Histology Flashcards
What are the 2 main functions of the kidney and urinary tract?
- Maintenance of water and electrolyte homeostasis, body fluid osmolarity and acid-base balance
- Excrete toxic metabolic waste products (mainly urea and creatinine) and drugs
Name another function of the kidneys
Act as an endocrine gland, producing renin and erythropoietin (a hormone that stimulates rbc production)
What components make up a ‘lobe’ of the kidney?
Each medullary pyramid and its associated cortical tissue
The renal medulla is divided into 8-18 medullary pyramids.
What is meant by the renal papillae?
The apices of the medullary pyramids which project into the minor calyces and drip urine
What is a renal column?
The parts of the renal cortex which separate two medullary pyramids
Branches of the renal artery enter the renal medulla upon entering the kidney. T/F?
False
Arteries avoid the medulla and head straight up the renal columns to the cortex
What is the renal sinus?
The space within the kidneys which is neither cortex not medulla - it is mainly filled with adipose tissue (fat)
The connective tissue that lines the renal sinus is continuous with…?
The fibrous capsule which covers the kidney
What sort of tissue makes up the…
- Outer layer
- Inner layer
.. of the renal capsule?
Outer layer: collagenous
Inner layer: more cellular
Each human kidney contains 600,000 - 1 million nephrons. What is a nephron and what 2 components is it formed from?
The nephron is the basic functional unit of the kidney
It is composed of the renal corpuscle and renal tubules
The renal corpuscle is found in the cortex and contains what 2 structures?
- The glomerulus (a tangle of capillaries)
- Surrounded by Bowman’s capsule (which the glomerulus has invaginated into like a fist into a balloon)
The renal tubules include the… (3)
- Proximal convoluted tubule
- Loop of Henle
- Distal convoluted tubule
What is the function of the renal corpuscle? (glomerulus + tubules)
Production and collection of glomerular filtrate
Describe the blood flow to and from the glomerulus
Blood is supplied via an afferent arteriole
It is drained via an efferent arteriole (not a vein!, this is unique)
What kind of cells form the Bowman’s capsule?
A single layer of epithelial cells called podocytes
Describe the structure of a podocyte
- Cell body
- Primary processes, give off branches known as…
- Secondary processes, give off branches known as…
- Tertiary processes = pedicels
- Pedicels interdigitate with pedicels from another podocyte, the spaces between the pedicels are known as the filtration slits
What is meant by the urinary space?
The space between the two cell layers of the Bowmann’s capsule (as the glomerulus pushes into and invaginates it like a fist into a balloon)
It is where the filtrate will collect
How does the structure of the podocytes help with filtration?
The podocytes have foot processes that form filtration slits for the blood to enter from the capillaries
How does the structure of the capillary endothelium help with filtration?
It is fenestrated
The basal lamina/basement membrane between the podocytes and the capillary endothelium is thinner/thicker than usual?
Thicker (~6x so)
What is the stalk-like core of the glomerulus formed from?
Mesangial cells and their ECM
What are mesangial cells?
Cells found in the core of the glomerulus which produce a connective tissue core called the mesangium
Each glomerulus has a X pole and a Y pole
X = vascular (where afferent and efferent arterioles connect) Y = urinary (where the renal tubule is found)
Recap the 3 components of the glomerular filter:
- Fenestrated endothelium of the capillary wall
- Thick basement membrane
- Filtration slits between pedicels (foot processes of the podocytes)
What is the function of the proximal convoluted tubule?
Reabsorption of important substances from the glomerular filtrate e.g., water, proteins, amino acids, carbohydrates and glucose
In the proximal tubule, what proportion of... - Na+ - H2O - Glucose - Amino acids ... are reabsorbed?
- Na+: 70% (active transport)
- H2O: 70% (follows Na+ by diffusion)
- Glucose and amino acids: almost all (co-transport)
Describe the structure of a proximal convoluted tubule (PCT) cell
- Simple cuboidal epithelial cells
- Microvilli form a brush border at the apical membrane to increase SA
- Contain lots of mitochondria for energy
- Substances are transported across the basal membrane to enter the interstitium/capillaries
What substances pass through the PCT cell?
H2O, Na+, Cl-, proteins, amino acids, glucose etc from the filtrate
How can the PC tubules be distinguished from the distal convoluted tubules on histology?
PC tubules are slightly larger and they have a less well defined luminal margin due to the brush border
The proximal convoluted tubule briefly straightens out to become the X.
It then enters the Y to form Z.
X - proximal straight tubule
Y - renal medulla
Z - the loop of Henle
What is the function of the loop of Henle?
To create and maintain the hyperosmotic environment in the medulla (i.e., the high salt conc.)
Describe the structure of the loop of Henle
- Short thick descending limb
- Long thin descending limb
(makes a hairpin turn then…) - Long thin ascending limb
- Short thick ascending limb
What sort of cells line the…
- Thin
- Thick
… limbs of the loop of Henle?
