LEARNING OBJECTIVES/ LECTURE OUTLINES
(Microscopic features of kidney)
• Identify the cortex, medulla, calyces, pyramids, renal papillae and ureter in a diagrams
or
section of a kidney.
• Explain with labeled diagrams the organization of a renal corpuscle.
• Specify the kidney components involved in producing urinary filtrate and state the role
of
each component.
• Name the major parts of a nephron.
• Recognize the proximal and distal convoluted tubules and thin-walled parts of loops of
Henle in section.
• State where juxtaglomerular cells are located and discuss their significance.
Describe the sequence of the blood vessels supplying the kidney beginning with the renal
artery
and ending with the renal vein
Kidneys are reddish brown paired organs, situated along the posterior abdominal wall.
They are covered by thin capsule, which is easily removed
The cut surface shows outer cortex and internal medulla.
Inside the medulla there is renal sinus containing cup shaped major and minor calycies
The renal medulla consists of pale, striated, conical renal pyramids.
Their bases are peripheral while their apices converging to the renal sinus where they
project into minor
. Each pyramid is capped by cortical tissue to form renal lobe.
The renal cortex is subcapsular; it also extends towards the renal sinus as renal columns.
The medullary tissue extends into the cortex as medullary rays.
Cortex close to the medulla is sometimes termed juxtamedullary cortex.
Microscopic Features
• Kidney is composed of many tortuous closely packed uriniferous tubules, bound by
little
connective tissue in which run blood , lymphatics and nerves.
• Each uriniferous tubule consists of two embryologically distinct parts.
1. The secreting Nephrons which elaborate urine and
2. Collecting tubule which convey urine to the calyceal system.
Kidney and Urineferous Tubule
• Each kidney consists of about 1,000,000 nephrons,
• The nephrons begins as double walled epithelial cup (Bowman’s capsule), which
surrounds a tuft
of capillaries, the glomerulus.
• Bowman’s capsule and the glomerulus together form the renal or Malpigian corpuscle.
Bowman’s capsule consists of two simple epithelial layers; an inner or visceral layer
covering the
glomerulus and an outer parietal layer. The region where the vessels enter or leave the
glomerulus is
the ‘vascular pole of the corpuscle.
‘Urinary pole’ of the corpuscle is roughly opposite the vascular pole; here the parietal
layer of
Bowman’s capsule becomes continuous with proximal tubule.
The Nephron and Renal Corpuscle
The cells of the visceral epithelium are specialized, called ‘podocytes’.
The nucleus and surrounding cytoplasm protrude into the capsular space and rests on the
processes that
extend from the cell body, branching one or more times to form a series of slender
processes on basal
The terminal processes or pedicles interdigitate with the pedicles of neighboring
podocytes; leaving a
series of slits between them.
Parietal Layer of Bowman’s capsule
Parietal layer of bowman’s capsule is lined by simple squamous epithelium.
This layer becomes continuous with the lining of proximal convoluted tubule.
Tubular Part of Nephron
The tubular portion of nephrons is composed of
Both proximal and distal tubules have a convoluted portion and a straight portion.
The straight portion of the proximal tubule and distal tubule together with the thin
segment are arranged
in a hairpin loop extending a variable distance into the medulla. These three segments
together are called
The convoluted portion of the distal tubule joins the arched collecting tubule which
begins system of the
ducts or ‘ collecting tubules’.
The proximal tubule is the longest section of the nephron (about 14 mm).
The convoluted part of the proximal tubules coils close to the glomerulus in the cortex.
The diameter of proximal tubules is ~65 µm. Their walls are formed by a low columnar
epithelium.
The eosinophilic cells of the epithelium have a wide brush border
They almost completely reabsorb substances of nutritional value from the glomerular
filtrate (glucose,
amino acids, protein, vitamins etc).
In the proximal tubules the volume of the glomerular filtrate is reduced by about 75%.
Sodium ions are actively resorbed from the glomerular filtrate. They are followed by
passively diffusing
chloride ions and the osmotic absorption of water.
The straight portion of the proximal tubule descends towards the medulla.
The straight portion of the proximal tubule merges with the intermediate tubule the thin
segment of the
A flattened, only ~1-2 µm high epithelium (simple squamous) forms the intermediate
tubule, which is
The diameter of the distal tubule expands to ~35 µm.
The distal tubule, is formed by low cuboidal cells without a brush border.
Basal infoldings and all the cell organelles are present.
Function of Distal convoluted tubule
Epithelial cells in distal tubules cells transport chloride (active) and sodium ions (passive)
into the
surrounding peritubular space.
Cells in the distal tubules are sensitive to the hormone aldosterone.
