Changes in the appearance of the structure of organs and tissues make the recognition of lesions difficult for students of necropsy. Some are considered incidental findings during necropsy. These changes may be categorised into five major groups as follows:

1) Normal anatomical variation
2) Physiologic changes
3) Senile changes
4) Agonal changes
5) Post mortem changes

NORMAL ANATOMICAL STRUCTURES AND SPECIES VARIATION

Veterinary students and practitioners should be aware that anatomic variations occur within and between species. The appearance of organs in one species group cannot be assumed to be similar for most species of the same group. Variations occur even within the same animal group. Following is a list of structures and other findings included under this category:

1) Mucoid material in renal pelvis of horses - this is due to the presence of mucin secreting goblet cells in the epithelia of renal pelvis and calyces.
2) Pallor of the kidney cortex in cats - this is due to the abundance of intracytoplasmic lipid in tubules of nephrons.
3) Nodules of Arantius - found at the center of the free edge of aortic and pulmonary valves of the heart in horses and pigs.
4) Valvular haematoma in atrio- ventricular valves in young ruminants - occur as blood-filled cyst on margins of the valve of the heart.
5) Os cordis in cattle - consist of bones embedded in the myocardium at the base of the aortic valve.
6) Unguiculate papillae - these are keratinised papillae at oesophageal groove of ruminants.
7) Torus pyloricus - this is a tongue- shaped epithelium covered bulge of tissue at gastro-duodenal junction in pigs and cattle.
8) Duodenal papillae - nodules in the mucosa of the proximal duodenum and these structures are present in most species. These are the sites where the bile duct and pancreatic ducts open into the duodenum.
9) Pigmentation of mesenteric lymph nodes and the presence of haemo- lymph nodes in ruminants.
10) Melanosis of organs - gray or black pigmentation of meninges, brain, parenchyma, kidney, adrenals, uterus, lungs, oesophagus, oral cavity, gastric and intestinal mucosa, and intima of great vessels of the heart in most species.
11) Os penis of dogs - consist of bone at the corpus cavernosum of the penis.
12) Absence of seminal vesicle and bulbo-urethral glands in dogs.
13) Discrete fine nodules in the pancreas of cats - these are pacinian corpuscles normally found in the pancreas of this species.
14) Presence of caseous material in the prepuce of boars.
15) Presence of whitish plaques in the oesophageal mucosa of animals that have not eaten for some time.
16) Conversion of red marrow into a gelatinous mass - seen in malnourished animals and is more pronounced in ruminants.
17) Hyperplasia of lymphoid follicle with formation of prominent nodules in pharynx and larynx of young horses.
18) Dilated lymphatics in epicardial surfaces of heart in horses.
19) Cysts in kidneys of pigs - maybe solitary or multiple, and most are congenital abnormalities of no clinical significance.

PHYSIOLOGICAL CHANGES

The normal functions of the animal body usually lead to some alterations in the gross appearance of organs and tissues. This may be affected by a host of factors related to the state of nutrition, endocrine influence, circulatory status, and stage of production. Tissue changes under this category include the following:

1) Post-prandial physiologic hyperaemia of stomach mucosa - occur as reddening of the stomach mucosa in most species. This change is more pronounced in the horse and pig. The visible absence of inflammatory exudates differentiates it from gastritis.
2) Copious amount of mucus in stomach mucosa in the horse and pig - this is due to continued secretion of mucus in the stomach even after a few minutes after death in these species.
3) Distended gall bladder in most species that have not eaten recently - the gall bladder contains watery pigmented bile, and this finding is more pronounced in dogs and ruminants.
4) Pallor of the liver in pregnant and lactating animals, and this is particularly seen in ruminants.
5) Uterine mucosal changes in appearance, texture and contents - this is part of the normal cyclical activity of the uterus following pregnancy and parturition. If the animal has recently given birth, pink- coloured sludge like material with no offensive odour may be seen contained in the uterus.
6) Atrophy of the prostate in male animals after castration - the prostate show great deduction in size and with shriveled connective tissue capsule.
7) Vascular congestion and hyperaemia of the gastrointestinal tract - seen in animals that have taken a meal shortly before death.
8) Yellow precipitate in renal papillae of piglets, usually associated with dehydration and/or insufficient urine flow.

