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Cell injury underlies all diseases. So to understand diseases one, has to start by knowing what cell injury is. When a cell is exposed to an injurious agent (i.e. the causes of diseases), the possible outcomes are: 1. The cell may adapt to the situation or 2. They cell may acquire a reversible injury or 3. The cell may obtain an irreversible injury and may die.
The cell may die via one of two ways: either by necrosis or by apoptosis. Which of these outcomes occur depends on both the injurious agent and on cellular factors. In other words, the result depends on the type, severity, and duration of the injury and on the type of the cell. This post is concentrated only on cellular adaptation other results will be discussed separately. The types of cellular adaptation include hypertrophy, atrophy, hyperplasia, and metaplasia
Hypertrophy
Hypertrophy is increase in the size of cells. Increased workload leads to increased protein synthesis and increased size and number of intracellular organelles which, in turn, leads to increased cell size. The increased cell size leads to increased size of the organ. Examples: the enlargement of the left ventricle in hypertensive heart disease and the increase in skeletal muscle during sternous exercise.
Hyperplasia
Hyperplasia is an increase in the number of cells. It can lead to an increase in the size of the organ. It is usually caused by hormonal stimulation. It can be physiological as in enlargement of the breast during pregnancy or it can pathological as in endometrial hyperplasia.
Atrophy
Atrophy is a decrease in the size of a cell. This can lead to decreased size of the organ. The atrophic cell shows autophagic vacuoles which contain cellular debris from degraded organelles. Atrophy can be caused by: Disuse, Undernutrition, Decreased endocrine stimulation, Denervation and Old age
Metaplasia
Metaplasia is the replacement of one differentiated tissue by another differentiated tissue. There are different types of metaplasia. Examples include:
1. Squamous metaplasia
This is replacement of another type of epithelium by squamous epithelium. For example, the columnar epithelium of the bronchus can be replaced by squamous epithelium in cigarette smokers
2. Osseous metaplasia
This replacement of a connective tissue by bone, for example at sites of injury.