Before you initiate your description, keep in mind that some tissue changes might require more descriptive details than others. It all depends on how complex the changes are. Think of a well-known piece of art or landmark such as Da Vinci’s Mona Lisa or the Statue of Liberty. You do not need too much effort to describe these two objects since you assume that virtually everyone knows these famous works of art. If you translate that into plain simple anatomic pathology, you can imagine yourself trying to describe edema to a veterinarian or a pathologist over e-mail, texting, or a phone conversation. You do not need too much information, the words tissues were distended by clear fluid will suffice for that purpose, the conversation would be short, and you would understand each other perfectly. Edema is a basic pathologic change that is well known by most if not all professionals engaged in biological sciences and even a layperson. In contrast, if you are trying to tell someone about the little house you lived in as a child, you will need to give away much more detailed information so that a person can try to picture an image of it. You should mention the landscape (city or countryside), neighborhood, architectural design, size, color, type of windows, doors, number of bedrooms, furniture, a cozy backyard with a couple of trees setting down a nice shade where you used to play with a little dog you had. Every bit of information is worth mentioning since you are the only one in that conversation that really knows what that house looked like. Now back to pathology, imagine you are trying to describe an adenocarcinoma in the large intestine of a horse to the referring veterinarian or to the owner that requested that examination. Now you have to realize that not every clinician has seen an intestinal adenocarcinoma, let alone the owner. Therefore, you will have to deliver that information in a more elaborate way so the listener will be able to picture how that intestine looked like. You should say that the intestine had a focally extensive, constricted, irregular, transmural, pale to dark red, firm area that narrowed down the intestinal lumen. You can also say that due to the luminal narrowing, the proximal portions of the intestine were markedly dilated by stagnant fluid and food material. Finally, you will talk about the cut surface and mention that the constricted intestinal portion was composed of extensive pale areas intermingled with multifocal irregular yellow foci that extended to the ulcerated mucosa. Based on that basic description (with no interpretation) you can discuss the reasons that led you to think that the horse had an intestinal adenocarcinoma (in this case, the typical gross appearance of an adenocarcinoma in a tubular organ and subsequent histologic evaluation of the intestine).

            A complete and accurate gross description should rely on almost all senses. You should be able to use sight, touch, smell, and hearing to describe a tissue change. A complete gross anatomic description should contain all or most of the following features: 1) location; 2) distribution; 3) color; 4) shape and demarcation; 5) size; 6) consistency; and 7) special features. A systematic approach addressing each one of these parameters can initially appear cumbersome, but your description will flow naturally after you become familiar with the system. A good description uses simple and straightforward terms to describe tissue changes, so the reader understands why a particular interpretation has been given to a particular change in a specific organ. This method will result in a multilayered compilation of information that will facilitate not only the description of tissue changes but also the thought processes that will lead to a presumptive and subsequent final diagnosis.


            This is one of the first things that may be easy to forget when describing a tissue change: its location. The most detailed description is not complete without organ or tissue identification, so it may be a good idea to automatically start out a description by listing the location of the tissue changes and only then moving on to the details. To identify the affected organ or tissue, a modern anatomy book should be used so the correct anatomic terms are identified. You should then list a few simple spatial coordinates to make it more specific (right, left, dorsal, ventral, cranial, caudal, etc.) and locate the tissue change in this context. Always try to be as specific as you can. It is always better to say that a particular change is occurring in the distal jejunum than just saying it is affecting the small intestine. Canine cardiac hemangiosarcoma is a classic example in which location is of paramount importance for the diagnosis (Fig. 1).

Image Fig. 1. Right auricular hemangiosarcoma in a dog. The exact location of the lesion in this case (right auricle) is a powerful description tool, as this is a classic anatomic site for cardiac hemangiosarcoma in dogs (Image D.R. Rissi).

These tumors typically occur in the right auricle, so if you see a dark red neoplasm in the right auricle, make sure the reader knows it is in the right auricle and not just the auricle or the myocardium or the heart. The sentence a dark red tumor in the right auricle automatically implies that the dog had a cardiac hemangiosarcoma.             


            The distribution is an objective assessment of the spatial arrangement of a tissue change. It is typically addressed using the natural or capsular surface and the cut surface of an organ. Distribution is classically referred to as focal, multifocal (or multiple), multifocal to coalescing, and diffuse. Each of these distribution patterns can have small variations that will complement on their specificity and even provide hints about the underlying pathogenesis or cause of a tissue change. A good way to evaluate the distribution of a tissue change is to imagine a brick wall with a couple of supporting pillars (Fig. 2).

Fig. 2. A. A brick wall with two supporting pillars (white) represent a tissue or an organ.
A focal change consists of one single affected area (one brick) surrounded by normal tissue.
A focally extensive change (multiple adjacent bricks) is technically a focal change that spans a larger area of the affected tissue.
Multifocal changes consist of two or more affected foci (multiple independent bricks).
When multifocal affected areas become more extensive and coalesce to each other, changes are referred to as multifocal to coalescing.
F. When multifocal changes are small and occur throughout the parenchyma, they can be referred to as disseminated.
A segmental change affects a well-defined anatomic portion of a tissue (only the supporting pillars in the wall) and may suggest an underlying vascular mechanism.
Symmetrical changes are a variation of multifocal or focally extensive changes, but occur when affected areas are associated with a specific anatomical or physiological unit (in our wall, symmetrical changes would have a predilection to occur only in bricks with similar colors).
Diffuse changes affect virtually 100% of the tissue surface or all the bricks in the wall (Image D.R. Rissi).

