Unlike breast cancers called carcinomas, which develop inside the ducts (milk-carrying tubes) or lobules (milk-producing glands) of the breast, phyllodes tumors start outside of the ducts and lobules. Phyllodes tumors develop in the breast's connective tissue, called the stroma. The stroma includes the fatty tissue and ligaments that surround the ducts, lobules, and blood and lymph vessels in the breast. It may be helpful to think of the stroma as the tissue that "holds everything together" inside the breast. In addition to stromal cells, phyllodes tumors can also contain cells from the ducts and lobules.