Histology 2: Urinary System

Question 1

Describe the histological structure of the kidney including all layers and regions. You should be able to recognize these features on microscopic slides and drawings.

Answer 1

a. Covered with CT capsule. Formed by dense irregular CT (collagen fibers, fibroblasts, myofibroblasts)
b. Hilum is medially located deep fissure through which blood vessels and nerves enter kidney.
c. Conducting system = calyces (minor and major) and renal pelvis

d. Cortex (external)
- i. Dark, red-brown compared to lighter medulla. Contains 90-95% of blood in kidney.
- ii. Consists of kidney tubules (renal corpuscles, blood vessels, and interstitial CT)
- iii. Convoluted tubules form cortical labyrinth
- iv. Medullary rays are straight tubules and collecting ducts in radial rays
- v. Renal columns split pyramids of medulla.

e. Medulla
-i. Mostly consists of renal tubules and collecting ducts.
-ii. Tubules form pyramids (8-12 in average human kidney). Base of pyramid faces cortex,
with apex (renal papilla) projecting into minor calyx. Can be further divided
–1) Outer medulla - pyramid closest to cortex. Further divided into inner and outer
stripes. Consists of different parts of loop of Henle
—a) Outer stripe = thick tubules and collecting ducts
—b) Inner stripe = thin limbs of loop and collecting ducts
–2) Inner medulla = thin tubules and collecting ducts.
–3) Renal Papilla - projects into minor calyx. Area cribrosa is where duct empties
renal papilla outer medulla into calyx.

Question 2

Describe the structure of the nephron, connecting tubule, and the collecting duct. Relate the structure of a renal corpuscle and renal tubules, including proximal convoluted tubule, distal convoluted tubule, and the loop of Henle, as well as their functions.

Answer 2

a. Consists of renal corpuscle and renal tubules
-i. Corpuscle - filters blood. Consists of glomerulus, which is surrounded by Bowman’s
Capsule.

• -1) Bowman’s Capsule = 2 layers
• –a) Parietal layer - formed by simple squamous
• –b) Visceral layer - formed by modified epithelium = Podocytes
• —i) Podocytes - form numerous processes covering capillaries that line glomerulus. Has large (primary) and small (secondary/pedicels) processes. Between pedicels = filtration slits, allowing filtration into primary urinary space.
• –c) Urinary space separates parietal and visceral layers. Filtrate enters proximal convoluted tubule. Has vascular and urinary poles.

–2) Glomerulus - cluster of fenestrated capillaries.
—a) Fenestrae do not have diaphragms, preventing formed blood elements
from passing through.
—b) Mesangial cells - located between capillaries. Several functions
—-i) Possess contractile components - REGULATE GLOMERULAR BLOOD FLOW (Angiotensin II REDUCES blood flow)
—-ii) Structural support
—-iii) Phagocytosis of residue and proteins trapped in basal lamina

• 1) Proximal convoluted tubule (PCT)- begins from urinary pole. Formed by large cuboidal cells with distinct BRUSH BORDER. Possesses basal striations. Fluid is isotonic when it leaves
• -a) FUNCTION:
• –i) reabsorb Aas, proteins, glucose, and ions.
• –ii) Eliminates: organic solutes, drugs and toxins.
• 2) Loop of Henle
• -a) Thick descending (proximal straight) - simple cuboidal epithelium. Similar in function to PCT, but not as specialized.
• -b) Thin descending - simple squamous - highly permeable for water. Water diffuses, NaCl and urea enter here. Filtrate = HYPEROSMOTIC
• -c) Thin ascending - simple squamous. Highly permeable for NaCl, not H2O. Becomes HYPOTONIC
• -d) Thick ascending - simple cuboidal. Similar to distal convoluted tubule. Impermeable to water and urea, transfers NaCl from lumen to interstitium. Filtrate becomes more hypotonic.
• 3) Distal convoluted tubule - cuboidal WITHOUT brush border. Numerous interdigitations and basal striations. Connect to connecting tubule. Reabsorbs more NaCl, HCO3

iii. Connecting tubule - intermediate between DCT and collecting duct.

iv. Collecting duct - lined with simple cuboidal epithelium. Located within both cortex
and medulla = cortical and medullary collecting ducts. Towards apex of pyramid = papillary ducts, which empty into minor calyx.
-1) Principal cells - main cell type. Respond to VASSOPRESSIN (ADH), which opens water channels. This causes water to move towards the highly osmolar interstitium from the lumen, decreasing volume and increasing urine concentration. Reabsorbs NaCl and secretes K as well.
-2) Intercalated cells - regulate pH by H+ and HCO3- transportation.

Question 3

Describe the differences between juxtamedullary and cortical nephrons.

Answer 3

a. Juxtamedullary nephrons - have renal corpuscles located close to medulla. These have long loops of Henle, with both descending and ascending thin segments. Projects into inner medulla.
b. Cortical nephrons - renal corpuscles closer to capsul. Short loop of Henle WITHOUT ASCENDING THIN SEGMENT. Only enters inner strip of outer medulla.

Question 4

Describe the components of the juxtaglomerular apparatus and relate the structure of these components to their function.

Answer 4

a. FXN - important in regulating blood pressure through RENIN production
b. 3 major components
i. Macula densa - modified epithelium of distal portion of thick ascending limb of loop of Henle. Senses Cl concentration in tubule.
ii. Juxtaglomerular cells - Contains granules with renin. Respond to low blood pressure, Renin-Aldosterone-angiotensin II
iii. Extraglomerularmesangialcells-providestructuralsupportandtransmitsignalsfrom macula densa to juxtaglomerular cells.

Question 5

Describe the histological structure of all the layers of the ureter, urinary bladder, and male and female urethra.

Answer 5

a. Ureter - transitional epithelium.
- i. Muscularis - 2-3 layers of smooth muscle (inner longitudinal, middle circular, maybe outer longitudinal)
- ii. Adventitia - formed by CT with adipose, nerves and blood vessels.

b. Urinary bladder -
- i. Transitional epithelium, as many as 6 layers when relaxed. Thick lamina propria.
- ii. Muscularis - thicker = detrusor muscle. Similar identifiable layers as ureter, with significant collagen fibers.
- iii. Serosa/adventitia - same as ureter.

c. Urethra -
- i. Male
- -1) Prostatic - lined with transitional epithelium
- -2) Membranous - contains skeletal muscle fibers. Lined with either pseudostratified or stratified columnar
- -3) Spongy - pseudostratified proximally, very distal end = non-keratinized stratified squamous.

• ii. Female - transitional proximally to pseudostratified/stratified distally. Extremely vascularized lamina propria.
• iii. Pockets of mucous glands = urethral glands.