Focal tissue changes are usually evident because they contrast well with the normal surrounding tissue on the background. As it is implied, focal changes consist of one single affected area surrounded by normal tissue (one brick in the wall). A cerebral abscess is an example of a focal lesion (Fig. 3).

Image Fig. 3. Focal abscess in the right globus pallidus of a foal. These abscesses typically occur due to septicemia in newborn animals (Image D.R. Rissi).

When a focal tissue change extends to a broader area it can be referred to as focally extensive (multiple adjacent bricks in the wall). This definition is a little subjective since a focally extensive change is technically still a focal change that has spanned a larger area of the affected tissue. Focally extensive areas of skeletal muscle necrosis (Fig. 4) are essentially still a focal lesion, but are much more extensive than the small abscess previously seen in the brain of the foal, so it is better to refer to those are focally extensive.

Image Fig. 4. Skeletal myonecrosis in a cow due to Senna occidentalis toxicosis. The pale white necrotic muscle (left) spans a more extensive area when compared with the previous focal abscess in the foal (Image D.R. Rissi).

Multifocal changes consist of two or more affected foci in the tissue parenchyma (multiple independent bricks in the wall). The multiple areas of suppurative inflammation in the kidney of a foal with septicemia (Fig. 5) are a good example of a multifocal change.

Image Fig. 5. Suppurative nephritis in a foal. Multifocal areas of suppurative inflammation typically occur in the kidneys of foals due to Actinobacillus equuli septicemia. The small nodules contrast well with the dark red renal parenchyma (Image D.R. Rissi).

The small foci are yellow and contrast well with the dark red renal parenchyma. In the liver, multifocal changes are typically referred to as random because they can occur anywhere within the hepatic lobule. These hepatic lesions (Fig. 6) result from infectious agents reaching the hepatic parenchyma randomly through the circulation, and thus have no predilection to the center or the periphery of the hepatic lobules. Over time, multifocal changes can become more extensive and coalesce to each other. At this point, these changes can be described as multifocal to coalescing. Thus, multifocal to coalescing changes may imply chronicity.

Image Fig. 6. Multifocal hepatocellular necrosis due to canine herpesvirus-1 infection in a puppy. The pale necrotic foci are random and occur throughout the parenchyma because the infection reaches the liver through the bloodstream (Image D.R. Rissi).

The lung shown in Fig. 7 is expanded with multifocal to coalescing areas of metastatic infiltration by a mammary carcinoma.

Image Fig. 7. Metastatic mammary carcinoma in the lung of a dog. Different than the isolated foci typical of multifocal lesions, the multiple neoplastic foci often coalesce to each other (Image D.R. Rissi).

When multifocal changes are exceedingly small (less than 1 mm in diameter) and occur throughout the parenchyma, they can be referred to as miliary because lesions resemble millet seed. An example of miliary changes is the embolic foci in the kidney of a newborn calf (Fig. 8). Each small focus consists of accumulations of neutrophils and cell debris (suppurative inflammation) that reached the renal vasculature secondary to septicemia. When a tissue change occurs across the entire parenchyma it can also be referred to as disseminated. The embolic foci showed in Fig. 8 are also disseminated throughout the kidney since the whole renal surface is affected by suppurative inflammation.

Image Fig. 8. Bacterial septicemia in the kidney of a calf. There are widespread, less than 1 mm in diameter yellow foci of suppurative inflammation throughout the renal parenchyma (Image C.S.L. Barros).

 A segmental tissue change, similar to a focal or focally extensive change, affects a well-defined portion of a tissue, but usually reflects an underlying vascular problem. Using the brick wall analogy, a segmental change would affect one or multiple adjacent bricks in the wall, but only those in the supporting pillars. Classic examples of segmental lesions include renal infarcts (Fig. 9) due to thromboembolism and cutaneous infarcts in pigs infected by Erysipelothrix rhusiopathiae or porcine circovirus-2 (Fig. 10).

Image Fig. 9. Renal infarct in a dog. The focally extensive necrotic area is segmental since it reflects a portion of dead tissue irrigated by a specific blood vessel that has been occluded by a thrombus (Image D.R. Rissi).
Image Fig. 10. Cutaneous infarcts due to Erysipelothrix rhusiopathiae infection in a pig. The rectangular areas of necrosis and hemorrhage are typical of this infection and are secondary to vasculitis and thrombosis caused by septicemia (Image D. Driemeier).

