DEGENERATIVE CHANGES

Calcification -
 
        Dystrophic calcification -  occurs in dead or degenerating tissues with subsequent calcium deposition.  The causes of calcium precipitation within the dead tissue is unknown but is not related to the calcium content of the blood.  Changes in alkalinity has been suggested to favor precipitation of calcium within the degenerating and/or dead tissues.

         Metastatic calcification - precipitation of calcium salts as a result of persistently high concentrations  of calcium in the blood is termed metastatic  calcification.

Dilatation - refers to an expansion of an organ, vessel, ductule, or tubule.

Erosion - Sloughing of epithelium from a surface, skin or mucus membrane, in which the basement membrane remains intact.

Fatty Change - also known as fatty metamorphosis or fatty degeneration describes the appearance of microscopically visible fat, either in the form of fine or large droplets.  The size of the droplets depends upon the proportion of neutral fat, large droplets contain little phospholipids.  Normally, fat is taken up in the form of fatty acids via pinocytosis.  The fatty acids are synthesized to triglycerides, bound to phospholipids and proteins, and delivered to the blood as lipoproteins.  Any disproportion between the amounts of triglycerides or of phospholipids or failure of energy coupling leads to accumulation of fat, i.e.  fatty change.

Hyaline - Hyaline is a descriptive term used to qualify any translucent, homogeneous, structureless material that stains with eosin.  Injury to certain cells can cause a hyaline change of varying degree in the cytoplasm.  Hyaline droplets in renal tubular epithelium, particularly in proximal convoluted tubules, appear as numerous, closely grouped, eosinophilic structures within the cytoplasm. This change can be a result of renal tubular injury or as a result of excessive protein passing through injured and abnormally permeable glomeruli and is being reabsorbed into tubular epithelium.

Necrosis - is defined as the non-programmed death of cells or tissues while the organism as a whole is still alive.  Causes of necrosis include poisons, both chemical and biological, lack of a proper blood supply and nerve innervation, pressure, and mechanical and thermal injuries. The three major types of necrosis are coagulation necrosis - in which cells are still somewhat recognizable, liquefactive necrosis - in which cells have lost their structure and the tissue architecture is totally disrupted, and caseous necrosis which is characteristic of Mycobacterial granulomata - with a center of amorphous granular debris surrounded by a wall of inflammatory cells and fibroblasts.

Vacuolar Degeneration -Cloudy swelling microscopically affected cells are swollen and the cytoplasm appears granular.  The swelling of the cell is mainly due to an increase in the water content of the cytoplasm.  The granular appearance is the result of changes in the mitochondria.  It has been postulated that the mitochondria are damaged which diminishes ATP output. The decrease in ATP levels impairs membrane permeability and the active transport mechanisms of the cell.  This results in an increase in sodium ions within the cell followed by an increase in water content.  The condition is reversible.

Hydropic degeneration - Hydropic degeneration is a more severe form of cloudy swelling.  The cells are swollen due to the imbibition of water into the cells. Cells can appear pale and vacuolated or reticulated. The clear vacuoles may be small and multiple or confluent.  When large vacuoles are present the term vacuolar degeneration is often used.  The vacuolar appearance is the result of vesicular distention of the endoplasmic reticulum and is more prominent than in cloudy swelling.  The condition is reversible.

Ulceration - An ulcer is a loss of epithelium from a surface, skin or mucus membrane, in which the basement membrane has been breached.  Typically, inflammation is associated with adjacent tissue.

GROWTH CHANGES

Atrophy - is the shrinkage in the size of a cell by loss of cell substance.  When numerous cells are involved the entire organ diminishes in size and is atrophic.

Dysplasia - is an alteration in adult cells characterized by variation in their size, shape and organization.  This term usually refers to epithelial cells that have undergone irregular, atypical proliferative changes in response to chronic irritation or inflammation.

Fibrosis - Abnormal formation or accumulation of fibrous tissue, predominantly collagen and fibroblasts, which can occur in various organs and tissues.  It is usually the result of chronic inflammation where normal structures have been destroyed.  These areas are sometimes grossly apparent as scars.

