Diffuse and localized hypertrophy of the breast occurs relatively frequently in both the male and the female. In gynecomastia the hypertrophy may be unilateral or bilateral, but it is usually diffuse. The unilateral type is the more common. The localized change (the so-called fibro-adenoma) is the most common form in the female breast. Diffuse hypertrophy of one or both breasts, known as virginal hypertrophy, is most frequently bilateral. Gynecomastia, fibro-adenoma, and virginal hypertrophy are in almost all instances benign. In extremely rare instances a fibrosarcoma is supposed to have developed in or from a preexisting clinically benign fibro-adenoma.
Histologically these benign lesions are characterized by an increase in the length and diameter of the ducts, an increase in the number of layers of epithelium lining the ducts, and a proliferation of the periductal connective tissue. The histologic picture is essentially an exaggeration of that observed in the normal female breast
Dean Lewis, Charles F. Geschickter. ESTRIN IN HIGH CONCENTRATION YIELDED BY A FIBRO-ADENOMA OF THE BREAST. JAMA. 1934;103(16):1212–1213. doi:10.1001/jama.1934.72750420001007