Gynaecomastia. Light micrograph of a section through male breast tissue showing gynaecomastia. The male breast is normally rudimentary and inactive, however changes in the level of female (oestrogen) hormones may result in mammary tissue cell proliferation (hyperplasia), resulting in breast formation. The usually simple mammary ducts (dark loops) become enlarged, often with thickening of the epithelium and an increase in periductal fibrous tissue. Many conditions can cause this, such as liver cirrhosis (where the liver cannot break down the natural oestrogen present), tumours, and some medications.