The urinary tract is the system in the body that is responsible for producing, storing, and eliminating urine. It includes the kidneys, ureters, bladder, and urethra. The kidneys filter waste products from the blood to produce urine, which then travels through the ureters to the bladder for storage. When the bladder is full, urine is expelled from the body through the urethra. The urinary tract plays a crucial role in maintaining the body's fluid balance and removing waste products from the bloodstream.

1. URINARY SYSTEM
Department of Histology Senior lecturer Khasanova Ilmira Raisovna

2. The intermediate mesoderm
forms a longitudinal elevation
along the dorsal body wall
called the urogenital ridge. A
portion of the urogenital ridge
forms the nephrogenic cord,
which gives rise to the urinary
system.
The nephrogenic cord develops
into three sets of nephric
structures:
1. The pronephros.
2. The mesonephros.
3. The metanephros.
Development

3. The pronephros develops by
the differentiation of
mesoderm within the
nephrogenic cord to form
pronephric tubules and the
pronephric duct. The
pronephros is the cranial-most
nephric structure and is a
transitory structure that
regresses completely by week
5. The pronephros is not
functional in humans.

4. The mesonephros develops by
the differentiation of
mesoderm within the
nephrogenic cord to
form mesonephric tubules and
the mesonephric duct (Wolffian
duct). The mesonephros is the
middle nephric structure and
is a partially transitory
structure. Most of the
mesonephric tubules regress,
but the mesonephric duct
persists and opens into the
urogenital sinus. The
mesonephros is functional for a
short period.

5. The metanephros develops
from an outgrowth of the
mesonephric duct (called the
ureteric bud) and from a
condensation of mesoderm
within the nephrogenic cord
called the metanephric
mesoderm. The metanephros is
the caudal-most nephric
structure. The metanephros
begins to form at week 5 and is
functional in the fetus at about
week 10. The metanephros
develops into the definitive
adult kidney.

6. The inductive influence of the
collecting ducts causes the
metanephric mesoderm to
differentiate into metanephric
vesicles, which later give rise
to primitive S-shaped renal
tubules, which are critical to
nephron formation. The S-
shaped renal tubules
differentiate into the
connecting tubule, distal
convoluted tubule, loop of
Henle, proximal convoluted
tubule, and Bowman capsule.
Tufts of capillaries called
glomeruli protrude into
Bowman capsule.

7. The urinary system consists of:
1. The paired kidneys.
2. The paired ureters.
3. The bladder.
4. The urethra.
This system’s primary
role is to ensure optimal
properties of the blood, which
the kidneys continuously
monitor.

8. Kidney
Kidney tissue consists of an
outer part called the cortex and
an inner part called the
medulla.
The renal cortex consists of two
parts. The tissue lying between
the bases of the pyramids and
the surface of the kidney forms
the cortical lobules. This part
of the cortex shows light and
dark striations. The light lines
are extensions of the pyramids
into the cortex and are called
medullary rays. The tissue,
lying between the pyramids, is
also a part of the cortex. This
part constitutes the renal
columns.

9. The medulla is made up of
triangular areas of renal tissue
that are called renal pyramids.
Each pyramid has a base
directed towards the cortex. An
apex (or papilla) which is
directed towards the renal
pelvis and opens into a minor
calyx. Pyramids show
striations that pass radially
towards the apex.

10. Nephron
Each kidney has about 1.3
million uriniferous tubules
surrounded by a stroma
containing loose connective
tissue, blood vessels,
lymphatics, and nerves.
Each uriniferous tubule
consists of two embryologically
distinct segments:
1. The nephron.
2. The collecting duct.

11. The nephron consists of two
components:
1. The renal corpuscle.
2. A long renal tubule.
The renal tubule consists of:
1. The proximal convoluted
tubule.
2. The proximal straight
tubule.
3. The loop of Henle.
4. The distal convoluted tubule,
which empties into the
collecting tubule.

12. Collecting tubules have three
distinct topographic
distributions:
1. A cortical collecting tubule,
found in the renal cortex as the
centerpiece of the medullary
ray.
2. An outer medullary
collecting tubule, present in
the outer medulla.
3. An inner medullary
segment, located in the inner
medulla.

