinary bladder — ✅

• Lined by transitional epithelium

• Has very thick smooth muscle layer (detrusor muscle)

• Highly folded lumen when empty → hallmarks of urinary bladder histology

• The organ’s function requires extreme expansion and contraction.

❌ Why The Other Options Are Incorrect

Kidney — ❌

• Lined mostly by simple cuboidal epithelium (tubules).

• No transitional epithelium lining large cavities.

• Cortical/medullary patterns—not thick smooth muscle walls.

Ureter — ❌

• Does have transitional epithelium,

• BUT muscle layer is moderately thick, not as massive as the bladder.

• Lumen is less folded and usually appears star-shaped.

Urethra — ❌

• Epithelium varies:

  • Transitional → pseudostratified columnar → stratified squamous.

• Does not have such a thick smooth muscle coat.

• Folds not as prominent and lumen shape is irregular.

Nephron — ❌

• Microscopic unit, not a gross organ.

• Lined by various epithelium types (simple squamous, cuboidal).

• No transitional epithelium + no thick muscular wall.