SENILE CHANGES

Changes in the structure and appearance of organs and tissues occur as the animal matures. These changes might be confused as lesions during necropsy. Senile tissue changes seen in domestic animals include the following:

1) Tension lipidosis in the liver - seen most commonly in cattle and horses. This is characterised by focal pallor of the liver parenchyma immediately next to the mesenteric attachment.
2) Nodules in the liver of old dogs - these nodules are of variable size, and may be deep in the parenchyma or sub capsular in location. They are composed of hyperplastic liver parenchyma.
3) Fatty cyst in old cats - these are brownish nodular masses found on the sub capsular surfaces or deep into the parenchyma of the liver.
4) Cystic hyperplasia of gall bladder mucosa in dogs - this takes the appearance of fleshy folds in the mucosa with fluid-filled cysts.
5) Nodular hyperplasia of pancreas and adrenal glands - these structures should be differentiated from neoplastic nodules.
6) Multifocal pleural fibrosis in cattle and sheep - these are observed more commonly on the diaphragmatic surfaces of caudal lobes of the lungs.
7) Focal grittiness in trachea, bronchi and lung parenchyma of old dogs and cats.
8) Anthracosis in dogs and cats - frequently seen in those animals living in the city. The lung parenchyma may show fine dark coloration. The draining lymph nodes may also show the same changes.
9) Cholesteatoma (Cholesterol granulomas) in choroid plexus in the lateral ventricle of the brain in horses - these are discrete nodular masses that may be tan, sometimes mineralised gritty nodules. These are accumulations of cholesterol and macrophages on the distal tips of the choroid plexus.
10) Medullary calcification in the kidneys of dogs and cats - occur as white gritty radial streaks in the kidney medulla. Sometimes, they appear as whitish streaks that border the cortex and the medulla.
11) Par ovarian cysts in old dogs - these may be solitary or multiple and situated on the surface of the ovary and in the adjacent mesosalpinx or mesometrium. These structures may contain colorless fluid.
12) Prostatic changes in dogs - there may be enlargement, or hyperplastic nodules may be seen. Laminated and calcified masses deep within the parenchyma may be noted.
13) Nodular endocardiosis in dogs - these changes occur in the atrio-ventricular valves and appear as shiny nodules at the edges of the valves. They may be multiple, and their significance should be assessed in terms of their effects on the patency of the affected valve.
14) Siderotic nodules and extra splenic tissues in dogs - Siderotic plaques are hard and gritty pale to yellowish nodules seen more commonly at the edges of the spleen. Extra splenic tissues occur as nodules of varying sizes scattered at the omentum near the spleen, and may represent previous traumatic injury to the spleen.
15) Osteoarthritis in dogs - this is common in large breeds of dog, and may take the form of erosion of the articular cartilages. Other findings include thickening of the joint capsule with deformation of the articular surfaces.
16) Conversion of red marrow into fat in most species.
17) Telangiectasis of liver in old debilitated cattle.
18) Small, atrophic lymph nodes in old sheep
19) Subendocardial mineralisation in ventricle or atria of adult horse.
20) Haemomelasma ilei in horses - consist of elevated subserosal plaques of variable size, shape and colour on the antero- mesenteric surface of distal small intestine and colon.
21) Fibrous tags attached to the liver capsule, particularly on diaphragmatic surface in horses and buffaloes.

AGONAL CHANGES

Agonal changes are those tissue changes that occur immediately before death or following cessation of vital functions. While dying, some changes occur in tissues and include the following:

1) Vascular congestion of most organs - seen particularly in the lungs and pancreas.
2) Pulmonary emphysema in ruminants and pulmonary oedema in dogs - the emphysema is due to labored breathing during the agonal period. The pulmonary oedema is due to agonal impairment of the venous return of blood.
3) Haemorrhages in the heart in most species - occur as streaks of haemorrhages and are more pronounced on the endocardial surfaces.
4) Adrenal congestion and haemorrhages - seen in most species and more commonly observed in cattle and horses.
5) Food materials in the lungs and airways - this is more commonly observed in ruminants and should be differentiated from ante mortem aspiration of foreign materials.
6) Congestion of the meningeal vessels in the brain of most species - may be related to redistribution of blood to vital organs.