13. Depending on the distribution
of renal corpuscles, nephrons
can be either cortical or
juxtamedullary. Renal tubules
derived from cortical nephrons
have a short loop of Henle that
penetrates just up to the outer
medulla. Renal tubules from
juxtamedullary nephrons have
a long loop of Henle projecting
deep into the inner medulla.

14. Renal corpuscle
The renal corpuscle, or
malpighian corpuscle, consists
of the capsule of Bowman
investing a capillary tuft, the
glomerulus.
The capsule of Bowman has
two layers:
1. The visceral layer, attached
to the capillary glomerulus.
2. The parietal layer, facing the
connective tissue stroma.

15. The visceral layer is lined by
epithelial cells called podocytes
supported by a basal lamina.
The parietal layer consists of a
simple squamous epithelium
continuous with the simple
cuboidal epithelium of the
proximal convoluted tubule.

16. A urinary space (Bowman’s
space or capsular space),
containing the plasma
ultrafiltrate (primary urine),
exists between the visceral and
parietal layers of the capsule.
The urinary space is
continuous with the lumen of
the proximal convoluted tubule
at the urinary pole, the gate
through which the plasma
ultrafiltrate flows into the
proximal convoluted tubule.
The opposite pole, the site of
entry and exit of the afferent
and efferent glomerular
arterioles, is called the
vascular pole.

17. Glomerulus
The glomerulus consists of
three cell components:
1. The podocytes, the visceral
layer of the capsule of
Bowman.
2. The fenestrated endothelial
cells, lining the glomerular
capillaries.
3. The mesangial cells,
embedded in the mesangial
matrix.

18. Podocytes
The podocytes are
mesenchymal-derived
postmitotic cells. They
are polarized cells with their
cell body bulging into the
glomerular urinary space. Long
primary processes, arising from
the cell body, branch and give
rise to multiple endings, called
foot processes or pedicels.
Pedicels encircle and attach to
the surface of the glomerular
capillary, except at the
endothelial cell-mesangial
matrix interface.

19. Podocytes and fenestrated
endothelial cells each
produce a basal lamina that,
when combined, constitute
the glomerular basement
membrane, a member of the
glomerular filtration barrier.
The
major components of the
glomerular basement
membrane are type IV collagen,
laminin, fibronectin, and
heparan sulfate-containing
proteoglycan.

20. The pedicels, derived from the
same podocyte or from adjacent
podocytes, interdigitate to cover
the glomerular basement
membrane. Pedicels are
separated from each other by
gaps, called filtration slits.
Filtration slits are bridged by a
membranous material, the
filtration slit diaphragm. The
filtration slit diaphragm is the
major size barrier to protein
leakage.

21. 1. The endothelium of the
glomerular capillaries is
fenestrated and permeable to
water, sodium, urea, glucose,
and small proteins.
2. The glomerular basement
membrane, a product of
endothelial cells and podocytes,
contains type IV collagen,
laminin, fibronectin, and
proteoglycans.
3. The pedicels are
interdigitating cell processes of
podocytes covering the
glomerular basement
membrane and coated by a
negatively charged glycoprotein
coat.
Components of the
filtration barrier

22. The mesangium is an
intraglomerular structure
interposed between
the glomerular capillaries.
It consists of two components:
1. The mesangial cell.
2. The mesangial matrix.
Mesangium

23. Mesangial cells are specialized
pericytes with characteristics
of smooth muscle cells and
macrophages.
Mesangial cells are:
1. Contractile cells.
2. Phagocytic cells.
3. Capable of proliferation.
Mesangial cells

24. Mesangial cells participate
indirectly in the glomerular
filtration process by:
1. Providing mechanical
support for the glomerular
capillaries.
2. Controlling the turnover of
the glomerular basement
membrane.
3. Regulating blood flow by
their contractile activity.
4. Secreting prostaglandins and
endothelins (vasoconstrictors
of the afferent and efferent
glomerular arterioles).
5. Responding to angiotensin II.

