Degenerations and Infiltrations - Pathology

()Map GENERAL PATHOLOGY (Map)

Introduction

• Degenerations and infiltrations are the morphological manifestation of an altered metabolism within the cell.
  • A particular kind of change within a cell or tissue may suggest that a specific type of alteration has occurred.
• Degenerations and infiltrations are types of structural changes.
  • These are best considered at a cellular level.
  • These structural changes are deviations from the cell's normal structure and function.
    • Parameters are outside the normal physiological range for the cell.
• Degeneration
  • The tissue cell shows some change in itself.
• Infiltration
  • Something accumulates in the cell or tissue.

Cellular Swelling

• Cellular swelling is
  • The earliest detectable degenerative change.
  • The mildest form of cellular degeneration.
  • The first stage in injury to a cell.
  • Caused by a variety of insults, e.g.
    • Lack of oxygen (anoxia) to a tissue.
    • Toxic influences.
• Is due to the impairment of the integrity of the cell membrane.
• Cellular swelling is characterised by a moderate swelling of the individual cells.
  • Due to an influx of water into the cell.

Gross Appearance

• Organs diffusely affected with cloudy swelling grossly appear pale.
  • This may be partly due to the swollen cells impeding the tissue's blood supply.
• Without cutting into an organ, it may be difficult to appreciate a gross enlargement of it.
  • Each individual cell is increased in size, meaning the entire volume of the organ is also increased.
  • E.g. on cutting the liver or kidney capsule, the underlying swollen parenchyma bulges outwards, making the cut ends of the capsule retract.
• The degree of gross swelling is not great.
  • Could be easily confused with early post-mortem changes in the organ.

Histological Appearance

• Individual cells appear somewhat swollen.
• The cytoplasm appears more red in colour in hematoxylin and eosin (H&E) stained sections.
• The nucleus of the cell remains normal.
• Cellular swelling is best histologically appreciated in the liver and kidney in damage caused by circulating toxins that are not powerful enough to actually kill the cells.

Significance of Cellular Swelling

• Cellular swelling is an important stage in degeneration.
  • Not commonly observed on its own without more serious changes
    • Not easy to identify at post-mortem unless the examination is performed very soon after the animal's death.
      • Early post-mortem (autolytic) change in dead tissue looks rather similar.
    • Cellular swelling is also reversible.
      • When the toxin is no longer exerting its effect, the tissue returns to normal.
    • Cellular swelling may be a transient stage in the more serious forms of degenerations which follow.

Hydropic Degeneration

• Hydropic degeneration often indicates severe cellular damage due to viruses.
  • Is a more severe or advanced form of cellular swelling.
• There are two types of hydropic degeneration, in which:
  1. The cells may swell up like a balloon prior to their destruction.
     • Ballooning Degeneration
  2. There is a discrete bleb (vacuole) of fluid within the cytoplasm.
     • Vacuolar Degeneration

Ballooning Degeneration

• May occur in a variety of conditions.
  • Is particularly seen in viral conditions of epithelial tissue.
• Foot and Mouth Disease is the best example.
  • Foot and Mouth virus attacks the stratum spinosum of the epithelium of the tongue and feet.
  • Affected cells balloon up with water containing the replicating virus, swelling until they burst.
    • The fluid contained in the cells then forms microvesicles (blisters) in the stratum spinosum.
      • Blisters may later burst, shedding vast quantities of the virus.
  • On bursting, the edges of the erosions look ragged.
    • Within weeks, the germinal epithelium at the base of the erosion regenerates the epithelium, leaving no trace of a scar.

Vacuolar Degeneration

• In vacuolar degeneration, excess water is transferred to the endoplasmic reticulum (ER).
• The ER swells and eventually fragments.
  • A fluid vacuole remains in the cytoplasm.
• Commonly occurs in cells that are very metabolically active and have well developed pumping mechanisms.
  • E.g. as the hepatocyte, renal tubular epithelium and pancreatic acinar cell.

Cellular Fatty Change

• DOES NOT REFER TO THE THE FAT STORES OF THE BODY!
  • Fatty substances accumulate or increase within the cytoplams of specific cells.
  • In some instances, the fat stores may be involved in the transfer of fat to these specific cells.
• Cellular fatty change is an important intracellular abnormality.
  • Principally concerns the intracellular fat in hepatocytes.
• Fatty change is commonly seen in three organs of the body.
  • Principally in the liver.
  • Also in the kidney and the heart.
  • This is because these organs are either:
    • Involved in the metabolism of fat, or
    • Dependant upon lipids as an energy source.
• Fatty change can be readily recognised at post-mortem.

