Causes The occurrence and development of the disease are closely related to the endocrine status of the ovaries. It is well documented that when ovarian endocrine disorders occur, excessive secretion of estrogen and a relative decrease in progesterone not only stimulate parenchymal hyperplasia, but also cause irregular hyperplasia of the terminal duct epithelium, resulting in ductal dilatation and cyst formation, as well as excessive proliferation and collagenization of mesenchymal connective tissue and lymphocytic infiltration due to the loss of the inhibitory effect of progesterone on estrogen. Pathogenesis The pathological features of simple mammary epithelial hyperplasia are: 1. Gross morphology. The lesion area of mammary hyperplasia is tough in texture, without envelope, and poorly defined from normal tissue, with a small grayish-white granular appearance on the cut surface. 2. Histological morphology. Microscopically, the terminal milk ducts and alveolar epithelium are seen to proliferate and detach, making the milk ducts swollen and distended; causing the ducts to dilate and form small cysts; fibrous tissue proliferation in the lobules of the breast, with interlobular fusion; and lymphocytic infiltration in the interlobular stroma.