Atrophy is characterized by decrease in the size and function of  a cell. It is often seen in areas of vascular insufficiency or chronic inflammation and may result from disuse of skeletal muscle.

Atrophy may be thought of as an adaptive response to stress in which the cell shrinks in volume and shuts down its differentiated functions, thus reducing its need for energy to a minimum.

 Upon restoration of normal conditions, atrophic cells are fully capable of resuming their differentiated functions, size increases to normal, and specialized functions, such as protein synthesis or contractile force, return to their original levels.

Atrophy occurs under a variety of conditions:

- Reduced Functional Demand : The most common form of atrophy follows reduced functional demand.

For example, after immobilization of a limb in a cast as treatment for bone fracture, or after prolonged bed rest, muscle cell atrophy and muscular strength is reduced.

With resumption of normal activity, normal size and function are restored.

- Inadequate Supply of Oxygen : Interference with blood supply  to tissue is known as ischemia. Total ischemia, with cessation of oxygen perfusion of tissues, results in cell death.

However, partial ischemia occurs after incomplete occlusion of  a blood vessel or in areas of inadequate collateral circulation following a complete vascular occlusion.

This results in a chronically reduced oxygen supply, a condition often compatible with cell viability.

Under such circumstances, cell atrophy is common.

It is frequently seen around the inadequately perfused margins   of ischemic necrosis (infarct) in the heart, brain, and kidneys following vascular occlusion in these organs.

- Insufficient nutrients : Starvation or inadequate nutrition associated with chronic disease leads to cell atrophy,  particularly in skeletal muscle. It is striking that reduction in mass  is particularly prominent in cells that are not vital to survival of the organism. One cannot dismiss the possibility that a portion   of the cell atrophy caused by partial ischemia reflects a lack of nutrients.

- Interruption of Trophic Signals : The functions of many cells depend on signals transmitted by chemical mediators. 

Examples include the endocrine system and neuromuscular transmission.

The demands placed on the cell by the actions of hormones or, in the case of skeletal muscle, by synaptic transmission, can be eliminated by removing the source of the signal.

This can be accomplished through, for example, surgical excision of an endocrine gland or denervation.

If the anterior pituitary is surgically resected, the loss of thyroid-stimulating hormone (TSH), adrenocorticotropic hormone (ACTH), and follicle stimulating hormone (FSH) results in atrophy of the thyroid, adrenal cortex, and ovaries, respectively.

Atrophy secondary to endocrine insufficiency is not restricted to pathological conditions -( Eg:  Atrophy of the endometrium caused by decrease in the estrogen levels following menopause).

Even cancer cells can be induced to undergo atrophy, to some extent, by hormonal deprivation. Androgen-dependent cancer  of the prostate partially regresses after castration. The growth of certain types of thyroid cancer is halted by inhibiting pituitary TSH secretion with thyroxin.

Neurologic conditions resulting in denervation of muscles, and thus in the loss of the neuromuscular transmission necessary for muscle tone, cause atrophy of the affected muscles.

The wasting caused by poliomyelitis or traumatic paraplegia  falls into this category.

Persistent Cell Injury :  Persistent cell injury is most commonly caused by chronic inflammation associated with prolonged   viral or bacterial infections.

Chronic inflammation may be seen in a variety of other circumstances, including immunologic and granulomatous disorders. Whether cell injury results from the inciting agent, the inflammatory process itself, or both, is not always clear. In any event, cells in areas of chronic inflammation are often atrophic.

Persistent toxic injury, as exemplified by the action of cigarette smoke on the bronchial mucosa, can also cause atrophy.

Even physical injury, such as prolonged pressure in inappropriate locations, produces atrophy.

Heart failure leads to increased pressure in sinusoids of the liver because the heart can not efficiently pump the venous return from that organ. Accordingly, the cells exposed to the greatest pressure - those in the center of the liver lobule - become atrophic.

Aging : Cell aging is a process independent of disease.

One of the hallmark of aging, particularly in non-replicating cells such as those of the brain and heart, is cell atrophy.

The size of all the parenchymal organs of the body decreases with age.

The size of the brain is invariably decreased, while in the very old the size of the heart may be so diminished that the term "senile atrophy" has been used.

Summary: 

Atrophy is characterized by reduction in size of the cell by loss of cell substance due to diminished work load, blood supply, endocrine stimulation etc.                                      

Example: (i) Disuse atrophy of paralyzed limb ; (ii) Senile atrophy of heart, kidney ; (iii) Menopausal atrophy of uterus ; (iv) Physiological atrophy of thymus.

Incomplete lipid-peroxidation by the atrophied cells often leads to accumulation of brown pigment "Lipofuscin", commonly seen in brown atrophy of heart.

Atrophy may be followed by apoptotic cell death.

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