The affected areas in these cases are essentially focal, focally extensive, multifocal, or multifocal to coalescing. However, these areas of coagulative necrosis are well-demarcated (triangular or rectangular in the case of renal infarcts and diamond-shaped in the case of cutaneous infarcts) and caused by ischemia due to vasculitis and thrombosis. Renal infarcts caused by obstruction of the interlobular artery are triangular, with the apex pointing at the corticomedullary junction. Infarcts secondary to obstruction of the arciform artery have a rectangular shape and occur in the cortex. Infarcts caused by obstruction of the interlobar artery are also triangular, but the apex is located in the renal medulla. The renal and dermal necrotic foci in these cases delineate areas supplied by the affected blood vessels that are no longer functional. Symmetrical changes are also a variation of multifocal or focally extensive changes, and occur when affected areas are associated with a specific anatomical or physiological unit.
Symmetrical changes in the wall would have a predilection to affect only the two green bricks. Symmetrical changes usually have a toxic or a metabolic basis. Centrilobular hepatic necrosis is typically caused by toxins that target hepatocytes at the center of the lobules (Fig. 11).

Image Fig. 11. Centrilobular hepatocellular necrosis in a sheep. The areas of necrosis are restricted to the center of all lobules (red areas) and reflect a symmetrical change caused by a toxin that targets primarily the hepatocytes around the central veins (Image D.R. Rissi).

Symmetrical lesions in the brain include those caused by Clostridium perfringens type D epsilon toxin in sheep and goats (Fig. 12), diminazene aceturate toxicosis in dogs, Centaurea spp. toxicosis in horses, and Aeschynomene indica toxicosis in pigs.

Image Fig. 12. Bilateral symmetrical encephalomalacia due to Clostridium perfringens type D epsilon toxin in a sheep. The symmetrical lesions occur equally in both sides of the brain and specifically target the basal nuclei and internal capsule (Image K. Thompson).

Diffuse changes affect virtually 100% of the tissue surface. A diffuse pattern can be difficult to evaluate since there may be no normal tissue to be compared with the affected tissue. In these cases, even organ recognition may be a challenge. A diffuse change in the wall would occur in all the bricks. Cases of canine parvovirus-2 infection or thymic hemorrhage in dogs can be segmental but necrosis and/or hemorrhage can eventually affect the entire organ (Fig. 13). While there is no normal intestine to be evaluated in this case, it is evident that the organ is abnormal.

Image Fig. 13. Diffuse necrotizing enteritis due to canine parvovirus-2 infection in a dog. The entire small intestine is necrotic and hemorrhagic. The lesion is very clear since changes in color are distinct (Image D.R. Rissi).


            While we think of color as an objective concept (what looks green to me looks green to you), variations in color perception may exist among different people. When describing the color of a tissue change, use simple primary colors with a few necessary variations, and avoid the use of the suffix “ish” (greenish, yellowish) or redundant terms such as “green in color”. If something is green, it is not greenish, it is green (maybe light green, maybe dark green, but still green). And if something is green, it must be green in color, not in shape, so the use of “in color” is superfluous. Keep in mind that the color of a tissue change can reveal the underlying process in that particular tissue. Red usually indicates the presence of blood or its by-products. The presence of too much blood in an organ may suggest congestion, hyperemia, or hemorrhage. Congestion and hyperemia can be difficult to differentiate since they only differ from each other by the fact that congestion is usually a passive event and hyperemia is usually associated with inflammation. A good example of congestion is the splenic enlargement in dogs anesthetized or euthanized with barbiturates (Fig. 14).

Image Fig. 14. Post-anesthetic splenic congestion in a dog. The spleen is diffusely enlarged and dark red due to massive accumulation of blood (Image D.R. Rissi).

In these cases, the spleen is diffusely dark red and a large amount of blood oozes out from the parenchyma when the organ is cut (Fig. 15).

Image Fig. 15. Post-anesthetic splenic congestion in a dog. A massive amount of blood oozes out of the parenchyma when the spleen is transected (Image D.R. Rissi).

While vascular congestion can be an important change in this and other particular situations (Fig. 16), it is one of the most over-interpreted gross and histologic changes in the diagnostic routine. It is usually an unimportant change used as a “filler” in descriptions where nothing important was found. Describe congestion only if it is meaningful to the case. For example, make sure your report the splenic congestion associated with anesthesia or euthanasia, or the classic chronic passive congestion in the liver of a dog with chronic right-sided heart insufficiency. On the other hand, do not waste time describing vascular congestion in a normal brain. It is probably a misinterpretation resulting from the contrast between blood-filled capillaries and the normal pale white neuroparenchyma.

Image Fig. 16. Colonic torsion in a dog. The twisted intestinal loop is dark red due to massive entrapment of blood within capillaries (congestion). The remaining intestinal loops are bright red due to hemoglobin imbibition (Image D.R. Rissi).

Active inflammation can lead to hyperemia, causing the engorged capillaries to be easily observed in the affected organs (Fig. 17). While congestion and hyperemia reflect the presence of too much blood within the vasculature, hemorrhage indicates that blood has leaked out of the blood vessels (Fig. 18 and 19).

Image Fig. 17. Herpesviral conjunctivitis in a cat. The conjunctival capillaries are engorged with blood (hyperemia) due to active inflammation following feline herpesvirus-1 infection (Image D.R. Rissi).
Image Fig. 18. Necrotizing meningoencephalitis due to bovine herpesvirus infection in a calf. Extensive, swollen red areas of hemorrhage and necrosis are present throughout the frontal cerebral hemispheres (Image D.R. Rissi).
Image Fig. 19. Intercostal hemorrhages in a horse. These dark red areas of hemorrhage are typically seen in cases of septicemia or endotoxemia in horses (Image D.R. Rissi).

