Hypertrophy (Gr. over, and nourishment), an excess of growth of a part without degeneration or alteration in the structure; the exact opposite to atrophy. Hypertrophy may depend on the excess of the materials of certain tissues in the blood; when this fluid contains habitually too much fat, there may be an abnormal increase of the adipose tissue; similar hypertrophy may thus be induced in other tissues, but there is no evidence that the muscles or nerves increase in bulk from the mere excess of their formative materials. Though an increased supply of blood is generally rather the consequence than the cause of excessive nutrition in a part, hypertrophy may arise from a mere increased circulation, and when one kidney cannot perform its functions, the other has been known to increase in size, owing to its increased activity as an excreting organ. This must be distinguished from the augmented bulk of long congested parts, in which there is not normal hypertrophy, but an addition of altered and inferior tissue. Hypertrophy is in most cases dependent on a preternatural formative capacity in the part, sometimes congenital (as in the abnormal growths of fingers and toes, and even entire limbs), but generally acquired.
The most striking instances of acquired nutritive activity are seen in the muscular system, consequent upon the excessive exercise of its functional powers. Muscular hypertrophy is most often seen in the involuntary muscles, whose action is in some way impeded; thus stricture of the urethra or stone in the bladder, obstructing the exit of the urine and calling for extra exertion to expel it, causes hypertrophy of the muscular coat of the bladder; so it happens with the gall bladder when its ducts are stopped by calculi, and with the intestines when a stricture exists in any portion. Hypertrophy of the ventricles of the heart is often dependent on narrowing of the cardiac orifices by disease of the valves, giving the organ double work to do, and increasing its activity, as in other muscles. (See Heart, Diseases of the.) When any of the voluntary muscles are specially exercised, hypertrophy is observed in them, as in the arm of the blacksmith or the legs of a professional dancer; and such hyper-trophied muscles generally cause an increased nutrition of the bones to which they are attached, and an enlargement of the points of origin and insertion.
There are certain enlargements of glands, in which their proper tissue is increased without structural change, which unite physiological hypertrophy with pathological tumors, as in the case of the mammary, thyroid, and prostate glands. Certain tumors of the uterus contain only an excess of the normal muscular and fibrous tissues of the organ, and yet cannot be regarded as examples of hypertrophy, as they observe no regular growth, subserve no physiological purpose, and constitute a positive deformity and disease; such abnormal growths may exist upon a uterus itself hypertrophied from increased functional activity, and must not be confounded with the latter. Supernumerary parts, as additional fingers and toes and various outgrowths developed during foetal life, must in like manner be referred to local hypertrophy from excess of formative activity. Dr. Carpenter sees in this whole series of abnormal production the operation of a similar power; that which in simple hypertrophy is confined to increasing the size of an organ by the development of new tissue according to the morphological type of the part, in the formation of supernumerary tissues also imparts to them an independent existence; on the other hand, while in ordinary hypertrophy the tissues in excess are incorporated in the affected organ, in the structure of a tumor the perfectly formed and independently growing tissues constitute a mass whose shape is determined more by surrounding conditions than by any tendency of their own - the formative power undirected by the normal morphological nisus.
In malignant growths, the development of tissues stops short of the limit by which formative power produces the normal tissues, and their vital endowments are not sufficient to resist the tendency to degeneration.