ANATOMICAL
CHARACTERISTICS
In carp, and most other cyprinids, there are two
macroscopically discernable separate kidneys, whereas,
in salmonids and clupeids this is not so dear. The
carp kidneys each have a very small anterior "head"
segment and the posterior or "trunk" segment is
fairly large and triangular in shape. The head kidney
consists of lymphoid tissue, and the nephrons, glomeruli,
arterioles and interstitial haematopoietic tissue
form the trunk kidney. Collecting ducts lead into
the ureters and each ureter leads into the common
cloaca, via a rudimentary urinary bladder.
HISTOLOGICAL
CHARACTERISTICS
Nephrons, glomeruli and collecting ducts
In histological sections of salmonid kidney there
is a more distinct difference between the head and
the trunk areas, but in carp and other cyprinids
this distinction is not so dear because nephrons
are often spread into both segments. However, tubules,
collecting ducts and glomeruli are more abundant
in the trunk segment. It is possible to differentiate,
under the light-microscope, the proximal and distal
segments of the nephron tubule. The proximal tubule
segment is lined with cuboidal cells with cilia
and is recognised by its brush border. As in salmon,
the brush border is not seen in the distal segments
of the nephron tubule. The epithelial cells of this
segment are more eosinophilic in the H&E stain.
In carp, the glomeruli are numerous and large, whereas
in marine fish, glomeruli are small and less numerous.
Juxtaglomerular cells are situated adjacent to the
walls of afferent arterioles on the edge of the
glomerulus. These cells contain granules which are
PAS+ve and have a secretory function.
The
haematopoietic tissue
The interstitial tissue is made up of lymphoid and
haematopoietic cells. The tissues are composed of
reticular cells and abundant capillaries. Immature
blood cells (blast cells) are present, and it is
usually possible to identify mitotic figures in
those cells, indicating blood formation processes.
The tissue contains varying numbers of cells containing
golden to dark brown pigmented material. This material
is made up of melanin and haemosiderin granules.
The melanin granules are scarce and less obvious
than in the salmonid kidney. However, aggregates
of macrophages containing pigment material are still
called melanomacrophage centres (MMC's). It is normal
to see ectopic thyroid follicles in the interstitial
kidney of carp and some other cyprinid species.
The
collecting ducts
The walls of the collecting ducts and ureter consist
of a thin outer-layer called the tunica adventitia,
a layer of circular smooth muscle, a layer of connective
tissue - the lamina propria - and an inner lining
of columnar epithelial cells.
The
interrenal gland
The
interrenal gland in carp is seen as well developed
glandular masses in the anterior part of the kidney.
Chromaffin cells are present amongst the interrenal
gland and stain pale with the H&E method.
PATHOLOGICAL
CONDITIONS
THE GLOMERULUS
The glomeruli can develop changes such as capillary
dilation and invasion by inflammatory cells "glomerulonephritis".
In some instances, atrophy of glomeruli within the
Bowman's capsule has been observed. The causes are
not always specific, but in some instances nutritional
and toxicity problems have been suspected. Thickening
of the capillary walls have been reported in Japan
in carp affected with both a diabetic syndrome known
as Sekoke disease (Yokote, 1970) and vitamin E deficiency
(Watanabe et al., 1970 ).
THE
RENAL TUBULE
Nephrocalcinosis is common to many species of fish,
but less so in cyprinids. It is seen as aggregations
of calcified material in the tubule lumens, which
stain basophilic with haematoxylin.
Occlusions
of eosinophilic, amorphous material within the lumens
of the tubules, are noted more often in the distal
segments of the renal tubules (this condition is
very similar to "pyelonephritis" commonly seen in
the laboratory rat.
Bilateral
swelling and Sphaerosporosis
Dilations and occlusions of tubules may be associated
with infections of the myxosporean Sphaerospora
sp., which is fairly common in carp and other
cyprinids. The species is usually identified as
S. cyprini..
In advanced
cases of sphaerosporosis, the tubules become extremely
dilated, the epithelia are thin and take on a cystic
appearance, making it difficult to identify that
it is kidney tissue that is being examined as glomeruli
are rarely seen. In these cases it is known as polycystic
kidney or nephrocystosis. This pathological abnormality
occurs in several cyprinid species, including ornamental
varieties such as koi and goldfish and in the UK
is common in crucian carp.
Cloudy
swelling
Cloudy swelling, with subsequent narrowing of the
tubule lumen, may be the result of exposure to pollutants
such as heavy metals or agricultural pesticides.
Sometimes the proximal segments of the renal tubule
epithelia contain masses of eosinophilic, hyaline
droplets. When the hyaline droplets are present
in such high amounts, necrotic changes can be seen
by the presence of pyknosis and cell vacuolation.
The droplets are accumulations of lipo-proteinaceous
material, possibly the result of a metabolic problem.
The tubule epithelia can also be vacuolated because
of increased glycogen or lipid.
Nephritis
Nephritis, is necrosis of the epithelial cells of
the tubules, and is usually accompanied by invasion
of lymphocytes and other inflammatory cells. In
extremely necrotic kidneys, the nephrons and glomeruli
disappear, being replaced by interstitial cells.
The basement membranes lining the tubules and glomeruli
can be thickened in such instances.
HAEMATOPOIETIC
TISSUE
Proliferation of interstitial cells
This condition can be seen in some disease conditions
as varying degrees in increases of lymphoid and
haematopoietic cells, pigment cells (forming melanomacrophage
centres, MMCs) and phagocytes. Pyknosis leading
to necrosis will be present in moribund fish . In
those instances, there will be active phagocytosis
and invasions of inflammatory cells denoting septicaemia.
When faced with such sections for examination, it
is worth applying a modified Gram's stain, to investigate
whether bacteria are present. Also, a Giemsa or
May-Grunwald Giemsa stain will help differentiate
early stages of myxosporeans, if they are present.
Spores are usually encysted and easily identified.
However, in cyprinids, it is quite common to identify
granulomata in the interstitial tissue. Whether
these are the result of parasite infections or perhaps
mycobacteria are difficult to make out, as organisms
are rarely seen in section.
NEOPLASM
Neoplasia, arising from kidney cells have been reported
in cyprinids, the most common form is adenocarcinoma
or nephroblastoma (Harshbarger, 1972). This condition
is fairly common in ornamental cyprinids ( Mawdesely
Thomas, 1972 ) . Lymphosarcoma in the head kidney
has been reported in roach ( Bucke & Feist, 1985
) . The histological picture resembled a "starry-sky
at night". The causes for most tumours in cyprinids
remain unknown, however, fish experimentally exposed
to nitrosamines have developed adenocarcinoma.
FIXATION
AND STAINING METHODS
(After Bucke, 1989 )
Fixatives
The routine fixatives such as 10% neutral buffered
formalin and Bouin's fluid are quite suitable for
paraffin embedded sections. However, the blood cells
of the haematopoietic tissue are not easy to recognise
after the latter fixative.
Staining
methods
The haematoxylin and eosin (H&E) technique is usually
adequate for most routine examinations of kidney.
However, the periodic-acid Schiff (PAS) will demonstrate
basement membranes and a trichrome stain can be
helpful for showing connective tissue. The Giemsa
or May-Grunwald Giemsa methods are necessary to
identify the blood cell types. Myxosporeans will
also be demonstrated with either of these methods.