Focus of cell alteration - these are considered pre-neoplastic lesions.  Cells take on a different appearance from that usually encountered in that tissue.  For example,  hepatocytes may take on a clear, pink (eosinophilic) or bluish (basophilic) appearance compared to the surrounding hepatocytes.

Granulation tissue - This tissue is found in wound healing, once the necrotic debris has been cleared away by and inflammatory response, fibroblasts and capillaries migrate into the lesion.  Macrophages are found among the many capillaries and fibrous tissue.

Giant cells - are large multinucleated cells that are usually formed by fusion of macrophages, rarely nuclear division.  The nuclei are sometimes arranged around the periphery in a horse shoe pattern forming the Langhans' type giant cell.  Giant cells also are formed in the presence of indigestible material or foreign bodies.  They are seen in Mycobacterium and fungal infections.

Hyperplasia - is an increase in the number of cells in an organ or tissue which may have increased in volume.

Hypertrophy - is an increase in the size of cells which may increase the size of the organ.  It is also used to describe the increase in the size of an organ.

Loss of - refers to the loss of cells or tissues normally present.

Metaplasia - is a reversible change in which one adult cell type, epithelial or mesenchymal, is replaced by another adult cell type.

Neoplasia - or tumors can be described as an abnormal mass of tissue, the growth of which exceeds and is uncoordinated with that of the normal tissues, and persists in the same excessive manner after cessation of the stimuli which evoked the change.  Growth activity usually predominates over function although the latter is not necessarily lacking.

 Benign tumors - are those that tend to grow slowly and expansively.  They can be well tolerated by the organism and do not necessarily interfere with the organisms overall health or shorten its life expectancy unless they are in some vital spot or interfere with an important organ.  They are usually composed of well differentiated mature cell types characteristic of the tissue of origin.  Example:  Papilloma - is a benign epithelial tumor which grows outward from an epithelial surface. Papillomas can be pedunculated or sessile, grow outward, and consist of finger-like processes of varying length, thickness, and number. Microscopically the neoplastic epithelial cells are situated externally around a branching core of connective tissue containing blood vessels. The type of epithelium, i.e.  squamous, transitional, columnar, or cuboidal, remains that of the surface of origin.  The tumor cells do not penetrate the basement membrane and invade the underlying tissues or form metastases.  Papillomas of the skin have been associated with viral, metazoal, and chemical contaminant origin.

 Malignant tumors - are generally more rapidly growing than benign tumors, tend to infiltrate and extend into normal structures and interfere with the health of the organism and at least in mammals will eventually cause death.  Malignant tumors are usually composed of cells less differentiated than the normal tissue of origin.  Anaplasia refers to cells which have major variations in size and staining characteristics within the neoplasm and compared to the normal tissue.

Regeneration - Repopulation of an injured tissue by the same population of cells with the same functional characteristics.

Thickened - This is a non-specific term referring to an increase in the size of a structure or organ without specifying what this increase is due to.

HEMODYNAMIC CHANGES

Congestion - refers to an excess of blood in vessels in a given region.  Congestion can occur by too much blood being delivered by the arteries (active) or too little being drained by the veins (passive).

Dilatation - refers to an expansion of an organ, vessel, ductule, or tubule (in this case refers to blood vessels.)

Edema - This lesion is characterized by the appearance of extracellular fluid in the stroma, resulting in the separation of parenchymal cells which looks like clear spaces  on tissue section..

Embolism - An embolus is a detached intravascular solid, liquid or gaseous mass that is carried by the blood to a site distant from its point of origin.

Hemorrhage - denotes escape of blood from the cardiovascular system, usually the result of rupture of a vessel.  Extravasation of erythrocytes due to their passage through apparently unruptured venular and capillary walls is known as hemorrhage by diapedesis.