25. The mesangial cell-matrix
complex is in direct contact
with endothelial cells.
Glomerular basement
membrane is not present at the
mesangium site. Instead,
cytoplasmic margins of
mesangial cells, containing
cytoskeletal contractile
proteins, are closely associated
to the endothelial cell surface.
Мesangial matrix

26. The proximal convoluted
tubules have a relatively small
lumen. They are lined by
cuboidal cells, have central
nuclei and very acidophilic
cytoplasm because of the
abundant mitochondria. The
cell apex has very many long
microvilli that form a
prominent brush border in the
lumen
that facilitates reabsorption.
Proximal convoluted
tubules

27. The apical cytoplasm of these
cells has numerous pits and
vesicles near the bases of the
microvilli, indicating active
endocytosis and pinocytosis.
Cells have many long basal
membrane invaginations and
lateral interdigitations with
neighboring cells. Both the
brush border and the
basolateral folds contain the
many types of transmembrane
proteins that mediate tubular
reabsorption and secretion.
Long mitochondria
concentrated along the basal
invaginations supply ATP
locally for the membrane
proteins involved in active
transport.

28. The PCT continues with the
much shorter proximal straight
tubule that enters the medulla
and continues as the nephron’s
loop of Henle. This is a U-shaped
structure with a thin descending
limb and a thin ascending limb.
Each limb is formed by a thick
segment and a thin segment.
The wall of the thin segments
consists only of squamous cells
with few organelles and the
lumen is prominent.
Loop of Henle

29. The thick descending segment
(proximal straight tubule) is a
continuation of the PCT. The
thick ascending segment (distal
straight tubule) is continuous
with the distal convoluted
tubule. The loops of Henle and
surrounding interstitial
connective tissue are involved in
further adjusting the salt
content of the filtrate.

30. The thin ascending limb of the
loop becomes the thick ascending
limb, with simple cuboidal
epithelium and many
mitochondria again, in the outer
medulla and extends as far as
the macula densa near the
nephron’s glomerulus.

31. Like the PCT, the thin
descending segment harbors
aquaporin channels and is highly
permeable to water. The thin
ascending segment is
impermeable to water
but reabsorbs salts. Salt moves
into the interstitium of the
medulla. Water is transported
towards the outer medulla and
juxtamedullary cortex where it
returns to the systemic blood
circulation.

32. The simple cuboidal cells of the
distal tubules differ from those
of the proximal tubules in being
smaller and having no brush
border and more empty lumens.
Cells of the DCT also have fewer
mitochondria than cells of
proximal tubules, making them
less acidophilic. The rate of Na+
absorption here is regulated by
aldosterone from the adrenal
glands.
Distal convoluted tubules

34. Where the initial, straight part
of the distal tubule contacts
the arterioles at the vascular
pole of the renal corpuscle of its
parent nephron, its cells
become more columnar and
closely packed, forming the
macula densa. This is part of a
specialized sensory structure,
the juxtaglomerular apparatus
that utilizes feedback
mechanisms to regulate
glomerular blood flow and keep
the rate of glomerular filtration
relatively constant.
Juxtaglomerular apparatus

35. Cells of the macula densa
typically have apical nuclei,
basal Golgi complexes, and a
more elaborate and varied
system of ion channels and
transporters.
Adjacent to the macula densa,
the tunica media of the
afferent arteriole is also
modified. The smooth muscle
cells are modified as
juxtaglomerular granular cells,
with a secretory phenotype
including more rounded nuclei,
RER, Golgi complexes, and
granules with the protease
renin.

36. Also at the vascular pole
are lacis cells, which are
extraglomerular mesangial
cells that have many of the
same supportive, contractile,
and defensive functions as
these cells inside the
glomerulus.

37. The kidney vasculature is
large, well-organized, and
closely associated with all
components of the nephron.
Blood vessels of
the kidneys are named
according to their locations or
shapes. Each kidney’s renal
artery divides into two or more
segmental arteries at the
hilum. Around the renal pelvis,
these arteries branch further
as the interlobar arteries,
which extend between the renal
pyramids toward the
corticomedullary junction.
Blood circulation

38. Here the interlobar arteries
divide again to form the arcuate
arteries that run in an arc along
this junction at the base of each
renal pyramid. Smaller
interlobular arteries radiate
from the arcuate arteries,
extending deeply into
the cortex. From the interlobular
arteries arise the microvascular
afferent arterioles, which divide
to form a plexus of capillary
loops called the glomerulus, each
of which is located within a renal
corpuscle where the blood is
filtered .