If the blood leaks into a body cavity, it can clot and form a hematoma (Fig. 20).

Image Fig. 20. Subdural hemorrhage in a dog. A focally extensive red blood clot covers the left telencephalic hemisphere of a dog that suffered physical trauma to the skull (Image D.R. Rissi).

Dark red contents in the urinary bladder (Fig. 21) is usually associated with conditions that lead to hematuria (blood in the bladder secondary to renal or urinary bladder lesions), hemoglobinuria (hemoglobin secondary to intravascular hemolysis), and myoglobinuria (myoglobin secondary to muscle necrosis). The identification of the type of contents (blood versus hemoglobin or myoglobin) should be made according to the other gross changes and confirmed in the clinical laboratory. Hemoglobin imbibition is usually a postmortem artifact caused by the destruction of erythrocytes following death. The released hemoglobin stains all organ surfaces and tissues become diffusely cherry red (Fig. 22). This change is often misinterpreted as congestion or hemorrhage by the untrained examiner.

Image Fig. 21. Myoglobinuria in a cow. The dark red contents in the urinary bladder of this cow (or any other animal species) indicate the presence of hemoglobin (red blood cell destruction), myoglobin (muscle necrosis), or blood (pyelonephritis or cystitis) (Image D.R. Rissi).
Image Fig. 22. Widespread hemoglobin imbibition in a dog. Postmortem decay leads to the destruction of cell membranes and release of hemoglobin from erythrocytes, which saturates and turns tissues bright red (Image D.R. Rissi).

Less often, hemoglobin imbibition can occur antemortem because of erythrocyte breakdown in cases of severe intravascular hemolysis. Yellow can be the normal color of tissues, such as fat and keratin, but can be suggestive of pathologic changes, such as icterus, edema in horses, and inflammation; it can also reflect biliary imbibition (another postmortem artifact). Adipose tissue is usually white in most animal species, but it is typically bright yellow in horses, Guernsey and Jersey cattle, and primates (including human beings). This change is referred to as pseudoicterus (Fig. 23), and it occurs due to the accumulation of dietary carotenoid pigments in the fat. Icterus occurs due to an increased concentration of bilirubin in the blood (hyperbilirubinemia) and its consequent accumulation within tissues.

Image Fig. 23. Subcutaneous pseudoicterus in a horse. The adipose tissue of horses and primates is normally yellow due to physiologic accumulation of dietary carotenoid pigments (Image D.R. Rissi).

Icterus is easily observed when affecting mucosal surfaces (Fig. 24) and the surface of tissues rich in elastin, such as the intimal surface of arteries (especially aorta and pulmonary artery), subcutaneous tissues (Fig. 25), articular surfaces, and brain.

Image Fig. 24. Oral icterus in a cat. The oral mucosa and lips are diffusely yellow due to hepatic failure (Image D.R. Rissi).
Image Fig. 25. Widespread icterus in a cat. The subcutaneous tissues and mesentery are diffusely bright yellow due to hepatic failure (Image D.R. Rissi).

The main causes of hyperbilirubinemia and icterus include hemolysis (pre-hepatic icterus), reduced hepatocellular activity with impaired capture, conjugation, and secretion of bilirubin (hepatic icterus), and bile stasis (post-hepatic icterus) due to intra-hepatic or extra-hepatic biliary obstruction. Icterus should be differentiated from pseudoicterus. This differentiation can be relied on the fact that carotenoid pigments will not be present in tissues other than fat.

Image Fig. 26. Left cerebrocortical granuloma in a horse. The main lesion (granuloma) is surrounded by yellow areas of edema that extend to the corona radiata (Image D.R. Rissi).

Edema fluid in horses, especially when contrasting with a pale white background (Fig. 26), can be yellow due to the fact that horses typically have yellow plasma under physiological conditions. Fat accumulation in organs such as liver (hepatic lipidosis) will also turn their normal color into a diffuse pale or bright yellow (Fig. 27).

Image Fig. 27. Hepatic lipidosis in a cat. The hepatic parenchyma is diffusely swollen and bright yellow due to massive accumulation of lipid within hepatocytes (Image D.R. Rissi).

In these cases, the hepatic parenchyma is usually swollen, greasy, and friable due to the large amount of lipid droplets within hepatocytes. Keratin is a naturally yellow material, so it is expected that lesions rich in keratin, such as keratin cysts and squamous cell carcinomas (Fig. 28) will be yellow.

Image Fig. 28. Metastatic squamous cell carcinoma in the lymph node of an ox. The yellow keratin produced by neoplastic cells expands and effaces a portion of the lymph node (Image C.S.L. Barros).

In cases of inflammatory exudate, fibrin admixed with neutrophils will usually look pale yellow (Fig. 29).

Image Fig. 29. Fibrinous enteritis due to Salmonella typhimurium infection in a cow. Bright yellow fibrillar material (fibrin) covers the necrotic intestinal mucosa (Image D.R. Rissi).

