The preceding sections dealt with the normal cell, the adapted cell, the reversibly injured (sick) cell and the irreversibly injured (dead or dying) cell. As you may recall, the normal cell may adapt to adverse influences in its environment and assume an "altered but steady state." However, when the challenge exceeds the capacity of the cell to adapt, it may become reversibly injured as reflected by cellular swelling (hydropic change) and fatty change. With more devastating challenges, the cell may become irreversibly injured and die as reflected by the sequence of changes known as necrosis.
The next three sections will deal with substances that accumulate in abnormal amounts within and outside of both normal and injured cells. These substances may result in damage to the cell, but in many instances, they do not cause cellular injury and merely serve as indicators of some abnormal occurrence.
At the conclusion of this section, the student should be able to perform the following tasks:
Under certain circumstances, normal or injured cells accumulate abnormal amounts of various substances. These accumulations most often occur in the cytoplasm and usually are contained within lysosomes.
In many instances, intracellular accumulations are composed of substances that are produced by the cell involved. These substances accumulate in excessive amounts when either their rate of production is increased or their rate of utilization and/or elimination is reduced.
In other instances, intracellular accumulations are composed of substances that originate in other organs or tissues and amass in cells because of an inability on the part of that cell to eliminate or metabolize the substance at a rate exceeding its acquisition.
In addition, some pathologic pigments are exogenous in nature meaning they come from outside the body.
As pointed out earlier, the term fatty change refers to an abnormal accumulation of fat within parenchymal cells. It currently replaces such previously used terms as fatty degeneration and fatty infiltration, because the process is not necessarily degenerative or infiltrative in nature. Fatty change represents an absolute increase in the amount of fat and it is due to an imbalance involving either production, utilization or mobilization of fat.
Fatty change occurs most notedly in hepatocytes. The liver plays a pivotal role in the metabolism of fat because it is largely responsible for the conversion of free fatty acids into a lipoprotein which is more readily utilizable by other cells. Normally, lipids are transported to the liver from either adipose tissue or from dietary sources. Once in the liver, they are used for several processes including: the formation of triglycerides, the production of cholesterol, incorporation into phospholipids or utilization as a fuel by mitochondria. Lipids leave the liver as lipoproteins formed by combining triglycerides with a lipid acceptor protein. With this in mind, it is possible to identify several potential mechanisms that, either singly or in combination, are responsible for hepatic fatty change.
Fatty change occurs in other tissues as well, including cardiac muscle. The pathogenesis in this case usually involves hypoxia and probably results from decreased oxidation of fatty acids by mitochondria.
There are other situations where lipids or lipoid substances accumulate in cells. Ceroid and lipofuscin are lipid in nature but they are discussed under pathologic pigments. Macrophages that phagocytose large amounts of lipid debris develop numerous lipid filled phagosomes and take on a foamy appearance. These cells are often referred to as foam cells.
Cholesterol and cholesterol esters accumulate in cells in certain circumstances. In atherosclerosis, cholesterol accumulates in smooth muscle cells and macrophages in the intima of arteries. In hereditary hyperlipemia, it accumulates in macrophages, usually under the skin, forming tumor-like structures known as xanthomas.
Accumulations of excessive amounts of a proteinaceous material within cells occurs primarily in epithelial cells of the proximal convoluted tubules of the kidney and in plasma cells. In the kidney, this excessive accumulation occurs subsequent to leakage of proteins from glomeruli into the glomerular filtrate. Usually the protein involved is albumin, but other proteinaceous substances such as hemoglobin and myoglobin are also encountered. The protein is then pinocytosed into the proximal tubular epithelial cell and the pinocytotic vesicle fuses with a lysosome forming a secondary phagolysosome. These appear as hyaline granules in the cytoplasm of the renal tubular epithelial cells.
Plasma cells actively engaged in the production of immunoglobulins sometimes become overloaded with these immunoglobulins and large eosinophilic inclusions, called Russell Bodies, appear in their cytoplasm. The inclusions are membrane bound and are localized within dilated cisternae of the endoplasmic reticulum.
Intracellular accumulation of excessive amounts of glycogen can be divided into two basic categories. These categories are "Glycogen Infiltration" and "Glycogen Storage." They differ mainly in their pathogenetic mechanisms. Whatever the cause, the glycogen appears as clear vacuoles in the cytoplasm of cells in H & E stains. It is best preserved in nonaqueous fixatives such as absolute alcohol. Special stains used to help identify glycogen include Best's carmine and periodic-acid Schiff (PAS) stains, both of which impart a rose to violet color to glycogen.
