Post on 03-Jul-2020
Urinary system
By
Dr. Sara Mohammed Naguib Assistant professor of histology
Minia university
Histological structure of the kidney
The kidneys are large, reddish, bean-shaped
organs situated retroperitoneally on the posterior
abdominal wall.
-The kidney, embedded in peri-renal fat. It has:
1. A convex border situated laterally.
2. A concave border, the hilum facing medially.
Branches of the renal artery and vein, lymph
vessels, and ureter pierce the kidney at its hilum.
The ureter is expanded at this region, forming the
renal pelvis.
Uriniferous tubules
This tubule consists of two parts, each with a
different embryological origin, the nephron and
the collecting tubule
Nephron
The nephron consists of four distinct parts:
1. The renal corpuscles
2. Proximal convoluted tubule
3. Loop of Henle.
4. Distal convoluted tubule
Renal corpuscle
Each renal corpuscle consists a tuft of capillaries, the
glomerulus, surrounded by a double-walled epithelial
capsule called glomerular (Bowman's) capsule.
-Each renal corpuscle has 2 poles:
Bowman'scapsule:
It is a double-walled epithelial capsule called glomerular (Bowman's)
capsule formed of:
The external layer forms the outer limit of the renal corpuscle and is called
the parietal layer of Bowman's capsule and composed of simple squamous
epithelial cells.
The internal layer (the visceral layer) of the capsule envelops the capillaries
of the glomerulus composed of epithelial cells that become modified and are
known as podocytes.
Between the two layers of Bowman's capsule is the urinary (Bowman's)
space, which receives the fluid filtered through the capillary wall and the
visceral layer.
These large cells have numerous long cytoplasmic extensions, primary
(major) processes.
Each primary process bears many secondary processes, known as
pedicels, arranged in an orderly fashion.
Pedicels completely envelop most of the glomerular capillaries by
interdigitating with pedicels from neighboring major processes of different
podocytes.
Interdigitation occurs in such a fashion that narrow clefts, 20 to 40 nm in
width, known as filtration slits, remain between adjacent pedicels.
Filtration slits are not completely open; instead, they are covered by a
thin slit diaphragm that extends between neighboring pedicels and acts as a
part of the filtration barrier.
Glomerular capillaries:
The glomerulus is composed of tufts of fenestrated capillaries
supplied by the afferent glomerular arteriole and drained by the
efferent glomerular arteriole. Their endothelial cells have fenestrae which are usually not
covered by a diaphragm.
The pores are large, ranging between 70 and 90 nm in
diameter; hence, these capillaries act as a barrier only to
elements of the blood and to macromolecules whose effective
diameter exceeds the size of the fenestrae.
Between the fenestrated endothelial cells of the glomerular
capillaries and the podocytes that cover their external surfaces
is a thick basement membrane.