Thrombosis - A thrombis is a mass formed from the constituents of the blood within the vessels or the heart during life.  The process of its formation is known as thrombosis.  A thrombus also referred to as an antemortem clot can be differentiated from a clot that forms extravascularly and from clots occurring within the vessels after death, post-mortem clot.  Three factors predispose to the formation of a thrombus: changes in the vessel wall, changes in blood flow, i.e.  slowing, stasis, and eddying of the current, and changes in the blood constituents.

Telangiectasia - This lesion is a gill specific change and appears as a dilation of the lamellus with packing of blood cells in the lumen.   The diagnosis for this change is thrombosis (Hemodynamic; thrombosis) and is considered more accurate than telangiectasia.

INFECTIOUS

Bacterial - This lesion can be due to local invasion of bacteria, such as in a skin ulcer, or can be a fulminant systemic infection causing colonization of multiple organs.  Bacteria usually appear bluish on routine H&E stains.  Grams and acid fast stain can help categorized which group the organism belongs to.

Fungal  - Fungal infections are often secondary to another disease or environmental problem.  In fish, the organisms tend to elicit strong inflammatory responses and often granulomata.  The organisms may appear brownish, but are often difficult to discern with out the use of special stains such as Grocott's methalamine silver stain.

Viral -  Few viral infections can be diagnosed via histopathology.  A notable exception is Lymphocystis, an irido virus which causes characteristic and massive cellular hypertrophy.

INFLAMMATORY CHANGES

Abscess - is a localized collection of pus usually caused by  pyogenic bacteria.  It is composed of neutrophils in a space created by separation of preexisting cellular elements or by liquefactive necrosis of the surrounding tissues.  As it develops, an outside zone of fibroblastic proliferation occurs eventually walling off the abscess limiting further spread.

Acute Inflammation - has a rapid course and is not proliferative.  Pathologic features include changes in vascular flow, changes in vascular permeability, and leukocytic exudation consisting mainly of neutrophils and monocytes.

Chronic Active Inflammation - The transition from acute to chronic inflammation is often difficult to pinpoint. Characteristics of acute and chronic inflammation can occur and is referred to as chronic active inflammation.  Chronic active inflammation can occur when the inciting stimulus is persistent invoking cellular infiltrations characteristic to both acute and chronic inflammatory processes.

Chronic Inflammation - is a process that is prolonged and proliferative, especially of connective tissues. Persistent inflammatory stimuli leads to chronic inflammation.  Predominate cell types associated with chronic inflammation are lymphocytes, plasma cells, and macrophages.  Eosinophilic granulocytic cells (EGC's) can also predominate in chronic inflammation especially during parasitic infections.

Granulomatous Inflammation - is in all instances a form of chronic inflammation, with a distinctive histologic appearance.  It is characterized by focal accumulations of mononuclear cells, mainly macrophages (histiocytes and epithelioid).  Lymphocytes, plasma cells, and giant cells may be associated with the granuloma.  Frequently a fibrous tissue encapsulation is present.  Granulomatous lesions can occur as a result of certain bacteria, Mycobacterium sp. and Flavobacterium sp., fungus, Nocardia sp., reactions to foreign bodies, cholesterol, silica ect., and parasites.  Generally, granulomas are characterized by epithelioid cells surrounding a central core of necrotic debris.  The degree of connective tissue layers, epithelioid cell formation, and composition of the central region are factors used in making a diagnosis.  In the case of parasitic granulomas identification of the parasite is dependent upon sections of the parasite being present within the central region of the granuloma.  Histologic identification of the parasites is often difficult, but Chitwood and Lichtenfels (1973) have described a number of parasites in section.

PARASITISM

Metazoan - Metazoa is a division of the animal kingdom which includes all multicellular forms in contrast to unicellular forms or Protozoa.

Arthropod - The phylum Arthropoda consists of crustacea which are bilaterally symmetrical animals with segmented bodies bearing jointed appendages.  The body is covered with a chitinous exoskeleton.

