Aplasia signifies a total failure of development of a part. Hypoplasia is an incomplete development.

Atrophy

Atrophy refers to a decrease in the size and in the functional activity of a part. It may be general or local.

In general atrophy the entire body wastes, a condition known as emaciation. It may be the result of lack of food, of starvation, or of disturbances of trophic influences with disorders of metabolism.

In local atrophy certain portions undergo changes which may be either simple, degenerative, or numerical, as the latter is sometimes called.

In the simple variety the individual cells undergo a decrease in size.

In the degenerative the number of cells is reduced as a result of disease. This is not considered a condition of true atrophy.

Atrophy may be brought about by there being no longer a demand made upon the part. Through lack of use the cells become smaller.

Old age is often accompanied by atrophy; is seen particularly in the sexual organs and in the loss of the elastic tissue of the skin.

Pressure is one of the commonest causes; occurs as a result of tight lacing, etc.

Interference with the blood-supply on account of the part not being supplied with a proper amount of nutrition.

Disturbances of the trophic functions, as in poliomyelitis.

The atrophied part will be smaller than normal, and frequently very irregular, causing elevations and depressions. Microscopically, the cells will be reduced in size, more or less degenerated, and frequently pigmented. The latter condition occurs commonly in the heart and is known as brown atrophy.

Degenerations

Degenerations of cells can be divided into two forms:

1. Infiltrations, in which abnormal substances are deposited within the cells.

2. Metamorphoses., in which the protoplasm of the cell is transformed into abnormal substances.

"It was thought, but now seems less certain, that we could distinguish two processes which might accompany each other: one, the change wrought in the cytoplasm itself, leading to the appearance in the cell of such changed products; the other characterized by the appearance in the cytoplasm of substances obtained from outside the cell, and, it may be, imperfectly handled by the cell. It was thought that the former were degenerations proper and the latter infiltrations, but further study shows that it is becoming increasingly difficult to separate the two; that, in fact, they are too closely related to permit of being considered apart. Especially does it seem to be that true infiltration by itself is a rare occurrence. 'Infiltrated' materials, as fat, glycogen, etc., probably are the result of synthetic processes".

The changes in the cell may also be either quantitative, as when a normal substance is present in an abnormal amount; or qualitative, when there is an abnormal substance present.

Necrobiosis refers to the molecular or cellular death of a part.