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Fish have some unique anatomical and physical characteristics that are different from mammals; however, they still possess the same organ systems that are present in other animals. All fish are poikilothermic and must be able to adapt to changes in water temperature. Fish live in a variety of temperatures ranging from less than 0 C to hot geothermal springs. Yet, each species of fish must live in its particular specific temperature range. Abrupt temperature changes in the water can be lethal to fish.
Organ systems of fish vary to some extent from that of mammals due to the aquatic environment they live in. The following are some of the important differences.

Fish do not have a keratin layer over the epidermis. A cuticle composed of mucus, mucopolysaccharides, immunoglobulins and free fatty acids covers these animals. The epidermis is composed of a stratified squamous epithelium of variable thickness (4 20 cells thick). The outermost epidermal cells (Malpighian cell layer) retain the capacity to divide. Other cells present in the epidermis are goblet cells (responsible for secreting the cuticle), large eosinophilic club cells or alarm cells (present in most species of fish), eosinophilic granular cells (unknown function), leukocytes and macrophages. The dermis is composed of an upper stratum spongiosum and a deeper stratum compactum. Numerous melanophores, xanthophores, and iridophores (give fish their silvery color) are observed scattered throughout the dermis. Scales are calcified plates originating in the dermis and covered by the epidermis. There are two types of scales: ctenoid scales and cycloid scales. Ctenoid scales of elasmobranchs have spicules extending from the external surface giving these fish a rough sandpaper like texture. Cycloid scales of teleost fish have a smooth outer surface and are laid down in concentric a ring that makes them useful in determining the age of some fish. Scales also represent a source of calcium for fish; some fish will utilize the calcium in the scales in preference to the calcium in their skeleton during times of starvation or prespawning activity.

The gills consist of four holobranchs that form the sides of the pharynx. Each holobranch has two hemibranchs projecting from the gill arch. The hemibranch are composed of rows of long thin filament called primary lamella. The primary lamella have their surface area increased further by the secondary lamella that are semilunar folds over the dorsal and ventral surface. Gas exchange takes place at the level of the secondary lamella. Epithelial cells bounded by pillar cells line the secondary lamella. A thin endothelial lined vascular channel lies between the pillar cells and is the site of gas exchange, removal of nitrogenous waste and some electrolyte exchange.

The pseudobranch lies under the dorsal operculum. This organ is a gill arch with a single row of filaments. The function of the pseudobranch is unknown, however it is believed that this structure supplies highly oxygenated blood to the optic choroid and retina and may have thermoregulation and baroreceptor functions.

Adrenal Gland

There is no true adrenal gland present in most fish (exception is sculpins). The adrenal cortical tissue in most fish is represented by the interrenal cells. These cells are pale eosinophilic cuboidal cells associated with major blood vessels in the anterior kidney. Both glucocorticoid and mineralocorticoid are secreted.

The adrenal medullary cells (chromaffin cells) may vary in location. These cells are usually found with the sympathetic ganglia in clumps between the anterior kidney and spine or in the interrenal tissue.

Thyroid Gland

The thyroid follicles are very similar to mammalian thyroid tissue. Thyroid follicles are distributed throughout the connective tissue of the pharyngeal area and may be observed around the eye, ventral aorta, hepatic veins and anterior kidney. It is important to realize that thyroid tissue can be widely distributed. Many times pathologist have erroneously considered this distribution of normal thyroid tissue to represent metastasis from a thyroid follicular cell tumor.

Endocrine Pancreas

The endocrine pancreas is present in most fish as islet of Langerhans and is associated with the exocrine pancreas. In some species the islets are very large and may be grossly visible (Brockman bodies). During the spawning season the size and number of islet will increase in some fish. These should not be confused with an adenoma..

Parathyroid Glands

The parathyroid glands are absent in fish; their function is taken over by other endocrine organs. (Corpuscles of Stannius)

Ultimobranchial Gland

This gland lies ventral to the esophagus in the transverse septum separating the heart from the abdominal cavity. This organ secretes calcitonin (lowers serum calcium levels) that acts with hypocalcin (secreted by the corpuscles of Stannius) to regulate calcium metabolism.

Corpuscles of Stannius

These are islands of eosinophilic granular cells located in paired organs on the ventral surface of the kidney. This organ secretes a protein called hypocalcin (teleocalcin) that acts with calcitonin to regulate calcium metabolism.


This is a neurosecretory organ found on the ventral aspect of the distal end of the spinal cord. These bodies are composed of unmyelinated axons terminating on a capillary wall. The function of the urophysis is unknown.

Pineal Gland

The pineal gland is a light sensitive neuroendocrine structure that lies in the anterior brain and is a well-vascularized organ. This gland secretes melatonin that may play a role in controlling reproduction, growth, and migration.

The digestive system of fish is similar to the digestive tract of other animals. Carnivorous fish have short digestive tracts when compared to herbivorous fish. The stomach and intestines contain submucosal eosinophilic granular cells. The function of these cells is unknown. Some species of fish (Salmonids) have pyloric ceca, which are occasionally confused with parasites. These ceca secrete the digestive enzymes required to digest some food. Fish without the pyloric ceca have digestive enzyme production in the liver and pancreas. It is not possible to divide the intestine into large and small intestine.

The liver does not have the typical lobular architecture that is present in mammals. In many species of fish there are areas of exocrine pancreas (hepatopancreas) that are present near the small veins off the hepatic portal vein.

The pancreas is scattered in the mesentery, primarily near the pylorus.

Fish do not have lymph nodes. Phagocytic cells are present in the endothelial lining of the atrium of the heart and in the gill lamella. There are no phagocytic cells (Kupffer cells) in the liver. Melanomacrophage centers are present in the liver, kidney and spleen. Melanomacrophage centers increase in number during disease or stress.

The fish thymus is the central lymphoid organ. This organ is located subcutaneously in the dorsal commissure of the operculum.

Fish have the ability to produce specific immunoglobulins (IgM only) and have both delayed and immediate hypersensitivity. Fish have the ability to produce virus neutralizing, agglutinating, and precipitating antibodies. Both B and T lymphocytes are present.

The heart is composed of two chambers, one ventricle and one atrium. Some authors also describe the sinus venosus as the third chamber and bulbus arteriosus as the fourth chamber. Blood flows from the heart through the ventral aorta and the afferent branchial arteries, to the gills for oxygenation. Oxygenated blood returns via the efferent arteries to the dorsal aorta. The dorsal aorta then carries the oxygenated blood to the body. Some oxygenated blood also leaves the dorsal aorta and goes to the pseudobranch to be highly oxygenated and then is sent to the retina which has a high oxygen demand.

The kidneys of fish develop from the pronephros and mesonephros. The function of the kidney is osmoregulation. In freshwater fish, the kidney saves ions and excretes water. In saltwater fish, the kidney excretes ions and conserves water. The majority of nitrogenous waste is excreted through the gills. The other function of the kidney is hematopoiesis with hematopoietic tissue located in the interstitium of the kidney. This function is primarily in the anterior kidney but can be found throughout the entire kidney.

Lateral line system

There are two types of lateral line organs. These are the superficial neuromast and the two lateral line canal organs. There are two types of superficial neuromast; these are located in pits in the epidermis located primarily on the head. Their function is not completely known but is believed to aid in movement and orientation.

The second lateral line organ is the lateral line canal system that runs the entire length of the fish with continuous extensions over the head. This organ is sensitive to hydrostatic stimuli and sound.

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