Gross Appearance of Fatty Change

• Liver
  • This is the main organ involved in fatty change.
  • May be greatly increased in size.
  • Is tan to yellowish in colour.
    • Is normally reddish brown.
  • Very prone to rupture with slight pressure (friable).
  • Parenchyma bulges outwards on being freed from the constraint of the capsule when cut.
  • Parenchyma is dull, yellowish and greasy.
• Kidney
  • The cortex appears paler.
    • N.B. This is normal in e.g. the cat!
    • Diffuse paleness is not the prominent feature, unlike in the liver.
• Heart
  • Anoxia, as a result of anaemia, causes fatty change.
  • The heart is flabby.
  • Fatty change may occur as streaks in the papillary muscles.
    • I.e. those muscles furthest away from the blood supply.
  • Contractile ability is reduced, and blood is therefore not pumped efficiently.

Histological Appearance of Fatty Change

• The fat either appears as globules or is contained in varying sizes of vacuoles in the cytoplasm.
  • In the heart, fat appears as groups of tiny vacuoles dispersed along the myofibrils.
  • In the liver and kidney, vacuoles tend to coalesce to form larger ones.
    • One or more large globules may fill the cytoplasm.
      • The nucleus is displaced to the periphery of the cell.
• The nucleus remains normal.
  • Nuclear changes are only seen if the degree of fatty change becomes incompatible with the continued existence of the cell.
• In hepatocytes, it is necessary to stain for fat in order to ellucidate if a vacuole in the cytoplasm is fat-containing.
  • Two further conditions may produce vacuoles in hepatocytes.
    • Vacuolar hydropic degeneration
    • Glycogen accumulation
  • Stains commonly used include Sudan 111, Sudan 1V, and Oil Red O.
    • Stain fat varying shades of orange to red.
  • Sections must be prepared differently to the routine paraffin embedding (used e.g. in H&E staining).
    • The strong solvents used in paraffin embedding dissolve the fat out of the cell.
    • When staining for fat, the tissue to be examined is frozen and sectioned in a cryostat before being stained.
      • These sections are more than twice as thick as those attained by sectioning paraffin blocks
        • There may be some overlap of cells on the section.
        • Individual cells are less clear.

Causes of Fatty Change

Dietary and Metabolic

1. Starvation
   • A reduction in dietary intake necessitates the increased mobilisation of fat from body fat stores to meet energy needs.
   • Fat from stores is transported in the blood as fatty acids.
     • The liver cannot cope with them all properly.
       • The fatty acids are stored in the liver as neutral fats.
2. Overeating
   • When the dietary intake is greater than the energy expenditure, the fat is temporarily stored prior to movement to the body fat stores.
     • Also occurs in fat-rich diets.
3. Lipotrope Derangement
   • Lipotropes are substances which hasten the removal of fat from the liver cells.
   • Lipotropes include the amino acids that allow conjugation of fat with proteins to form the lipoprotein that is excreted from cells.
     • E.g. choline, methionine.
     • Dietary deficiency of these leads to fatty change within the cells.
   • Some poisons also prevent stages of lipoprotein formation.
     • E.g. CCl4, phosphorus and alcohol

Metabolic diseases

• Certain metabolic diseases may result in deranged carbohydrate metabolism.
• Glucose is not made available for uptake into the tissues.
  • The cells still require energy, and so alternative pathways are resorted to.
    • This leads to fatty change.
• Examples:
  • Diabetes mellitus in dogs
    • Deficiency of the hormone insulin required for cellular glucose utilisation.
  • Ketosis in ruminants
    • The body is exhorted to find another source of energy following drainage of the glucose reserves.
      • Fat reserves are mobilised and transported to the liver.
    • E.g.
      • Twin lambs in sheep
        • The condition is known as Pregnancy Toxaemia
      • Milk producion in high-yielding dairy cattle shortly after parturition.
        • Acetonemia

Anoxia

• Any condition that reduces the oxygen supply to the tissues will cause fatty change in the

liver.

