L23: Microanatomy of the Kidney

Microscopic structure of the outer cortex

- renal corpuscles with glomerulus

- pars convoluta with proximal and convoluted tubules

- pars radiata (medullary rays) collecting ducts

- blood vessels

Functional unit of the kidney

nephron/renal tubule

Renal corpuscle

blood-filtering component of the nephron

Glomerulus

capillary plexus which is enveloped by Bowman's capsule, responsible for blood filtration

Bowman's Space

surrounds the glomerulus creating the renal corpuscle

Proximal convoluted tubule

first & longest part of the proximal tubule

Proximal straight tubule

continues from the proximal convoluted tubule & descends into the medulla becoming the thin descending limb

Loop of henle

section of the nephron tubule that conserves water and minimizes the volume of urine

Distal convoluted tubule

shorter than the proximal convoluted tubule & no brush border/microvilli

Papillary duct

formed by the merging of several medullary collecting ducts

What is the functional unit of the kidney?

the nephron

Where is the proximal convoluted tubule located?

in the cortex

What is the area surrounding the glomerulus called?

Bowman's space

Kidney capsule is formed by

dense connective tissue

The outer cortex of the kidney appears

granular, uniform or lobated

The inner medulla of the kidney appears

smooth, mostly contains tubular parts of the nephron and collecting system

Kidney gross histology

Kidney cortex histology

How can you differentiate proximal convoluted tubules from distal convoluted tubules?

proximal convoluted tubules: less distinct lumen, contains microvilli

distal convoluted tubules: more distinct lumen, does not contain microvilli

Kidney medulla histology

What is contained within the medulla of the kidney?

collecting ducts, descending thin & ascending thin segments of loop of Henle, ascending thick sengment, vasa recti, peritubular capillaries

Considering the structure of the nephron, which structure will appear only in the section through the medulla of the kidney?

thick limb of Henle's loop & thin limb of Henle's loop

Nephron anatomy

Juxtamedullary nephron

type of nephron located near the boundary of the cortex & medulla, concentrates urine

Cortical nephron

nephrons located almost entirely in the renal cortex, loop of Henle is shorter

What structure makes up the peritubular capillary network?

efferent arterioles

What makes up the parietal layer of the Bowman's capsule?

simple squamous epithelium

What makes up the visceral layer of the Bowman's capsule?

podocytes

What occurs in the Bowman's space?

pre-urine filtrate is collected in this space

Vascular pole of the Bowman's capsule

the side of the capsule where the afferent and efferent arterioles enter and exit the glomerulus

Urinary pole of the Bowman's capsule

the side of the glomerulus where the filtrate exits and enters the proximal convoluted tubule

What is the function of the Glomerulus?

filtration of the blood (20%), 99% is reabsorbed

What is in the filtrate from the Bowman's capsule?

water, glucose, nutrients, bicarbonates, amino acids, small proteins, lipids, electrolytes

What is the purpose of the porous capillaries in the glomerulus?

allows substances of certain sizes to pass through

Glomerulus composition

- porous capillaries

- basement membrane with collagen type 4 layer

- surrounded by podocytes

What is the function of the larger lumen of the afferent arteriole and the smaller lumen of the efferent arteriole?

creates a pressure gradient encouraging filtration into the bowman's space

Podocytes

form the visceral layer of the bowman's space, act as a filtration barrier to control amount of filtrate produced

- contain cell body, primary processes, secondary processes

What do the podocytes form?

filtration slits that allow for filtration

Mesangial/intercapillary cells

holds the capillary tuft in place, connects the distal convoluted tubule at the vascular pole

- help to maintain GFR

- produce extracellular matrix

- regulate capillary blood flow

- phagocytosis

- secrete growth factors

Renal corpuscle histology

What epithelium is present in the parietal layer of the Bowman's capsule?

simple squamous epithelium

Proximal convoluted tubule & proximal straight tubule composition

- simple columnar epithelium with Acidophilic cytoplasm

- microvilli/brush border and basal striations

- microvilli less distinct in PST

Proximal convoluted tubule is responsible for how much reabsorption?

65%

Proximal convoluted tubule histology

Hele's loop: descending thin limb

simple squamous epithelium, nuclei are somewhat flattened & protrude into the lumen

Henle's loop: ascending thick limb

reabsorption of Na,Cl,K,Ca,Mg

What segment of the nephron reabsorbs most of the water from the glomerular filtrate?

the proximal convoluted tubule (65%)

Macula densa of the juxtaglomerular apparatus

modified cuboidal to columnar cells, sensitive to Cl ion and regulate filtration rate

Juxtaglomerular cells (JG) in afferent arteriole

modified smooth muscles, produce renin

Juxtaglomerular apparatus

specialized structure formed by the distal tubule & afferent arteriole, regulates blood pressure & glomerular filtration rate through the secretion of renin

The smaller collecting ducts are lined by

- simple cuboidal epithelium

- lateral borders are clear

The larger collecting ducts are lined by

simple columnar epithelium

The papillary duct is lined by

two epithelium layers & transitional epithelium

Calyces and renal pelvis are lined by

transitional epithelium and underlying loose connective tissue layer

What is responsible for the cloudy-like appearance of the urine in horses?

mucous glands present under the epithelium in the calyces and renal pelvis