Cestodes - are a class of organisms within the phylum Platyhelminthes.  The cestodes are endoparasitic with at least one intermediate host in their life cycle. The body (strobila) is subdivided into a number of segments (proglottids) each containing a single set of reproductive organs.  An attachment organ (scolex) is present at the anterior end.  Adult worms are usually white in color and may be very elongate.  They are parasitic in the intestines of the host.  Larval cestodes are commonly found in fish, often encysted in the viscera or musculature.  The scolex of these larvae is fully developed but the strobila is usually short and unsegmented.

Mollusc - Glochidia (larval forms) of  molluscs can be found on the gills of fish.

Nematode - Nematodes are grouped into the phylum Aschelminthes.  They are bilaterally symmetrical, pseudocoelomate, and possess a gut.  Nematodes are elongate worms with a cylindrical body tapering at both ends.  The mouth is terminal anteriorly.  The gut is divided into an esophagus and an intestine.  Sexes are separate.  At least one other host is required in the life cycle in species that are parasitic to fish.

Trematode - Trematodes or flatworms are in the phylum Platyhelminthes are dorsoventrally flattened, bilaterally symmetrical, and acoelomate.  They usually lack an anus and specialized skeletal, circulatory, and respiratory systems.  Two classes of parasitic flatworms occur in fishes, monogeneans and digeneans. The monogeneans are mostly ectoparasitic with no intermediate hosts involved in the life cycle.  They possess a posterior organ of attachment (haptor) comprised of hooks, clamps, or suckers.  Most have some form of an attachment organ anteriorly as well. Digeneans are endoparasitic and have a life cycle that involves at least one intermediate host.  Both adult and larval metacercarial stages can be found in fish. The majority of the digeneans have two suckers on their body surface, one at the anterior end and one ventrally located in the anterior half of the body.

Protozoan - The phylum Protozoa consists primarily of unicellular organisms, although some are colonial. Reproduction is typically asexual by fission, binary or multiple (schizogeny), however, conjugation and sexual reproduction can occur.  With some exceptions, most protozoans have a direct life cycle in which the infective stages released into the water.  Protozoans observed in section are classified either as amoebae, apicomplexa, ciliates, flagellates, microsporidians, or myxosporidians.

MISCELLANEOUS

Cyst - A cyst is a cavity that may contain a fluid or a semisolid material and is surrounded by a definite wall.  Cysts are typically lined by epithelial cells.  They can arise due to a blockage of ducts or tubules, parasites, tumors, developmental errors, or hemorrhage (hematoma) that has become encapsulated and is termed a pseudocyst.

Pigment Accumulation - refers to the accumulation of pigments in or around cells.  These pigments may normally be present in lesser amounts.

OBSERVATIONS

Ectopic Thyroid - Thyroid tissue in bony fish is unencapsulated and is capable of widespread ectopic growth in various organs.  Ectopic thyroid is not considered a lesion but is recorded for reference when observed.  This finding is not included in statistical analysis.

Basophilic Tubules - These tubules have been described as developing immature tubules.   They are extremely basophilic and contain large nuclei with frequent mitotic figures.   They have been noted in young, rapidly growing fish as well as fish that have been moved from fresh water to sea water.   In adult fish they may indicate that the kidney is responding to some type of damage.

Macrophage Aggregates - also known as melano macrophage centers are commonly associated with organs in teleost fishes, primarily the spleen, kidney, and liver.  The migration of macrophages to a point of accumulation suggests that the function of macrophage aggregates is to centralize foreign material.  Several reasons for centralizing material has been postulated.  These include, presenting antigens to cells capable of producing humoral antibody, to remove toxic substances and centralize their detoxification, and to store reusable substances.  The use of macrophage aggregates as a possible monitor of fish health has been presented.  However, these structures are known to increase with respect to age, starvation, disease, and dietary imbalances.  As such the number of macrophage aggregates are scored for reference but are not included in statistical analysis.

Liver glycogen - The amount of glycogen contained in hepatocytes can be considerable in well nourished animals.  A foamy appearance of the cytoplasm is common and varies with the amount and time of last carbohydrate consumption.  This finding is noted as an observation and is not included in statistical analysis.

Prominence - This is a non-specific term referring to a structure or cell type that has become more apparent in the sections examined.