Light micrograph of a section of breast tissue showing an intracanalicular papilloma, a non-cancerous tumor of the breast. A canaliculus is a small duct (white) in the breast. The papilloma (purple, center) is a mass of connective tissue made up of the cells lining the duct. Duct papillomas are wart-like growths within the ducts of the milk producing glands. They are harmless but can turn malignant in rare cases. The most common symptom is a bloody discharge from the nipple.