As the inflammation progresses, more neutrophils are recruited to the site and the exudate can turn hemorrhagic and appear red or brown (Fig. 30).

Image Fig. 30. Diffuse fibrinous pericarditis in a pig. In this case, fibrin appears red due to presence of blood and degenerate neutrophils (Image D.R. Rissi).

Suppurative and granulomatous inflammation will also look pale to bright yellow, such as in abscesses (Fig. 31) and areas of caseous necrosis (Fig. 32), respectively.

Image Fig. 31. Cerebral abscess in a lamb. Two abscesses containing yellow to green pus expand the left cerebral hemisphere (Image D.R. Rissi).
Image Fig. 32. Granulomatous lymphadenitis due to Mycobacterium bovis infection in a pig. Bright yellow areas of caseous necrosis diffusely efface the nodal architecture and also infiltrate the pulmonary parenchyma (Image D. Driemeier).

Urinary sediment in the bladder of animals that have not been able to urinate before death typically appears as a turbid, sandy yellow fluid (Fig. 33).

Image Fig. 33. Urinary sediment in the bladder of a dog. These yellow, sandy sediments accumulate when urination is impaired and urine accumulates in the bladder (Image D.R. Rissi).

Yellow or green pigmentation can also be observed as bile imbibition (Fig. 34). This is a postmortem artifact and occurs due to leakage of biliary pigment through the decaying gall bladder wall. Biliary imbibition is more evident in tissues that are in close contact with the gall bladder.

Image Fig. 34. Bile imbibition in a cat. The bile leaks through the decaying gall bladder wall after death and deposits on the tissues around it (Image D.R. Rissi).

Black indicates the presence of endogenous pigments such as melanin (and thus commonly seen in melanocytic neoplasms), exogenous pigments (carbon or tattoo ink), digested blood in the gastrointestinal tract, infection by pigmented fungi, and pseudomelanosis. Melanin is an endogenous black pigment that protects the skin from ultraviolet light. Thus, it is expected that areas with accumulation of melanin will be black, such as in areas of melanosis (Fig. 35).

Image Fig. 35. Leptomeningeal melanosis in the brain of an alpaca. The black pigmented areas reflect normal accumulations of melanocytes in the leptomeninges (Image D.R. Rissi).

Melanosis occurs as areas with physiologically excessive deposition of melanin. They occur more often in the intimal surfaces of large arteries in sheep, leptomeninges in sheep and cattle, esophageal mucosa in dogs, and lungs of pigs. Naturally, melanocytic neoplasms can be dark brown or black (Fig. 36), unless tumors are amelanotic (non-pigmented), in which case they will likely lack the dark pigmentation. Inhalation of carbon by-products will be deposited in the airways and will be drained to regional lymph nodes, where the pigment will permanently stain the affected tissue. This condition is referred to as anthracosis (Fig. 37) and is considered an incidental finding that occurs mainly in dogs that live in urban areas or in a smoking household.

Image Fig. 36. Malignant subungual melanoma in a dog. The neoplasm is diffusely black due to heavy pigmentation of neoplastic melanocytes (Image D.R. Rissi).
Image Fig. 37. Nodal and pulmonary anthracosis in a dog. The tracheobronchial lymph nodes are diffusely black, and there are multiple, pinpoint, black subpleural spots due to aspiration of carbon by-products (Image C.S.L. Barros).

Digested blood in the gastrointestinal tract is an important finding in cases of gastric ulcers. The blood that comes out of the ulcers will be digested when in contact with gastric enzymes and will be converted to a dark red (Fig. 38) and subsequently black, tar-like material (Fig. 39) that can be present in the stomach, intestine, or perianal area (Fig. 40).

Image Fig. 38. Gastric ulceration in a dog. The blood originating from the mucosal ulceration is typically dark red (Image D.R. Rissi).
Image Fig. 39. Gastric ulceration in a dog. Over time, the blood is digested by gastric enzymes and becomes black (Image D.R. Rissi).
Image Fig. 40. Bloody diarrhea in a dog. Digested blood can be eventually seen around the anus (Image D.R. Rissi).

A similar color is seen in areas of hemorrhage that have healed over (Fig. 41).

Image Fig. 41. Hemomelasma ilei in a horse. These are common serosal changes in the intestine of horses that reflect previous foci of hemorrhage.

Infection by certain pigmented (dematiaceous) fungi can cause affected tissues to become brown or black (Fig. 42).

Image Fig. 42. Necrotizing meningoencephalitis due to Cladophialophora bantianum infection in an alpaca. The pigmented fungal hyphae tinge the affected tissues in dark green or black (Image J. Stanton and B.J. McHale).

Pseudomelanosis manifests as dark green (Fig. 43) or black (Fig. 44) areas and is a postmortem artifact.

Image Fig. 43. Diffuse subcutaneous pseudomelanosis in an alpaca. Pseudomelanosis is the result of postmortem hemoglobin degradation by bacteria and occurs as dark green, gray, or black areas in multiple tissues (Image D.R. Rissi).
Image Fig. 44. Splenic pseudomelanosis in a dog. Pseudomelanosis is the result of postmortem hemoglobin degradation by bacteria and occurs as dark green, gray, or black areas in multiple tissues (Image D.R. Rissi).

