Under normal conditions cells are in a homeostatic or steady state. When stimulus arrived to the cell, first the cell will adapt but if this stimulus crosses the boundary of adaptation it will cause cell injury. So cells respond to the stimulus by the following two ways:

Cellular adaptation:

Adaptation is an adjustment of the cell to the environmental change which disturbs the homeostasis. Cellular adaptations are of the following types.

A.     Atrophy:

A reduction in the size of the cell due to loss of cell contents which leads to decrease in the size of an organ or tissue. The number of cells remains unchanged. Cell shrinks and become smaller in size.

Causes:

        I.            Ischemia:

The atrophy may be due to low blood flow to organ, tissue or cell. Such type of ischemia is called ischemic atrophy.

Example:

Narrowing of coronary arteries results in ischemic atrophy of myocardium

      II.            Disuse (reduced functional activity):

The atrophy due to disuse or immobilization of an organ, tissue or cell is called disuse atrophy. As we now that blood flow to the tissue, organ or cells depends on the metabolic needs and inactivity or immobilization reduces the nutritional demand.

Example:

Atrophy in paralytic limb.

Obstruction of gland duct leads to atrophy of gland.

   III.            Interrupted (discontinue) nerve supply:

Skeletal muscle depends on their nerve supply for normal function and structure. Damage to the nerve supply leads to rapid atrophy of the muscle fibers supplied by that nerve. This type of atrophy is called denervation atrophy.

Example:

Atrophy of skeletal muscle after destruction of nerves in poliomyelitis.

    IV.            Endocrine deficiency:

Hormonal deficiency causes reduced metabolic activity leading to atrophy.

Example:

Deficiency of pituitary hormones leads to atrophy of thyroid, adrenal glands, gonads and genital organs.

      V.            Pressure:

Atrophy is produced by persistent pressure on a tissue or organ which may either causes injury to the cell or interferes with its blood supply or lymphatic drainage.

Example:

Tumor pressuring on the surrounding tissues

    VI.            Lack of nutrients:

Atrophy is produced by lack of nutrients such as in protein calorie malnutrition.

Example:

Iron deficiency anemia

 VII.            Senile atrophy:

The atrophy which occurs at old age is called senile atrophy.

Normal old age of male is above 60 years while that of female is after menopause (45 years).

B.     Hypertrophy:

Increase in the size of the cells resulting in an enlargement of a tissue or organ without any change in the number of cells is called hypertrophy.

Types of hypertrophy:

        i.            Physiological hypertrophy:

The growth of uterus during pregnancy stimulated by estrogen hormone is called physiological hypertrophy. Hypertrophy in the muscle cells of the heavy exercising individuals like body builders.

      ii.            Adaptive hypertrophy:

It is enlargement of cardiac and skeletal muscle due to over work. They enlarge because they are unable to form more cells by mitotic division to share the work. 

a.       Left ventricular hypertrophy in hypertension

b.      Right ventricular hypertrophy in pulmonary hypertension

C.     Hyperplasia:

Increase in the number of cells resulting in an increased volume of the organ or tissue. Hyperplasia can only occur in the cells capable of mitotic division in postembryonic life, when stressed or stimulated to increase activity.

Types of hyperplasia:

        i.            Physiological hyperplasia:

It is further divided into:

a.       Hormonal:

Hyperplasia of female breast at puberty, during pregnancy and lactation

Uterus during pregnancy shows hyperplasia in response to increased level of ovarian steroids

b.       Compensatory:

Hyperplasia in the remaining kidney when the other is removed or destroyed due to increased demand on the remaining kidney.

      ii.            Pathologic hyperplasia:

It occurs due to excessive hormonal stimulation of the target cell e.g.

a)      Adenomatous hyperplasia of endometrium due to excessive estrogen stimulation

b)      Thyroid hyperplasia: In primary hyperthyroidism

c)       Epidermal hyperplasia: In chronic irritation of the skin

Note: Hyperplasia may produce clinical diseases e.g. endometrial bleeding, thyroid hyper function etc

D.    Metaplasia:

·         The replacement of normal cell type by abnormal cell type is called Metaplasia.

·         It is reversible change in which one adult cell type (epithelial or mesenchymal) is replaced by another adult cell type.

·         The normal pseudo stratified columnar ciliated epithelium of the trachea and bronchi are replaced by stratified squamous epithelium in response to chronic irritation in chronic cigarette smokers.

·         Squamous epithelium of esophagus is replaced by columnar secretory epithelium as a result of acid reflux into esophagus causing Barrett’s esophagus).

In metaplasia there is chance of malignancy. The only difference in metaplasia and Neoplasia (cancer) is that the metaplasia is reversible while the Neoplasia is irreversible.