Monocytes form about 0.1 % of the circulating leucocytes
and are partially differentiated end cells which under appropriate conditions will develop into mature cells of the mononuclear phagocyte system. Monocytes are 9–25 μm in
diameter with a lighter staining cytoplasm than small
lymphocytes and contain small granules with a large
nucleus.
Differentiation of phagocytes into macrophages usually
takes place when they become extravascular, migrating from
the vessels into the tissues and therefore usually not seen in
circulation. Their phagocytic capability is well documented,
but resting macrophages in tissues are difficult to distinguish
from fibrocytes in H&E stained sections.
2.2.2 Secondary Circulation
Studies indicate that fish lack a true lymphatic system but a
second vascular system derived from and connected to the
primary circulation is present. Analogies between the lymphatic and the secondary system have been noted and
observed in skin, fins, gills, oral mucosa and lining of the
peritoneum, however it remains to be determined whether
the secondary system is the antecedent of a lymphatic system or a coincidentally similar structure.
The spleen is usually dark red or almost black in colour
and a discrete organ with sharply defined edges located near
the greater curvature of the stomach. A thin serous capsule
covers the surface. The spleen functions as a haematopoietic
organ, a temporary blood bank and as a remover of
circulating antigens and effete blood cells. Occasionally,
two or more spleens may be recorded and the organ may
also be located elsewhere in the abdominal cavity
(Fig. 2.14). The spleen structure is provided by a capillary
and a connective tissue meshwork among which the cells fill
up the interspaces i.e. erythroblasts, mature and immature
erythrocytes, lymphocytes, monocytes and macrophages.
The parenchyma of the spleen is composed of white pulp,
namely lymphoid tissue, surrounding small arteries which
diffusely intermeshes with the haematopoietic red pulp,
composed of a reticular cell network and supporting bloodfilled sinusoids. There is no sharp demarcation between red
and white pulp as the parts rich in erythrocytes and those rich
in lymphocytes are intermingled (Fig. 2.15). The ellipsoids
form the main elements of the spleen and are a thick-walled
filter capillary network gradually forming from the artery
which enters the organ. Each ellipsoid comprises a thick
basement membrane-bound tube within which the vessels
run and is separated from the membrane by a layer of
sheathed components. Degradation products of senescent
erythrocytes are stained yellow by H&E and known as
haemosiderin, a common feature in the spleen parenchyma.
Perl’s staining is used to differentiate haemosiderin deposits
(see Fig. 4.22). Variable numbers of melanomacrophages
may be found scattered in the spleen tissue. Phagocytic
cells capable of trapping large quantities of particulate matter from the circulating blood, once replete migrate to the
ellipsoids to the melanomacrophages.
2.3
Integument System
The structure of the skin varies to some extent among species but is basically composed of two layers: an outer epidermis and an underlying dermis. The epidermis constitutes
the barrier between the body and the aquatic environment
and can be divided into two additional layers. The outer
epithelium is composed of stratified squamous cells and a
basal layer of undifferentiated cuboid germinal cells. The
Fig. 2.14 Spleen in abdominal cavity of Atlantic salmon, note duplicate organ in this case
Fig. 2.15 Spleen of adult Atlantic salmon. Medium power
2.3 Integument System
11
and are partially differentiated end cells which under appropriate conditions will develop into mature cells of the mononuclear phagocyte system. Monocytes are 9–25 μm in
diameter with a lighter staining cytoplasm than small
lymphocytes and contain small granules with a large
nucleus.
Differentiation of phagocytes into macrophages usually
takes place when they become extravascular, migrating from
the vessels into the tissues and therefore usually not seen in
circulation. Their phagocytic capability is well documented,
but resting macrophages in tissues are difficult to distinguish
from fibrocytes in H&E stained sections.
2.2.2 Secondary Circulation
Studies indicate that fish lack a true lymphatic system but a
second vascular system derived from and connected to the
primary circulation is present. Analogies between the lymphatic and the secondary system have been noted and
observed in skin, fins, gills, oral mucosa and lining of the
peritoneum, however it remains to be determined whether
the secondary system is the antecedent of a lymphatic system or a coincidentally similar structure.
The spleen is usually dark red or almost black in colour
and a discrete organ with sharply defined edges located near
the greater curvature of the stomach. A thin serous capsule
covers the surface. The spleen functions as a haematopoietic
organ, a temporary blood bank and as a remover of
circulating antigens and effete blood cells. Occasionally,
two or more spleens may be recorded and the organ may
also be located elsewhere in the abdominal cavity
(Fig. 2.14). The spleen structure is provided by a capillary
and a connective tissue meshwork among which the cells fill
up the interspaces i.e. erythroblasts, mature and immature
erythrocytes, lymphocytes, monocytes and macrophages.
The parenchyma of the spleen is composed of white pulp,
namely lymphoid tissue, surrounding small arteries which
diffusely intermeshes with the haematopoietic red pulp,
composed of a reticular cell network and supporting bloodfilled sinusoids. There is no sharp demarcation between red
and white pulp as the parts rich in erythrocytes and those rich
in lymphocytes are intermingled (Fig. 2.15). The ellipsoids
form the main elements of the spleen and are a thick-walled
filter capillary network gradually forming from the artery
which enters the organ. Each ellipsoid comprises a thick
basement membrane-bound tube within which the vessels
run and is separated from the membrane by a layer of
sheathed components. Degradation products of senescent
erythrocytes are stained yellow by H&E and known as
haemosiderin, a common feature in the spleen parenchyma.
Perl’s staining is used to differentiate haemosiderin deposits
(see Fig. 4.22). Variable numbers of melanomacrophages
may be found scattered in the spleen tissue. Phagocytic
cells capable of trapping large quantities of particulate matter from the circulating blood, once replete migrate to the
ellipsoids to the melanomacrophages.
2.3
Integument System
The structure of the skin varies to some extent among species but is basically composed of two layers: an outer epidermis and an underlying dermis. The epidermis constitutes
the barrier between the body and the aquatic environment
and can be divided into two additional layers. The outer
epithelium is composed of stratified squamous cells and a
basal layer of undifferentiated cuboid germinal cells. The
Fig. 2.14 Spleen in abdominal cavity of Atlantic salmon, note duplicate organ in this case
Fig. 2.15 Spleen of adult Atlantic salmon. Medium power
2.3 Integument System
11