Histology
Dr. Abdulla A. H
Urinary System
The urinary system comprises the kidneys , ureter , urinary bladder and urethra .
Function :
1- the main functions of the urinary system are :
a- excretion of metabolic waste products and salts .
b- elimination of toxic substances .
c- regulation of fluid , salt, and aid –base balance .
d- modulation of blood pressure .
2- the urinary system accomplishes its functions by the production , storage ,
and elimination of urine .
a- an ultrafiltrate of blood is produced in the renal corpuscles and is
subsequently sub-stantially modified by selective reuptake in uriniferous
(renal) tubules to produce urine .
b- the renal calyces , pelvis , and ureter conduct the urine from the kidney to the
urinary bladder .
c- urine is stored in the urinary bladder and is eliminated through the urethra .
kidney :
General organization :
1- the kidney has an outer cortex surrounded by :
A- connective tissue capsule and an inner medulla projecting into renal calyces
or renal pelvis.
a-the renal cortex is composed of the cortical labyrinth and medullary rays .
1- the labyrinth contains renal corpuscles , proximal and distal convoluted
tubules , and arched collecting ducts .
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Histology
Dr. Abdulla A. H
2- the medullary rays are groups of parallel straight proximal and distal tubules
and collecting ducts .
b- the renal medulla is subdivided into an an outer and inner zone .
1- the outer zone contains proximal and distal straight tubules , thin tubules and
collecting ducts .
2- the inner zone contains only thin tubules and collecting ducts .
C-the cortical and medullary interstial connective tissue , which surrounded the
renal tubules and blood vessels , consists of fibrocytes and reticular fibers .
*** A renal lobe comprises a renal medullary pyramid and its associated
cortical tissue . a unilobar kidney is present in cats , doge , horses , sheep , and
goats , bovine and pig kidney are multilobar.
B- Renal tubules (uriniferous):
The structural and functional units of the kidney are the renal tubules , each
tubule has two components .the tubular Nephron and the collecting duct into
which it empties .
1-Nephron :
The nephron starts as the renal corpuscle and continues in the proximal convoluted
tubule ,proximal straight tubule , thin tubule and distal straight and convoluted
tubules .
The straight portions of the proximal and distal tubules and the thin tubule form
Henle’s loop in the medulla .
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Histology
Dr. Abdulla A. H
Anatomical structure of nephron
Type of Nephron :
1- cortical or subcapsular : which have short loops of Henle’s ( as bovine
kidney ) .
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Histology
Dr. Abdulla A. H
2- Juxtamedullary Nephrons : they have long loops of Henle’s that extend
into the inner region of the pyramid this type is essential for concentrating
urine ( as camel kidney ) .
3- Intermediate Nephron : have their renal corpuscles in the mid region of
the cortex . they are of intermediate length .( as Human kidney ) .
The renal corpuscle :
is composed of a tuft of capillaries, the glomerulus, which is invaginated
into Bowman’s capsule, The glomerulus is supplied by the short, straight
afferent glomerular arteriole and drained by the efferent glomerular arteriole
. The glomerulus is formed as several tufts of anastomosing capillaries that
arise from branches of the afferent glomerular arteriole. The connective
tissue component does not enter Bowman’s capsule, and is replaced by a
specialized cell type known as mesangial cells. There are two groups of
mesangial cells: Extraglomerular mesangial cells are located at the vascular
pole, and pericyte-like intraglomerular mesangial cells are situated within the
renal corpuscle.
The glomerulus contacts the visceral layer of Bowman’s capsule, composed
of modified epithelial cells called podocytes. The outer wall surrounding
Bowman’s space, is the parietal layer.
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Histology
Dr. Abdulla A. H
*** The Juxtaglomerular Apparatus :
Is located at the vascular pole of the renal corpuscle and comprises Juxtaglomerular
cells , extraglomerular mesangial cells , and the Maculadensa.
1- components and histological organization:
a- Juxtaglomerular cells are epitheloid modified smooth muscle cells , primarily
in the wall of the afferent arteriole .
b- Extraglomerular mesangial cells ( lacis cells ) are located between the
maculadensa and the two glomerular arterioles .
The macula densa: is a group of thin , tall and tighly packed epithelial cells
of the distal tubule
The juxtaglomerular apparatus.
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Histology
Dr. Abdulla A. H
Filtrate leaking out of the glomerulus enters Bowman’s space through a
complex filtration barrier composed of the endothelial wall of the capillary,
the basal lamina, and the visceral layer of Bowman’s capsule.
Podocytes bear numerous long, tentacle-like cytoplasmic extensions, primary
(major) processes, which follow the longitudinal axes of the glomerular
capillaries. Each primary process bears many pedicels, which completely
envelop most of the glomerular capillaries by interdigitating with pedicels from
neighboring major processes of different podocytes.
The interrelationship of the glomerulus, podocytes, pedicels, and basal laminae.
Proximal thick segment :
Which consist of proximal convoluted tubule and proximal straight tubule .
This segment characterized by :
1- the major site of reabsorption.
2- Has a brush border ( long microvilli ) .
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Histology
Dr. Abdulla A. H
3- Has lateral and basal interdigitations .
4- Acidophilic cytoplasm ( due to mitochondria) .
5- Pyramidal cells or cuboidal .
6- Narrow lumen .
7- Less cells (few ) .
8- Longer than the distal .
1- Distal thick segment :
which consist of distal convoluted tubule and distal straight segments .
this segments characterized by :
1- has cuboidal cells .
2- the number of cells were more than the proximal convoluted tubule .
3- no brush border .
4- cytoplasm not acidophilic .
5- shorter than the proximal convoluted tubule .
6- the lateral margins were indistinct .
Collecting duct system : this system begins with the arched colleting duct in
the cortical labyrinth . it continues as the straight collecting duct with in a
medullary ray , joins other collecting ducts in the medulla to form a papillary
duct which opens on the renal papilla or calyx .
A- components and histological organization:
1- arched collecting duct : (connecting tubules ) connect the distal convoluted
tubules and straight collecting duct .
2- straight collecting ducts : are lined with a simple epithelium comprising
principal cuboidal cell and intercalated cells of varying shapes .
2- papillary ducts : are lined by simple or stratified columnar or transitional
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Histology
Dr. Abdulla A. H
3- epithelium .
Vasculature of the kidney:
The urinary passages :
the urinary passages include the calyces ,renal pelvis , uretres , urinary bladder ,
and urethra .
a- general histological characteristics:
1- the mucosa in lined by transitional epithelium on a properia , submucosa of
loose connective tissue ..
2- in most areas , the tunica muscularis has an inner longitudinal , middle
circular , and outer longitudinal layer .
3- where structures are embedded in other tissue , the outer most layer is a
tunica adventitia of connective tissue .
4- where structures project into the peritoneal cavity they are surrounded by
tunica serosa of loose connective tissue with a peritoneal mesothelial
covering ..
b-special organ characteristics :
1- mucosa glands are present in the calyces and proximal ureter of the equine.
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Histology
Dr. Abdulla A. H
2- contraction of the tunica musuclaris result in a stellate profile of the ureteral
lumen in most histological sections .
3- in the urinary bladder , the appearance of the transitional epithelium varies with
the degree of filling . small bundles of smooth muscle form a discontinuous lamina
musuclaris in all domestic mammals except the cat . the tunica musuclaris consists
of an irregular network of smooth muscle bundles rather than distinct layers .
4- near the external urethral orifice is lined by stratified squamous epithelium .
Urinary bladder
ureter
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