Bacteria present in the intestines produce hydrogen sulfide, which reacts with the iron released by the postmortem breakdown of erythrocytes (see hemoglobin imbibition above), producing a precipitate that impregnates the surrounding tissues, giving them the dark green and then black appearance. Green implies the presence of endogenous substances such as bile (Fig. 45).

Image Fig. 45. Bile is naturally green and can be observed through the gall bladder wall in this cat (Image D.R. Rissi).

Eosinophilic inflammation in cases of eosinophilic myositis in cattle is characterized by green areas within the affected skeletal muscle. Similar green areas of muscle necrosis occur in cases of deep pectoral myopathy in poultry and at injection sites in ruminants and horses (Fig. 46).

Image Fig. 46. Focally extensive skeletal myonecrosis at an injection site in a horse. These lesions are typically green and reflect a mixture of necrotic tissue and foreign particles from the injection (Image C.S.L. Barros).

Infection by certain algae such as Chlorella spp. may cause the affected tissue to become green due to deposition of a pigment produced by the organisms. Light green crystals in the urinary bladder of dogs indicate deposition of ammonium biurate (Fig. 47) secondary to hepatic disease, including portosystemic shunts and cirrhosis.

Image Fig. 47. Ammonium biurate crystals in the bladder of a dog. These crystals typically indicate the presence of hepatic disease (Image L Chen).

As we have discussed, pseudomelanosis can be either green, when in early stages, or black. Translucent tissue changes usually reflect the accumulation of transudate or clear and watery edema fluid. The edematous abomasal folds of ruminants with hypoproteinemia (Fig. 48) are a great example of a translucent tissue change.

Image Fig. 48. Diffuse abomasal edema due to hypoproteinemia in an ox. The abomasal folds are translucent due to accumulation of edema fluid (Image D.R. Rissi).

Other changes that can lead to accumulation of clear fluid include cysts caused by obstruction of the normal outflow of secretory or excretory products (Fig. 49) or by parasitic infestations (Fig. 50).

Image Fig. 49. Renal cysts in a cat. The multifocal to coalescing cysts expand the renal parenchyma and are filled with clear fluid (Image D.R. Rissi).
Image Fig. 50. Hepatic cyst due to Cysticercus fasciolaris infection in a rat. Parasitic cysts are typically filled with translucent fluid (Image D.R. Rissi).

Abnormally white tissues or organs may reflect a lack or complete absence of blood (anemia) or the presence of adipose tissue, inflammation, necrosis, fibrosis, neoplasia, and mineralization. Diffusely pale white mucosal surfaces (Fig. 51) is a strong indication that you should start your quest for the cause of the underlying anemia.

Image Fig. 51. Conjunctival pallor in a goat. The conjunctival mucosa is diffusely white due to anemia (Image D.R. Rissi).

Adipose tissue (Fig. 52), and consequently adipose tissue neoplasms (Fig. 53), are naturally white.

Image Fig. 52. Subcutaneous adipose tissue in a dog. Adipose tissue is naturally white in most animal species (Image D.R. Rissi).
Image Fig. 53. Subcutaneous lipoma in a dog. Lipomas are neoplasms of adipose tissue and thus appear as white subcutaneous nodules (Image D.R. Rissi).

Lymphoplasmacytic or granulomatous inflammation, which can be observed in cases of malignant catarrhal fever, systemic granulomatous disease due to Vicia villosa toxicosis and other causes in cattle, or in cases of feline infectious peritonitis (Fig. 54) are good examples of inflammatory lesions that appear white.

Image Fig. 54. Granulomatous nephritis and vasculitis due to feline infectious peritonitis virus infection in a cat. Multifocal to coalescing white areas of inflammation and vasculitis surround renal blood vessels throughout the cortex and medulla (Image D.R. Rissi).

Fibrin with a low degree of neutrophilic inflammation, hemorrhage, or necrosis can also appear white grossly (Fig. 29).

Pale white areas of necrosis are easily observed in the skeletal muscle of animals suffering from vitamin E and selenium deficiency or that ingested specific toxins (Fig. 55).

Image Fig. 55. Focally extensive skeletal myonecrosis (Senna occidentalis toxicosis) in an ox. The pale white areas correspond to muscle degeneration and necrosis (Image D.R. Rissi).

Tissue loss and replacement with fibrous connective tissue can also be observed as single or multiple pale white or gray areas (Fig. 56) or throughout the entire organ (Fig. 57).

Image Fig. 56. Hepatic capsular fibrosis in a cow. The hepatic capsule is thickened and white due to multiple previous hepatic biopsies (Image D.R. Rissi).
Image Fig. 57. Diffuse hepatic fibrosis in a horse. Pale white, reticular areas of fibrosis secondary to cholelithiasis and chronic biliary obstruction are distributed throughout the hepatic parenchyma (Image D.R. Rissi).

Neoplastic infiltration especially in cases of lipoma (see Fig. 53) and lymphoma (Fig. 58) or other round cell tumors usually appears pale white.

