Cell adaptation refers to the changes taking place in a cell in response to environmental changes.

Normal functioning of the cell is always threatened by various factors such as stress, chemical agents, diseases and environmental hazards. Yet, the cell survives and continues the function by means of adaptation. Only during extreme conditions, the cell fails to withstand the hazardous factors which results in destruction and death of the cell.

Cellular adaptation occurs by any of the following mechanisms.

1. Atrophy

2. Hypertrophy

3. Hyperplasia

4. Dysplasia

5. Metaplasia.


Atrophy means decrease in size of a cell. Atrophy of more number of cells results in decreased size or wasting of the concerned tissue, organ or part of the body.

Causes of Atrophy

Atrophy is due to one or more number of causes such as:

i. Poor nourishment

ii. Decreased blood supply

iii. Lack of workload or exercise

iv. Loss of control by nerves or hormones

v. Intrinsic disease of the tissue or organ.

Types of Atrophy

Atrophy is of two types, physiological atrophy and pathological atrophy. Examples of physiological atrophy are the atrophy of thymus in childhood and tonsils in adolescence. The pathological atrophy is common in skeletal muscle, cardiac muscle, sex organs and brain.


Hypertrophy is the increase in the size of a cell. Hypertrophy of many cells results in enlargement or

overgrowth of an organ or a part of the body. Hypertrophy is of three types.

1. Physiological Hypertrophy

Physiological hypertrophy is the increase in size due to increased workload or exercise. The common

physiological hypertrophy includes:

i. Muscular hypertrophy: Increase in bulk of skeletal muscles that occurs in response to

strength training exercise

ii. Ventricular hypertrophy: Increase in size of ventricular muscles of the heart which is advantageous

only if it occurs in response to exercise.

2. Pathological Hypertrophy

Increase in cell size in response to pathological changes is called pathological hypertrophy. Example is the ventricular hypertrophy that occurs due to pathological conditions such as high blood pressure, where the workload of ventricles increases.

3. Compensatory Hypertrophy

Compensatory hypertrophy is the increase in size of the cells of an organ that occurs in order to compensate the loss or dysfunction of another organ of same type. Examples are the hypertrophy of one kidney when the other kidney stops functioning; and the increase in muscular strength of an arm when the other arm is dysfunctional or lost.


Hyperplasia is the increase in number of cells due to increased cell division (mitosis). It is also defined as

abnormal or unusual proliferation (multiplication) of cells due to constant cell division. Hyperplasia results in gross enlargement of the organ. Hyperplasia involves constant cell division of the normal cells only. Hyperplasia is of three types.

1. Physiological Hyperplasia

Physiological hyperplasia is the momentary adaptive response to routine physiological changes in the body. For example, during the proliferative phase of each menstrual cycle, the endometrial cells in uterus increase in number.

2. Compensatory Hyperplasia

Compensatory hyperplasia is the increase in number of cells in order to replace the damaged cells of an organ or the cells removed from the organ. Compensatory hyperplasia helps the tissues and

organs in regeneration. It is common in liver. After the surgical removal of the damaged part of liver,

there is increase in the number of liver cells resulting in regeneration. Compensatory hyperplasia is also

common in epithelial cells of intestine and epidermis.

3. Pathological Hyperplasia

Pathological hyperplasia is the increase in number of cells due to abnormal increase in hormone secretion. It is also called hormonal hyperplasia. For example, in gigantism, hypersecretion of growth hormone induces hyperplasia that results in overgrowth of the body.


Dysplasia is the condition characterized by the abnormal change in size, shape and organization of the cell. Dysplasia is not considered as true adaptation and it is suggested as related to hyperplasia. It is common in epithelial cells of cervix and respiratory tract.


Metaplasia is the condition that involves replacement of one type of cell with another type of cell. It is of two types.

1. Physiological Metaplasia

Replacement of cells in normal conditions is called physiological metaplasia. Examples are transformation of cartilage into bone and transformation of monocytes into macrophages.

2. Pathological Metaplasia

Pathological metaplasia is the irreversible replacement of cells due to constant exposure to harmful stimuli. For example, chronic smoking results in transformation of normal mucus secreting ciliated columnar epithelial cells into non-ciliated squamous epithelial cells, which are incapable of secreting mucus. These transformed cells may become cancerous cells if the stimulus (smoking) is prolonged.

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