Normal Salmonid Histology
Diane G. Elliott, Western Fisheries Research Center
Low magnification view of normal skin (scaled region of body, cross section) and
skeletal muscle of a juvenile chinook salmon Oncorhynchus tshawytscha. The skin is an
important first line of defense against pathogens. Note that the scales are located in the
dermis; for scale loss to occur, the epidermis must be torn. The epidermis is normally
covered by an acellular mucous secretion (cuticle) that is constantly sloughed and
renewed; the cuticle is usually lost during histological processing. Hematoxylin and eosin
stain (H&E). Magnification 100.
Stratum spongiosum (dermis)
Stratum compactum (dermis)
Muscle fiber muscle
Higher magnification view of normal skin (epidermis and dermis) and skeletal muscle of
a juvenile chinook salmon. Keratinization of the epidermis to produce the horny surface
layers characteristic of land vertebrates is rare among fishes; fish epidermis is generally
metabolically active throughout all its layers. The bulk of the dermis is made up of
fibrous connective tissue, and also contains blood vessels, nerves, scales, and adipose
(fat) tissue (in hypodermis). Skeletal muscle fibers are very large cells with multiple
peripherally located nuclei. H&E stain. Magnification 200.
Low magnification view of a portion of a gill arch of a rainbow trout Oncorhynchus
mykiss showing parts of two filaments (primary lamellae) supporting numerous lamellae
(secondary lamellae). The filaments are sectioned longitudinally through the cartilaginous
supporting rods. H&E stain. Magnification 100.
Higher magnification view of a portion of a normal gill filament of a rainbow trout. In
longitudinal section, the gill lamella is comprised of two layers of epithelial cells that
cover a planar sheet of pillar (pilaster) cells. Cytoplasmic extensions (flanges) between
adjacent pillar cells form the vascular space occupied by erythrocytes (red blood cells) ,
which have highly eosinophilic (pink) cytoplasm. Chloride cells or ionocytes are
common at the bases of lamellae and are involved in ion regulation. H&E stain.
Normal heart of a sockeye salmon Oncorhynchus nerka. H&E Stain. Blood flows
sequentially through the sinus venosus (not shown), the atrium, the ventricle, and the
bulbus arteriosus. The thin-walled atrium and thick-walled ventricle are comprised in part
of cardiac muscle, whereas the walls of the bulbus arteriosus are made up of elastic
connective tissue and smooth muscle. H&E stain. Magnification 50.
Normal cardiac muscle of the ventricle of a sockeye salmon. Cardiac muscle fibers differ
from skeletal muscle fibers in that they are branched and the nuclei are centrally located.
H&E stain. Magnification 400.
Fish kidneys are among the least readily recognized of fish organs because of their lack
of gross structural similarity to mammalian kidneys. However, nephrons similar to those
of higher vertebrates are present in the majority of fish kidneys. The photo shows a
portion of the posterior (renal) kidney of a rainbow trout. Urinary elements are embedded
in hematopoietic tissue. H&E stain. Magnification 200.
Portion of the anterior kidney (also called the pronephros or head kidney) of a rainbow
trout. The anterior kidney contains hematopoietic (blood cell forming) tissue but no renal
elements. A large vein containing erythrocytes (red blood cells) and leukocytes (white
blood cells) is visible; veins are distinguished from arteries (see posterior kidney) by
thinner walls with fewer smooth muscle and elastic components. H&E stain.
Spleen of a juvenile rainbow trout showing the nodular arrangement of the germinal
nodes (white pulp) and stroma of connective tissue with red blood cells (red pulp). This
nodular arrangement is less pronounced in some salmonids. The splenic capsule is much
thinner than in mammals. H&E stain. Magnification 50.
Higher magnification view of a portion of the spleen of a rainbow trout. The spleen
functions in hematopoiesis and red blood cell storage as well as blood cell destruction,
and may have immunological functions similar to lymph nodes in mammals. H&E stain.
Low magnification view of normal liver tissue of a rainbow trout. The appearance of the
hepatocytes can vary widely with such factors as season, nutritional status, age, and sex.
The hepatocytes in this fish are largely devoid of cytoplasmic glycogen vacuoles. H&E
stain. Magnification 200.
Upper photo: Higher magnification view of normal liver tissue of a rainbow trout. Note
that the hepatocytes are arranged in laminae or cords two cells thick; the sinusoids
between the cords are lined with endothelial cells. Lower photo: Liver of another rainbow
trout showing hepatocytes with abundant glycogen vacuoles in the cytoplasm. H&E stain.
glands Feed in
Longitudinal section of a normal cardiac (anterior) stomach of a sockeye salmon. All
regions of the stomach have a thick wall of longitudinal and circular muscle, and this
region of the stomach has gastric glands, which open into gastric pits. Note the feed in the
lumen of the stomach. H&E stain. Magnification 100.
Portion of the ascending intestine and pyloric caeca of a sockeye salmon. The pyloric
caeca are extensions of the ascending intestine. Diffuse exocrine pancreatic tissue is
visible between the caeca. H&E stain. Magnification 50.
Cross section of a pyloric caecum of a sockeye salmon. The intestine, including the
pyloric caeca, has a much thinner muscular wall than the stomach. The columnar
epithelium includes mucous (goblet) cells and wandering lymphocytes. The lamina
propria is a connective tissue layer beneath the epithelium. H&E stain. Magnification
Salmonid pancreatic tissue (H&E stain).
Top photo: Exocrine (acinar) and
Zymogen endocrine (islets of Langerhans)
granules pancreatic tissue of a sockeye salmon. In
salmonids, pancreatic tissue is dispersed
throughout the adipose tissue that
surrounds the pyloric caeca, but in other
other teleosts, exocrine pancreatic tissue
may be found in locations such as the liver
(hepatopancreas) or spleen. Magnification
200. Left photo: Eosinophilic-staining
zymogen (proenzyme) granules in
basophilic-staining pancreatic acinar cells
of a rainbow trout. The exocrine pancreas
produces digestive enzymes.