Image Fig. 58. Gastric lymphoma in a cat. The gastric wall is thickened by a pale white neoplasm. The mucosa is covered with dark digested blood (Image D.R. Rissi).

White areas of mineralization can be observed in many different tissues and characteristically have a chalky or gritty consistency on cut surface (Fig. 59 and 60).

Image Fig. 59. Aortic mineralization in a lamb. The white, irregular areas of mineralization on the intimal arterial surface are granular and chalky on cut (Image D.R. Rissi).
Image Fig. 60. Lens mineralization in a dog with cataracts. Similar to the areas of arterial mineralization, the lens is granular and chalky or gritty on cut (Image D.R. Rissi).

Areas of lymphoid hyperplasia, especially those seen in the colon of dogs also appear as white foci that can be seen through the serosa (Fig. 61).

Image Fig. 61. Colonic lymphoid hyperplasia in a dog. Lymphoid follicles appear as white circles throughout the serosa (Image D.R. Rissi).

Chylous effusion in the thorax (chylothorax) is common following the rupture of the thoracic duct, and appears as a milky white fluid filling the thoracic cavity (Fig. 62).

Image Fig. 62. Chylothorax in a cat. Abundant white milky fluid fills the thorax and compresses the lungs (secondary atelectasis) (Image D.R. Rissi).

Brown tissue changes can indicate suppurative inflammation, especially when neutrophils and blood are admixed with necrotic debris (Fig. 63).

Image Fig. 63. Fibrinous peritonitis due to feline infectious peritonitis virus infection in a cat. Abundant yellow fibrillar fluid fills the abdomen and covers the serosal surface of multiple organs (Image D.R. Rissi).

The cyanotic mucosal membranes seen in animals that developed hypoxia before death is a classic example of a blue tissue change (Fig. 64).

Image Fig. 64. Oral cyanosis in a cat. The oral mucosa is partially blue due to hypoxia secondary to cardiac failure (Image D.R. Rissi).


            Tissue changes occur in a wide range of shapes, so again, the best thing to do is to keep it simple. This is not supposed to be a Rorschach test. Try to give the reader a three dimensional image of the tissue change by using well-known shapes such as round, rectangular, triangular, or irregular, among others. Then finalize your description using terms like flat, raised (elevated), or depressed. The demarcation of a tissue change (well-demarcated versus poorly-demarcated) indicates how easily it can be seen when contrasted with the adjacent normal tissue. It is an important descriptive term in cases of neoplasia.

Flat tissue changes cannot be noted upon touch since they are neither elevated nor depressed, but at the same level as the normal adjacent tissue. These changes usually suggest a recent event in which there has been no time for inflammation or healing to take place. Examples of flat changes include areas of epicardial hemorrhage (Fig. 65) and recent areas of renal necrosis (Fig. 66). These renal hemorrhages are well-demarcated since they easily contrast with the adjacent renal parenchyma. Poorly demarcated changes can be more difficult to be seen because they do not contrast well with the adjacent tissues.

Image Fig. 65. Multifocal epicardial hemorrhages in a puppy. The red hemorrhagic foci are flat and do not displace the epicardial surface (Image D.R. Rissi).
Image Fig. 66. Multifocal renal hemorrhages due to canine herpesvirus-1 infection in a dog. The hemorrhagic foci represent areas of hemorrhage and necrosis (Image D.R. Rissi).

Cerebral astrocytomas in dogs are a classic example of poorly demarcated lesions that appear to blend into the adjacent parenchyma (Fig. 67). Raised or elevated changes indicate that something (fluid, inflammatory cells, neoplastic cells) has been added to that specific site (Fig. 68).

Image Fig. 67. Cerebral astrocytoma in a dog. The neoplasm expands the right putamen and lacks clear demarcation from the adjacent tissues (Image B. Porter).
Image Fig. 68. Focal metastatic renal sarcoma in a dog. The focal, pale yellow, raised nodule contains massive infiltration by neoplastic cells and slightly displaces the adjacent parenchyma (Image D.R. Rissi).

On the other hand, depressed tissue changes indicate that something has been removed from that particular site (Fig. 69). They usually indicate a more chronic process in which enough time has passed for necrosis and healing to develop. The more necrotic tissue is removed from the site, the more depressed the affected site will appear.

Image Fig. 69. Necrotizing meningoencephalitis due to bovine herpesvirus infection in a calf. The necrotic cortical tissue from the depressed frontal portions of the brain has been removed by inflammatory cells (Image D.R. Rissi).


            Galileo Galilei said, measure what is measurable, and make measurable what is not so. This is a useful concept to keep in mind when describing the size of a tissue change. Organs increase in size mainly due to cell swelling, hypertrophy, hyperplasia, and infiltration with inflammatory or neoplastic cells, among other causes. Importantly, particular organs are physiologically more dynamic than others and can change their size according to their functional activity or metabolic demands in a particular period of time. For example, students often and blamelessly describe things like “the urinary bladder wall was thickened” without realizing that this is a normal finding if the bladder is empty. The “thick” wall needs to be thick so it can distend greatly when the bladder is full. Changes in size can be easily noticed if an organ is markedly increased or decreased in volume. It is easy to notice a swollen kidney when you have the other kidney for comparison (Fig. 70).