Thin limbs: simple squamous epithelium
Thick limbs: simple cuboidal epithelium
How does the difference in the structure of the thin and thick limbs of the loop of Henle help to create and maintain the hyperosmotic interstitium?
The permeability to water and ions and the active transport of ions varies in the different parts of the loop of Henle
A normal blood supply entering from the top and exiting from the bottom of the medulla would carry away the salts and destroy the high osmotic gradient.
How is this problem solved?
Loops of thin walled blood vessels follow the loop of Henle down into the medulla and back up into the cortex
What are the loops of blood vessels that follow and supply the loop of Henle called?
The vasa recta
How can the vasa recta be told apart from the tubules on histology?
- They are less circular in shape
- They may contain blood within them
The thick ascending limb of the loop of Henle exits the medulla and becomes the X, then the Y
X - distal straight tubule
Y - distal convoluted tubule
What is the function of the distal convoluted tubule?
Maintaining acid-base and water balance
through the absorption of H2O, Na+ and HCO3-, and the excretion of K+ and H+
Describe the structure of the distal convoluted tubule cell
- Simple cuboidal cell
- Apical membrane has occasional microvilli but it relatively smooth
- Lots of mitochondria
- Invaginations at the base of the cell
- Sits on a basal lamina as usual
How can the DC tubules be distinguished from the proximal convoluted tubules on histology?
The distal convoluted tubules are slightly smaller and have a more obvious luminal margin because they only have sparse microvilli
Reabsorption of Na+ in the DCT is controlled by the hormone…?
Aldosterone
released by the adrenal cortex to increase BP by retaining H2O and Na+
The DCT is the last part of the nephron. What does it empty into?
The collecting tubules and collecting ducts
What is the function of the collecting tubules and the collecting ducts?
Controlled reabsorption of water under the control of ADH
Describe how ADH controls the reabsorption of water in the collecting tubules and ducts on a hot day or when their is low water intake
- ADH is released by the posterior pituitary
- This increases the permeability of the collecting ducts to water
- Due to the high salt content in the medulla, water passes out of the tubules
- This results in more water reabsorption and more concentrated urine
The renal papillae are the apices of the medullary pyramids which project into the minor calyces. This is where the collecting ducts have their terminal openings to drip out urine.
What are these terminal openings of the collecting ducts into the renal papillae called?
The ducts of Bellini
What structures make up the collecting and storage elements of the urinary system?
Minor calyces Major calyces Renal pelvis Ureter Bladder Urethra
Almost all of the conducting parts of the urinary tract are lined by what sort of epithelium?
Transitional epithelium aka urothelium
Describe the structure of transitional epithelium/urothelium
Why is it called transitional epithelium?
- 3-6 stratified cell layers
- Cells at the luminal surface (facing the urine) are domed and have a thickened, inflexible membrane
Called transitional epithelium because the cell layers are transitioning from thick to thin membraned
In transitional epithelium/urothelium, what are the domed cells facing the luminal surface called and why do they have a special structure?
Umbrella cells (due to their domed shape)
Reason for special structure:
- To provide a highly impermeable barrier
- Thickness will vary depending on different states of distention (e.g., thinner when the bladder is full and distended)
What surface structure on the umbrella cells helps them to cope with distention?
They have thick plaques on their apical surface which allow the cells to fold/invaginate away when the bladder is non-distended, then re-emerge when the bladder is distended
The smooth muscle around the X and Y is well organised
X - ureter
Y - bladder
Describe the histology of a cross-section of the ureter. Start with the lumen in the centre and move outwards
- Lumen
- Transitional epithelium/urothelium
- Lamina propria (connective tissue)
- Longitudinal layer of smooth muscle
- Circular layer of smooth muscle
- Adventitia or serosa
Which layers of the ureter make up the mucosa?
The transitional epithelium/urothelium and lamina propria (connective tissue)
How does the structure of the ureter change as it nears the bladder?
There are 3 layers of smooth muscle instead of 2, an inner longitudinal layer, a middle circular layer, and an outer longitudinal layer
Describe the histology of a cross section of the bladder
- Bladder lumen
- Transitional epithelium/urothelium
- Lamina propria (connective tissue)
- Smooth muscle layers forming the detrusor muscle
Describe the female urethra in terms of length and type of epithelium
3-5 cm length
Initially lined by transitional epithelium, transitions to stratified squamous epithelium near its termination
Describe the male urethra in terms of length and type of epithelium in the 3 sections
~20cm in length
Prostatic urethra (bladder + through prostate): 3-4cm, transitional epithelium Membranous urethra (prostate to bulb of penis): ~1cm, transitional epithelium changing to stratified columnar Penile urethra: ~15cm, stratified columnar epithelium, becomes stratified squamous near the tip of the penis
