Mitotic rate is high within the neoplastic epithelial cells, with atypical figures being frequent (arrowheads). (x100)

Mammary adenocarcinomas typically originate within ductular epithelium. This epithelium forms nodules (arrowhead) of tissue that protrude into the luminal space. (x200)

In this part of the tumour, irregular neoplastic glandular acini have invaded into surrounding fatty tissue (arrowheads). The neoplastic glands are surrounded by a oedematous stroma and excessive fibrous tissue. (x 40)

Malignant tumour cells invading lymphatic vessels (arrowheads) (x 100)

Mammary adenocarcinoma:

These tumours are typically locally invasive and evoke a prominent schirrous reaction, and a variable inflammatory (primarily lymphocytic) response.
Tumours arise from ductular epithelium and when they metastasise, they typically invade lymphatics and spread to local lymph nodes and to the lungs. Tumours with large amounts of cartilage and bone formation, produced by myoepithelial cells are more likely to be benign. Mammary carcinomas in cats are typically malignant, in dogs malignant and benign tumours occur in equal proportion.