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).
||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).
||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
||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).
||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).
||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).
||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.
If the blood leaks into a body cavity, it can clot and form a
hematoma (Fig. 20).
||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
||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).
||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.
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.
||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.
||Fig. 24. Oral icterus in a cat. The oral mucosa and
lips are diffusely yellow due to hepatic failure (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
||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
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).
||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
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.
||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).
||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).
||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.
||Fig. 31. Cerebral abscess in a lamb. Two abscesses
containing yellow to green pus expand the left cerebral
hemisphere (Image D.R. Rissi).
||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).
||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
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.
||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).
||Fig. 35. Leptomeningeal melanosis in the brain of an
alpaca. The black pigmented areas reflect normal
accumulations of melanocytes in the leptomeninges (Image
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.
||Fig. 36. Malignant subungual melanoma in a dog. The
neoplasm is diffusely black due to heavy pigmentation of
neoplastic melanocytes (Image D.R. Rissi).
||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.
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).
||Fig. 38. Gastric ulceration in a dog. The blood
originating from the mucosal ulceration is typically
dark red (Image D.R. Rissi).
||Fig. 39. Gastric ulceration in a dog. Over time, the
blood is digested by gastric enzymes and becomes black
(Image D.R. Rissi).
||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).
||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).
||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.
||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
||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).
||Fig. 45. Bile is naturally green and can be observed
through the gall bladder wall in this cat (Image D.R.
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).
||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.
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.
||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.
||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).
||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).
||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.
||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.
||Fig. 52. Subcutaneous adipose tissue in a dog. Adipose
tissue is naturally white in most animal species (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
||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.
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.
||Fig. 55. Focally extensive skeletal myonecrosis (Senna
occidentalis toxicosis) in an ox. The pale white areas
correspond to muscle degeneration and necrosis (Image
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).
||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).
||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
Neoplastic infiltration especially in cases of lipoma (see
Fig. 53) and lymphoma (Fig. 58) or other round cell tumors
usually appears pale white.
||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).
||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
||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.
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).
||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).
||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).
||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
The cyanotic mucosal membranes seen in animals that developed
hypoxia before death is a classic example of a blue tissue
change (Fig. 64).
||Fig. 64. Oral cyanosis in a cat. The oral mucosa is
partially blue due to hypoxia secondary to cardiac
failure (Image D.R. Rissi).