Glycogen infiltration refers to an accumulation of glycogen that occurs due to excessive amounts of glucose in the circulation (hyperglycemia). Hyperglycemia is usually encountered in Diabetes mellitus and it is due to insufficient insulin concentrations or tissue insensitivity to insulin. The tissues usually involved in glycogen infiltration include: the epithelial cells of the distal portion of the proximal convoluted tubule and in the loop of Henle in the kidney, leukocytes within inflamed or necrotic tissue, the liver and, on rare occasions, in cardiac muscle fibers.
In the liver, glycogen accumulates in the cytoplasm of hepatocytes under normal and abnormal circumstances. In well nourished animals, there may be enough glycogen in the cytoplasm of hepatocytes to give the cytoplasm a finely granular appearance. This "normal" increase in glycogen is difficult to distinguish from pathologic accumulations. In Diabetes mellitus, glycogen accumulates excessively and this is referred to as glycogen infiltration.
Glycogen infiltration in renal tubular epithelial cells develops due to reabsorption of glucose from the glomerular filtrate. This glucose is stored in the form of glycogen within the tubular epithelial cells.
Accumulated glycogen is not harmful to cells, but its presence is usually indicative of some abnormality in either glucose or glycogen metabolism.
Glycogen within leukocytes occurs when dead or dying cells within a necrotic area release glycogen into the intercellular medium. Subsequently, this glycogen is absorbed or phagocytized by invading leukocytes.
Glycogen Storage Diseases, also known as glycogenoses, are autosomal recessive genetic disorders characterized by defective catabolism of glycogen with accumulation of glycogen in lysosomes. Several enzymes are involved in the catabolism of glycogen and the various glycogenoses are caused by a lack of one or more of these enzymes. Eight different syndromes have been described in the human literature with such names as: Von Gierke's disease, Pompe's disease, Cori's disease, McArdle's Syndrome, etc.
Storage diseases are diseases characterized by abnormal storage of various substances. They are inherited problems and the accumulating substances are usually endogenous materials that are normally found in cells in small amounts. Generally these substances accumulate due to the lack of an enzyme that is necessary for their metabolism. When these substances accumulate in excessive amounts, they are usually localized within lysosomes and thus these diseases are also referred to as "lysosomal storage diseases." The glycogen storage diseases are somewhat similar to these conditions but, with the exception of Pompe's disease, the glycogen is not stored in lysosomes.
Usually, these substances collect within cells throughout the body, but mainly in reticuloendothelial cells, neurons, myocardial fibers and the parenchymal cells of the liver and kidneys. The list of substances involved includes glycogen, sphingolipids, mucopolysaccharides, mucolipases, complex carbohydrates, cholesterol esters and triglycerides. The diseases carry such names as Tay-Sachs disease, Niemann-Pick disease, Gaucher's disease, etc.
Various kinds of inclusion bodies may develop either in the nucleus or the cytoplasm of cells under pathologic conditions. These inclusions (or abnormal masses) are usually of major significance in making an etiologic diagnosis.
Inclusions induced by viral infections may occur within the nucleus or cytoplasm. They may be eosinophilic or basophilic in H & E sections. Many of these inclusions represent aggregates of virions which tend to have characteristic stain affinities. By using the Feulgen (for DNA) and acridine-orange (for single vs double stranded nuclei acids) techniques, inclusion bodies can be identified as being either DNA or RNA in nature. Also, such viral inclusions can be specifically identified with the use of immunologic reagents such as fluorescent antibody techniques. Protein-lattice inclusion bodies are found in viral infections, metabolic diseases and as incidental nonspecific changes in degenerating cells. These are usually large rhomboid structures which are particularly common in cells that normally contain fibers (myocytes, neurons, etc.). Protein is laid down in repeating units and the inclusions have a linear structure with defined periodicity.
In lead poisoning, acid fast intranuclear inclusions may develop in renal tubular epithelial cells. These inclusions are intranuclear protein matrices upon which metallic ions are deposited.
The Chediak-Higashi syndrome is an autosomal recessive genetic disorder characterized by, among other things, the formation of large intracytoplasmic granules that are often referred to as inclusions. These granules represent abnormally fused versions of normal granule subpopulations and they have been described in nearly every cell type, except mature RBCs. The disease has been reported in humans, mink, cattle, mice, cats and a killer whale.
A progressive neuronal myoclonus epilepsy known as Lafora's Syndrome has been reported in humans and in dogs. It is inherited as an autosomal recessive trait and is characterized by the development of large PAS positive intracytoplasmic inclusions within neurons and occasionally hepatocytes and myocardial fibers. Ultrastructural studies have shown that the inclusions contain fibrils with associated endoplasmic reticulum and ribosomal material.
Crystalline protein inclusions ("brick inclusions") occur in apparently normal cells of the liver, kidneys and gonads of many species. They occur within organelles or free with the nucleoplasmic or cytoplasmic matrix. Although striking in appearance, these inclusions have no known significance.
After completing this section, each student should be in a position to provide appropriate answers to the following questions.