• Examples:
  • Anaemia
    • Reduced numbers of red blood cells circulating in the blood
    • Caused by sustained loss of erythrocytes from the vessels by
      • Chronic haemorrhage
      • Excessive destruction of erythrocytes (haemolysis).
  • Circulatory disorders
    • Ischaemia
      • Reduced blood supply to a tissue
    • Chronic venous congestion
      • Slowing of blood flow through the vasculature e.g. due to a failing heart.

Toxins

• Many toxins will cause fatty change in the Liver.
  • In these cases fatty change may be considered to be a more severe form of cellular swelling.
• Examples:
  • Bacterial and fungal toxins
    • May be:
      • Produced in the bloodstream by circulating bacteria (septicaemia/bacteraemia)
      • Produced elsewhere and absorbed into the bloodstream.
  • Chemical toxins
    • For example, CCl4, phosphorus, arsenic and lead.
  • Plant toxins
    • Some plant toxins will cause fatty change in the very early stages of poisoning.

Distribution of Fatty Change in the Liver

• Fatty change in the liver tends to be throughout the whole lobule.
• Occasionally there is a preferential localisation - this may give some clue as to the inciting cause.
  • E.g. in chronic venous congestion
    • Due to a failing heart (a cause of anoxia).
    • Blood pools in the centrilobular area (due to ineffective blood flow back to the heart), as well as fatty change being induced.
      • Gives a striking gross appearance - areas of yellow interspersed with red.
        • Described as a 'nutmeg' liver.
    • When found post-mortem examination, indicated the heart should be examined for the cause.

Significance of fatty change

• Fatty change is reversible, provided that the underlying cause is brought under control.
• Necrosis
  • From the distribution of fat in a cell, it may be difficult to decide whether the fatty change is due to a toxic or metabolic defect.
    • In toxic effects, the fatty change can be considered as a more serious form of cellular swelling.
      • There may be evidence of necrosis.
    • If a metabolic defect is prolonged, cellular function may be impaired by the substantial amount of fat.
      • Necrosis may also result in this instance.
• Wallerian Degeneration
  • A special form of fatty change in the nervous system.
  • Damage to myelinated nerves results in the degeneration of the myelin that ensheaths them.
• Extracellular accumulation of lipids
  • Necrosis of cells containing lipid may release lipid into the extracellular space.
    • Haemorrhage or tissue damage may result in cholesterol being released from cells or pooled from lipoproteins in crystalline form (cholesterol clefts).

Mucoid Degeneration

• Mucoid degeneration is also known as mucinous or myomatous degeneration.
• Mucoid degeneration involves changes in epithelial tissue or the extracellular matrix/ ground substance.
• An extracellular phenomenon of some specific cells.
  • Tend to show a bluish tinge in H&E stained sections.

Epithelium

• The specific cells involved in mucoid degeneration in the eptihelium are:
  • The goblet cells of wet mucous membranes.
  • The mucous glands themselves.
• This is not really a degeneration, but an increased production of mucin.
  • It is a beneficial reaction; the product
    • Is important as a lubricant.
    • Soothes inflamed surfaces.
    • Traps and dilutes harmful agents.
    • Carries specific antibodies against infectious agents.
    • Provides a means for removal of infectious agents.

Connective tissue

• Here, the mucin forms part of the ground substance between the fibroblasts that produce it.
• A disturbance in the metabolism of the fibroblasts under some circumstances,means the ground substance takes on a bluish hue in H&E sections.
• Mucoid degeneration in the heart valves of middle-aged and older dogs is a common example.
  • Causes endocardiosis.
    • A condition specific to the dog.

Endocardiosis

• Tends to affects middle-aged and older dogs.
• Primarily occurs in the mitral valve.
• Results in slowly developing heart failure
• The valves become swollen and misshapen
  • The heart cannot pump blood effectively to the circulation from the left ventricle.
    • Blood passes back into the left atrium, compromising the filling from the pulmonary vein.
      • Leads to back pressure on the pulmonary capillaries.
        • Oedema forms in the lungs, and can be heard as moist sounds on auscultation.
        • Reduces the oxygenation of blood leading to exercise intolerance.
    • Failure of the left side eventually compromises the function of the right side.
      • There is pooling of blood in the venous system i.e. in the liver.