POST MORTEM CHANGES

Post mortem changes result from degradation of tissues associated with the release of proteolytic lysosomal enzymes from the cells. These processes (called post mortem autolysis) occur automatically after death of the animal. It must be remembered that autolysis also occur following death of a limited portion of an organ or tissues (necrosis). The term post mortem autolysis appropriately distinguishes the same processes that occur after death of the whole animal.

Post mortem autolysis is influenced by a host of factors. It is particularly enhanced by the action of bacteria. Bacteria that form part of the microbial flora of surface mucosa particularly in the gut proliferate soon after death. This possibly occurs due to alterations in the environment, where the usual defense mechanism of the animal body ceases to operate. Also, the bacterial proliferation may be due to the availability of a large amount of growth promoting substrates. Following their proliferation, invasion of organs and tissue occurs possibly through the vessels and lymphatics. Animals that died of bacterial diseases usually show post mortem changes rapidly than those that died of other causes.

FACTORS INFLUENCING THE RATE OF POST MORTEM AUTOLYSIS

The factors that influence the rate of post mortem changes include the following:

1) Temperature - The mechanism of enzyme action is temperature- dependent. Ambient environmental and body temperature of the carcass accelerates post mortem autolysis.

2) Post Mortem Interval - Post mortem autolysis occur immediately after death of the animal. The degree of post mortem changes expected to be seen in an animal cadaver depend on the time elapsed before necropsy. Post mortem autolysis is progressive with time, and the longer the time elapsed the greater the degree of post mortem changes in organs and tissues occur.

3) Cause and Mode of Death - Generalised infection such as septicaemic bacterial diseases hasten the appearance of post mortem changes.

4) Condition of the Animal before Death - The condition of the animal before death, particularly its nutritional status influences the rate of post mortem autolysis. Obese animals tend to show an accelerated appearance of post mortem changes. This condition is probably related to the insulating effect of the fat layer that retards cooling of the body after death.

5) Tissue-related Factors - The degrees of the expression of post mortem changes vary from tissues to tissues. The presence of bacterial flora, enzyme secretions, and the availability of moisture and substrates influences the rate of post mortem autolysis.

SPECIAL TERMS USED TO DESCRIBE SPECIFIC POST MORTEM CHANGES

1) Rigor mortis - refers to the contraction and stiffening of muscles after death. Most literatures consider the fall in the availability of adenosine triphosphate (ATP) as the possible cause of terminal muscle fiber contraction after death. Other plausible explanation includes the influx of calcium ions after cessation of the sodium pump.

Classically, rigor mortis begin from 1-6 hours after death and passes off in 24-48 hours. However, several factors influence the onset of rigor mortis and these include the following:

a) Nutritional status of the animal
b) Environmental and body temperature of the cadaver
c) Cause of death

Well-fed animals have large glycogen reserves, and may show a delay in the onset of rigor, while cachectic animals may develop rigor quickly. Animal cadavers that are exposed to hot temperatures may develop and passes off rigor mortis quickly. Those that died of septicaemic diseases may not develop rigor mortis at all. Once a part of the animal body that has passed into rigor is moved, that part will pass off rigor mortis. This has some significance in human medico-legal cases.

2) Algor mortis - Gradual cooling of the animal body after death, and is associated with a fall in ATP.

3) Livor mortis - The settling of blood to the down side of the animal body. Gravitational force causes this to happen. This gravitational settling of blood and body fluids results to intense reddish colouration of the organs and tissues at the down side of the cadaver.

4) Haemoglobin imbibition - Pinkish to reddish colouration imparted to tissues due to the lysis of red blood cells. This is most evident on the surfaces of large arteries and in outer surfaces of visceral organs.

5) Bile imbibition - Golden yellow colouration imparted on tissues following seepage of bile. Discolouration is most evident on the surfaces of organs in contact with the gall bladder, and on duodenal mucosa.

6) Pseudomelanosis - Greenish gray to dark colouration of tissues. This is due the action of bacteria to haemoglobin forming hydrogen sulphide. Pseudomelanosis suggests a more advanced stage of post mortem degradation of tissues.