39. Blood leaves the glomerular
capillaries, not via venules,
but via efferent arterioles, which
at once branch again to form
another capillary network,
usually the peritubular
capillaries profusely distributed
throughout the cortex. From
the juxtaglomerular corpuscles
near the medulla, efferent
arterioles branch repeatedly to
form parallel tassel-like bundles
of capillary loops called the vasa
recta that penetrate deep into
the medulla in association with
the loops of Henle and collecting
ducts.

40. Blood leaves the kidney in
veins that follow the same
courses as arteries and have
the same names. The
outermost peritubular
capillaries and capillaries in
the kidney capsule converge
into small stellate veins that
empty into the interlobular
veins.

41. Renal function involves specific
activities:
1. Regulation of the balance
between water and electrolytes
and the acid-base balance.
2. Excretion of metabolic
wastes along with excess water
and
electrolytes in urine, the
kidneys’ excretory product
which passes through the
ureters for temporary storage
in the bladder before its
release to the exterior by the
urethra.
Functions

42. 3. Excretion of many bioactive
substances, including many
drugs.
4. Secretion of renin, a
protease important for
regulation
of blood pressure by cleaving
circulating angiotensinogen
to angiotensin I.
5. Secretion of erythropoietin, a
glycoprotein growth factor
that stimulates erythrocyte
production in red marrow
when the blood O2 level is low.

43. 6. Conversion of the steroid
prohormone vitamin D,
initially produced in the skin,
to the active form.
7. Gluconeogenesis during
starvation or periods of
prolonged fasting, making
glucose from amino acids to
supplement this process in the
liver.

44. The wall of the ureter has
three layers:
1. Mucosa.
2. Muscularis.
3. Adventitia.
Ureter

45. The mucous membrane has a
lining of transitional
epithelium. It rests on a layer
of fibrous tissue containing
many elastic fibers.
The muscle layer has an inner
layer and an outer spiral layer
of smooth muscle. The thick
muscularis of the ureters
moves urine toward the
bladder by peristaltic
contractions and produces
prominent mucosal folds when
the lumen is empty
The outer fibrous coat consists
of loose connective tissue. It
contains numerous blood
vessels, nerves, lymphatics,
and some fat cells.

46. The wall of the urinary bladder
consists of:
1. Mucosa.
2. Submucosa.
3. Muscularis.
4. Serosa/adventitia.
Urinary bladder

47. The mucosa is lined by the
uniquely stratified urothelium
or transitional epithelium.
Cells of this epithelium are
organized as three layers:
1. A single layer of small basal
cells resting on a very thin
basement membrane.
2. An intermediate region
containing from one to several
layers of cuboidal or low
columnar cells.
3. A superficial layer of large
bulbous or elliptical umbrella
cells, which are highly
differentiated to protect the
underlying cells against the
potentially cytotoxic effects of
hypertonic urine.

48. Urothelium is surrounded by a
folded lamina propria and
submucosa, followed by a
dense sheath of interwoven
smooth muscle layers and
adventitia.

49. The urethra is a tube that
carries the urine from the
bladder to the exterior. The
urethral mucosa has
prominent
longitudinal folds, giving it a
distinctive appearance in cross
section. In men, the two ducts
for sperm transport during
ejaculation join the urethra at
the prostate gland.
Urethra

50. The male urethra is longer and
consists of three segments:
1. The prostatic urethra, 3-4
cm long, extends through the
prostate gland and is lined by
urothelium.
2. The membranous urethra, a
short segment, passes through
an external sphincter of
striated muscle and is lined by
stratified columnar and
pseudostratified columnar
epithelium.
3. The spongy urethra, about
15 cm in length, is enclosed
within erectile tissue of the
penis and is lined by stratified
columnar and pseudostratified
columnar
epithelium.

51. In women, the urethra is
exclusively a urinary organ.
The
female urethra is a 3- to 5-cm-
long tube, lined initially with
transitional epithelium which
then transitions to
nonkeratinized stratified
squamous epithelium
continuous with that of the
skin at the labia minora.
The middle part of the urethra
in both sexes is surrounded by
the external striated muscle
sphincter.