Image Fig. 70. Unilateral hydronephrosis in a dog. The right kidney is markedly distended by urine; that change is easily seen because you can compare the size and shape of the affected kidney with the normal contralateral kidney (Image D.R. Rissi).

Same thing with the eyes, thyroid glands, lungs, adrenal glands, and others. However, it can be challenging to assess changes in size when lesions are subtle or no contralateral organ is part of the game (Fig. 71).

Image Fig. 71. Cerebral oligodendroglioma in a dog. The right hemisphere is slightly swollen due to the presence of the mass (Image D.R. Rissi).

Here is where measurements become more important than ever. Another question that should be raised when assessing size is whether organ X is bigger than its counterpart Y or organ Y is actually smaller than X. In such cases, the recognition of additional features, such as scar tissue (which would likely make an organ look smaller) and your professional experience will be crucial for a proper description and interpretation of the tissue changes. Ultimately, the best thing to do is to cover your bases with objective measurements because you will need to convince the reader that whatever you observed was really there.


            Consistency is evaluated by touching the organ and the tissue change. After evaluating everything else and saving your formalin-fixed and fresh tissue samples, you should spend some time touching and squeezing normal tissues to feel their consistency (that will be your internal control for this parameter). Then you should compare that with tissues affected by specific pathologic processes. Consistency is a feature that reflects tissue homogeneity, firmness, adherence, resistance, density, and viscosity. You should assess the entire organ, but the organization or lack of organization can often be much more easily appreciated on its cut surface.

            Tissue changes with pasty or gooey consistency and no organization most likely reflect the presence of exudate or necrosis; solid tissue changes likely reflect fibrous connective tissue, chronic inflammation, or neoplasia. Think about it this way: could you spread it on a slice of bread? If yes, it is probably pasty (Fig. 72); if not, it is probably solid (Fig. 73).

Image Fig. 72. Focally extensive cerebral abscess in a lamb. The abscess is filled with pasty yellow suppurative exudate (Image D.R. Rissi).
Image Fig. 73. Cardiac histiocytic sarcoma in a dog. The neoplasm is soft and partially effaces the right ventricular wall and lumen (Image D.R. Rissi).

Solid tissue changes can be further characterized according to their consistency as soft (Fig. 74), firm (Fig. 75), and hard (Fig. 76). To describe consistency correctly, compare it to your ear lobe (soft), tip of the nose (firm), and forehead (hard).

Image Fig. 74. Multiple cutaneous lipomas in a dog. The tumors are soft and composed purely of adipose tissue (Image D.R. Rissi).
Image Fig. 75. Pharyngeal mast cell tumor in a dog. The neoplasm is white, organized, and firm (Image D.R. Rissi).
Image Fig. 76. Proximal tibial osteosarcoma in a dog. Neoplastic cells produce bone and thus the neoplasm is hard (Image D.R. Rissi).

The consistency of a fluid should not be a challenge. Examples of soft changes include cysts, abscesses, or some neoplasms (such as lipomas); firm changes can include fibrosis and neoplasms; hard tissue changes include bone neoplasms and mineral. Examples of fluid or semifluid changes include hemorrhage, edema, or exudates (Fig. 77).

Image Fig. 77. Mucopurulent sinusitis in a dog. The exudate is brown due to the presence of mucus and neutrophils admixed with blood (Image D.R. Rissi).


            Special features are not always assessed, and include aspects such as weight, sound, and odor. Weighing the organ will help you to confirm your findings if the change is subtle (e.g. mild cardiac hypertrophy). While you may be sure that something is bigger and heavier than normal (Fig. 78), other changes may be too subtle to be noticed (Fig. 79) and will need more evidence for confirmation.

Image Fig. 78. Cerebral oligodendroglioma in a dog. The focally extensive neoplasm expands the left lateral ventricle and adjacent neuroparenchyma (Image D.R. Rissi).
Image Fig. 79. Cerebellar herniation through the foramen magnum. The lesion is subtle and the herniated area is slightly below the line where the dorsal aspect of the foramen lies (Image D.R. Rissi).

Sound can indicate the presence of gas, as seen in cases of black leg in cattle (Fig. 80).

Image Fig. 80. Focally extensive skeletal myonecrosis and hemorrhage due to Clostridium chauvoei infection (black leg) in a cow. There is hemorrhage and multiple gas bubbles that can be felt during palpation as areas of crepitation (Image D.R. Rissi).

Odor is a more difficult parameter to describe since it may be subjective and prone to confirmation bias (you think there is chronic hepatic disease so you think you smell the sweet odor known as fetor hepaticus in the skin even if no such aroma is present). More specific odors, such as ammonia in the oral cavity of dogs with chronic renal disease (Fig. 81) and necrotic debris admixed with digested blood in the classic “parvovirosis smell” in dogs may be easily noticed.

Image Fig. 81. Bilateral lingual ulcers in a dog with chronic renal insufficiency. The oral cavity typically contains a strong ammoniac odor (Image D.R. Rissi).

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