Hypertrophy, generally speaking, means an enlargement or overgrowth of any kind. It is usually divided into true and false hypertrophy, or hyperplasia, as the latter is called.

True hypertrophy is a uniform enlargement of a part, dependent upon an increase in size of all of its component elements. Accompanying the enlargement there is an increase in the functional power of the part involved.

The hypertrophy may be either congenital or acquired. It may also be either physiologic or pathologic. The former, however, may come under the latter heading when it reaches a degree that is not normal to the individual.

Hypertrophy is called compensatory when one organ takes upon itself the amount of work that was primarily carried on by two; is known as vicarious when another function increases at the expense of one that has been destroyed.

Etiology

1. Congenital causes, in which case there are marked overgrowths of portions of the body, especially of the fingers and toes.

2. Exercise calls for an increased amount of energy. This demand is met by a greater supply of food with a subsequent increase in size, and is seen in the enlarged muscles of a blacksmith, or in a kidney when the other one is diseased or absent. This latter is an example of compensatory hypertrophy.

3. Nervous influences in some indefinite way play a part in hypertrophy, as is seen in the enlargement and increased function of the mammary glands during pregnancy.

4. Disease of the hypophysis cerebri apparently causes the condition called acromegaly, in which the tissues of the face and extremities hypertrophy.

Morbid Anatomy

The part affected is uniformly increased in size.

Microscopically hypertrophy may be divided into the simple or true and the numeric (hyperplasia).

In the simple there is an increase in the size of the individual cell. This is seen particularly in the pregnant uterus, where at term the unstriped muscle cells may be eleven times as long and four times as broad as normal.

In the numeric variety the cells increase in number, but not necessarily in size; may even be smaller than normal.

Hyperplasia, or false hypertrophy, is a condition in which there is an increase in number of the cells with usually an asymmetric enlargement of the tissue. It is an excess of one constituent of an organ without a corresponding growth of the other elements.

The fibrous connective tissue is most commonly involved.

Etiology

I. Irritation is the most common cause, if not too severe in character. In that case inflammation with consequent degeneration results.

The irritation may be mechanical, such as results from intermittent pressure exerted by tight shoes, or from the presence of a foreign body. Chemical irritants, such as alcohol, will bring about an increase in the amount of connective tissue, particularly in the liver, in which case there is also an increase in the number of bile capillaries.

2. Nervous influences, such as bring about the condition known as pseudohypertrophic muscular paralysis. In it there is not an increase in the muscle itself, but the fat has undergone a hyperplasia. There is also a fatty degeneration, with atrophy of muscle-fibers.

3. Compensatory, such as occurs when, on account of the decrease in size of an organ, the surrounding tissues have undergone a hyperplasia in order to supply the deficiency.

Morbid Anatomy

The part involved may be much larger than normal or, on account of the contraction of the newly formed connective tissue, be much smaller. In either case the change is not symmetric.

In elephantiasis the part involved will be irregularly enlarged as a result of the obstruction of the lymphatics and the increase in the number of cells.

In hyperplasia of the connective tissue of the liver the organ may be smaller than normal and have a roughly granular surface.

Metaplasia refers to the transformation of one tissue into one of another variety. The new variety must, however, be one derived from the same blastodermic layer. There is not, however, a development of less specialized tissues into a higher. type; a simple epithelium cannot, in the vertebrates, give rise to a more complex glandular tissue or to nerve-cells. Columnar epithelium may become converted into stratified squamous epithelium with keratosis, as in the uterus, gall-bladder, or larnyx. This change is more common in the connective-tissue group, as in the formation of fat from areolar tissue, of bone from fibrous tissue, etc.

Heteroplasia is the development of a new tissue in a locality where it is not normally found. This is seen particularly in connection with neoplasms.

Anaplasia refers to the reversion of a cell to a less specialized stage. It is a change occurring preparatory to an increased proliferation. Is applied mainly to tumor formation. Hanse-mann's studies of tumor cells show that unequal, asymmetric and multipolar mitosis, and destruction of chromosomes is of frequent occurrence, especially in the more malignant tumors. The term "anaplasia" is applied to these cells, signifying a loss of normal differentiation, of specific function, and of organization.

Anaplastic cells are, therefore, not embryonal cells, but a new type which has lost its place in the old organization.