The term hypertrophy means overgrowth or excessive growth. Looking to the cellular constituents of the tissues, Virchow has drawn a distinction between an increase of tissue due to an enlargement of the cells and that due to a numerical increase, applying to the latter the term Hyperplasia. This distinction, however, cannot be carried out, as in many cases both substantial and numerical increase may be present. The term hyperplasia may, however, be used where it is intended to convey the meaning that cell-division or proliferation is present.

Normal growth, as seen in the tissues during the period of adolescence, is determined by impulses inherent in the impregnated ovum. Already, at this early period, the sex and details of structure are implicitly inherent in the embryo, which enters on its career of development with a predetermined plan. The tissues cease to grow at the period of maturity not because their powers of new-formation have been exhausted, but because this plan has been fulfilled. Hypertrophy may occur because of some error in the embryonic arrangements, or it may be due to some stimulus acting on the tissues after birth. In the latter case the effect will be greater when the stimulus is applied during the period of normal growth than after the state of maturity has been reached. In all cases the new-formed tissue is in structure and function essentially similar to the normal tissue of its kind, and forms an addition to the existing active tissue of that kind.