7) Chicken Fat Clot - An old term referring to the gelatinous mass formed by separation and coagulation of plasma proteins from the component of blood. This is usually seen inside the major blood vessels and the heart. They sometimes give a cast of the ramifications of the vessels. To prevent misconceptions, the use of this term should be avoided.

8) Currant Jelly Clot - An old term applied to coagulated blood. The term becomes obsolete and should be avoided.
 

COMMENTS ON SPECIFIC POST MORTEM CHANGES IN VARIOUS ORGANS AND TISSUES

Gastrointestinal Tract

Segments of the gastrointestinal tract undergo rapid post mortem changes. This is due to the presence of digestive juices, bacterial flora, and substrates needed for the growth of microorganisms. Immediately after death, contractions of the villi occur and result to sloughing of the intestinal epithelia. Grossly, a mucoid whitish sludge may be observed, and this may be stained yellow with bile. As time progresses, gaseous distensions occur. This invariably results to stripping of the mucosa, perforation and rupture. Occasionally, stagnant blood may be lysed and impart a reddish colouration to the contents. Haemoglobin imbibition may impart a reddish colour to the serosa. In ruminants, sloughing of the epithelia of the forestomach occurs.

The Liver and Gall Bladder

Haemoglobin imbibition imparts a shiny reddish discolouration on the surface of the liver. The liver parenchyma close to the gall bladder may be stained yellowish or greenish because of bile imbibition. As time progresses, the liver may loss its turgidity, become soft and clay-like in colour and consistency. The presence of gas-distended bubbles on the parenchyma usually suggests a more advanced stage of post mortem decomposition. The gall bladder mucosa easily detaches after death.

The Pancreas

Post mortem change occur rapidly in the pancreas. It may be discoloured red from haemoglobin imbibition, lose its lobular pattern, become soft and translucent. The portion that is facing the gall bladder may show bile imbibition. In advanced stages of post mortem degradation, the pancreas turns into a sac-like structure containing red-tinged fluid. It may even disappear leaving a flimsy membranous structure.

The Kidneys

The kidneys may be discoloured red from haemoglobin imbibition, and later discoloured black (pseudomelanosis). The demarcation between cortex and medulla becomes not apparent. As time progresses, the organ becomes soft and gas bubbles form at the peri-renal tissues.

Spleen and Lymph Nodes

The spleen may be discoloured gray to black, becomes soft and mushy. Advanced stages of post mortem degradation usually turn the spleen into a sac-like structure containing liquefied parenchyma that oozes when cut. The lymph nodes may be discoloured, become soft and pulpy.

Lungs and Airways

Normal lungs are pale pink in colour. After death, post mortem congestion becomes evident apart from livor mortis. Blood and fluid may ooze when the surface is cut. The airways may contain considerable quantities of froth and fluid. The organ becomes soft, collapsed, discoloured red to gray, and gas bubbles may appear on the parenchyma as time progresses.

The Heart and Great Vessels

The heart becomes flabby, and may contain coagulated plasma separated from the red components of blood. The endocardial surfaces and the intima of great vessels may show reddish discolouration following haemoglobin imbibition.

The Brain

The brain, being located farther from the gut show delayed post mortem changes. Vacuolation of the brain parenchyma due to seepage of cerebrospinal fluid may be observed. With advanced stages of post mortem degradation, the brain turns into a soft mushy mass. Later, the brain liquefies and will ooze when the calvarium and meninges are opened.

Muscles

Discolouration of the muscles, fascia and tendon sheath occurs after death because of seepage of myoglobin. Intense colouration of the muscles may be seen at the down side of the animal. Air pockets demarcating muscle groups and fascial places occur in advanced stages of degradation.

The Eye

The cornea of the eyes turns opaque due to absorption of the aqueous humor. The globe may either protrude or collapse. When opened, detached retina may be noted.

Body Cavities and Orifices

Surfaces of body cavities may show discolouration following haemoglobin imbibition and pseudomelanosis. The cavities may contain red-tinged fluid and may be copious in its amount. Gaseous distensions render the carcass to appear bloated. It is not uncommon to find post mortem rectal or vaginal prolapse because of developing internal pressure. Straw-coloured fluid may be seen oozing from body opening in advance stages of post mortem decomposition.