Hyaline Degeneration

• The term "hyaline degeneration" is applied to several types of degeneration or infiltration.
  • "Hyaline" is a descriptive term meaning "glassy".
    • Used in pathology when structureless material appears in section, and stains red with eosin
      • Describes a variety of conditions in which structureless materials are present.
• Is applied to both extra- and intra-cellular degenerations/ infiltrations. ** Extracellular
  • • Protein hyaline casts in the renal tubules.
      • From excessive protein passing through glomerulus.
      • When fresh urine is examined under the microscope, casts may be seen as elongated glassy tubes.
    • Hyaline membranes
      • Proteinaceous effusions into pulmonary alveoli
      • Prevent gaseous exchange. ** Intracellular
    • Hyaline degeneration of skeletal muscle.
      • In vitamin E/ selenium deficiency.
• There are two forms of hyaline degeneration that deserve their own mention.
  • Fibrinoid degeneration.
  • Amyloid infiltration.

Fibrinoid Degeneration

• Fibrinoid degeneration features a material which is fibrin-like.
• Is essentially a focal death of cells in the walls of small blood vessels (usually arterioles).
  • Parts of the vessel wall become replaced by a granular material.
    • Pinkish-red (i.e. eosin-staining).
    • Has some of the appearance and staining properties of fibrin.
    • Consists partly of degenerated muscle and elastic fibres, and partly of an increased amount of protein ground substance around the degenerated fibres.
    • Looks like a red smudge in the vessel wall when viewed histologically.
• The presence of the fibrin-like material may suggest
  • A local hypersensitivity reaction
  • Hypertension

Amyloidosis

• Also known as amyloid infiltration
• Deposition of a proteinaceous hyaline substance in extracellular sites.
  • Sites of deposition vary with species.
• The kidney is a common site of deposition.
  • Amyloid is deposited under the capillary endothelium and progressively increases in volume.
• There are various categories of amyloid.
  • Is essentially an abnormal protein produced in the body
  • In most cases, it is produced in response to sustained antigenic stimulation caused by a chronic suppurative process.
    • E.g. a foot abscess, mastitis.
• Amyloid is a relatively inert substance.
  • When it accumulates, it is not easily removed.

Glycogen Infiltration

• Glycogen is normally present in substantial amounts in the liver and muscle.
  • Is a readily utilisable source of energy.
• Moderate glycogen infiltration in the liver:
  • Grossly - doesn't have much effect.
  • Histologically - shows up as foamy cytoplasmic vacuoles, similar to that of fat.
• Some conditions may result in an increase in glycogen deposits.
  • Diabetes mellitus
    • Gives an increase in hepatic glycogen stores.
    • May be overshadowed by the increased fat in the hepatocytes, and therefore difficult to see.
      • Renal tubular deposits are more easily observed.
  • Excessive glucocorticoids
    • Could be due to:
      • Hyperadrenocortism
      • Animals being maintained on glucocorticoid therapy over a long period of time.
        • Has more dramatic effect.
    • Huge amounts of glycogen infiltration.
      • Liver appears larger and paler.
      • Vacuoles may be so extensive that the rest of the cytoplasm appear as pink strands passing from the nucleus to the plasma membrane.
        • A "feathery appearance" or "web-like effect".
  • Glycogen storage diseases
    • Due to an inherited deficiency of an enzyme required for the breakdown of glycogen to glucose.
      • Cells continuously accumulate glycogen.
    • Seen in all tissues of the body but exerts its major effect in the CNS.
• Selective staining must be employed to distinguish glycogen vacuoles from fatty vacuoles in the liver.
  • Alcohol fixation is preferred.
  • Best's Carmine is the commonly used stain.
    • Stains the intracellular glycogen red.

Cellular Inclusions

Viruses

• Either in the nucleus or cytoplasm.

Storage Products

• Due to hereditary storage diseases in which there is a missing or defective cellular enzyme. There are many types known in domestic animals.
  • Break down/ build up of intracellular substances is halted.
    • The intermediate substance accumulates in the lysosomes.
  • Affects all tissues
    • The central nervous system is particularly vulnerable.
• Due to age
  • Lysosomes will accumulate non-degradable products
    • E.g. lipofuscin - the 'ageing' or 'wear and tear pigment'.
      • Commonly seen in middle-aged cats in the hepatocytes nearest the centrilobular veins.

Intracellular Bacteria

• For example, Mycobacterium tuberculosis in macrophages.

Protein Accumulation

• In the renal tubular epithelium.
• Occurs when there is leakage of protein out through a damaged glomerulus.