- Circumscribed tumor demarcated from the normal mammary tissue by a fibrous capsule.
- The loose stroma contains fibroblasts and collagen fibers. In the stroma inclusion of duct-like epithelial structures of various forms and sizes. In part, regular, open, round to oval ducts. Some ducts are compressed by proliferating stroma. These ducts appear on cross-sections as slits or irregular star-like structures.
- Usually, two cell layers can be distinguished within the ducts an inner epithelial and an outer myoepithelial layer. The latter faces the well-defined, intact basement membrane.
Phyllodes tumor: Typically, leaf-like slits and clefts are observed. The stroma exhibits an increased cellularity. In malignant variants, a high mitotic activity and anaplastic cells are observed in the stroma.