Juxta-glomerular Complex
The distal tubule contacts the glomerulus forming a specialized section of tubular
epithelium, the macula
At the point of contact with the glomerulus, the distal tubule is always in close contact
with the efferent
and afferent arterioles of the glomerulus.
The juxtaglomerular cells surround the afferent arteriole (they are modified smooth
muscle cells), which
produce and secrete renin.
Third component of the juxtaglomerular complex is extra-glomerular mesangial cells
Renin activates angiotensinogen, a precursor found in the blood stream, leading to the
formation of
Angiotensin I is converted to angiotensin II.
Angiotensin II is the most potent vasoconstrictor known.
Renin also stimulates the secretion of aldosterone
Blood supply of kidney
Renal artery is a branch of abdominal aorta.
Near the hilum it divides into anterior & posterior branches.
These branches further divide into segmental arteries‡ apical upper, middle lower &
posterior.
1. Interlobar arteries
2. Arcuate arteries
3. Interlobular arteries
4. Afferent arteriole
6. Efferent arteriole
Cortical and medullary capillaries →interlobular vein →arcuate vein→renal vein
The minor calyces merge to form major calyces within the kidney.
Major calyces in turn merge to form the renal pelvis (still within the kidney).
The urine flows through these structures to the ureter and is channeled to the bladder.
The basic structure of all these components is the same.
There are three layers in the wall:
The mucosa is lined with a transitional epithelium, the epithelium is virtually
impenetrable to any
components of the urine.
Part of Wall of Ureter
1. The lamina propria of the mucosa consists mainly of dense connective tissue, with
many bundles
of coarse collagenous fibers.
2. The muscularis usually consists of an inner longitudinal and outer circular layer of
smooth
In lower parts of the ureter and the bladder an additional outer longitudinal layer of
muscles is added to
Outermost layer is adventitia, as throughout its course it lies retroperitoneally.
Part of Wall of Ureter
a) Transitional epithelium
b) Lamina propria
3) Serosa/adventitia
Transitional epithelium
3---6 layers of cells, Basal: cuboidal/low columnar, Intermediate layer: Polygonal
Superficial: Umbrella shaped may be binucleate
Plasmalemma; unusually thick due to membrane internalization
Impermeable, with occluding junction
When the epithelium is stretched, as when the bladder is full of urine, the epithelium is
only three or
four cells in thickness, and the superficial cells become squamous.
The superficial cells of the transitional epithelium have a special membrane of thick
plates separated by
narrow bands of thinner membrane.
When the bladder contracts, the membrane folds along the thinner regions, and the
thicker plates
invaginate to form fusiform cytoplasmic vesicles.
These vesicles represent a reservoir of these thick plates that can be stored in the
cytoplasm of the cells
of the empty bladder and used to cover the increased cell surface in the full bladder.
Lamina propria consists of connective tissue, vessels n nerves.
Muscle layer (detrusor muscle) consists of inner and outer longitudinal layers of smooth
muscles and
The urethra is a tube that carries the urine from the bladder to the exterior
Towards the neck of bladder the smooth muscle fibers form the internal urethral
sphincter, which leads
The female urethra is a tube 4-5 cm long, lined with stratified squamous epithelium and
areas of
Pseudostratified columnar epithelium.
• In females, mucosa is surrounded by a layers of circular smooth muscle.
• The mid part of the female urethra is also surrounded by an external striated voluntary
In male, three divisions:
1. Prostatic urethra,
2. Membranous urethra,
3. Spongy or penile urethra.
In males urethra also discharges semen
1- Prostatic Part of Urethra
In the dorsal and distal part of the prostatic urethra, there is an elevation, the
verumontanum (from
Latin, meaning mountain ridge), that protrudes into its interior.
A closed tube called the prostatic utricle opens into the tip of the verumontanum; this
tube has no
The ejaculatory ducts open on the sides of the verumontanum.
The seminal fluid enters the proximal urethra through these ducts to be stored just before
ejaculation.
The prostatic urethra is lined with transitional epithelium
2- Membranous Urethra
The membranous urethra extends for only 1 cm and is lined with stratified or
pseudostratified columnar
Surrounding this part of the urethra is a sphincter of striated muscle, the external
sphincter of the
3-Penile or Spongy Urethra
The Penile or spongy part of the urethra is located in the corpus spongiosum of the penis.
The urethral lumen dilates distally, forming the fossa navicularis.
The epithelium of this portion of the urethra is mostly pseudostratified columnar, with
stratified
Littre's glands are mucous glands found along the entire length of the urethra but mostly
in the spongy
The secretory portions of some of these glands are directly linked to the epithelial lining
of the urethra;
others have excretory ducts.
Their function is lubrication of urethra.
Image of urinary system