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A malignant mammary tumor is a tumor originating from the mammary gland tissues that has been classified as malignant (invasive and capable of spreading). Although some of these cancers are cured by surgical removal, others recur and some spread to other parts of the body (metastasis). A dog that has a tumor of this type often has or will develop further tumors in other mammary glands.

Mast cells originate in the bone marrow but complete their development in peripheral tissues. They are found in all tissues of the body but are concentrated in the skin, respiratory tract and digestive tract. Mast cells interact with cells of the immune (protective) system producing allergic type antibodies (IgE), presenting foreign molecules (antigens) to immune sytem cells and recruiting certain cells (phagocytes) to engulf foreign or invading material. As well as being a cellular barrier to external agents, mast cells have a regulatory function on cutaneous nerves, blood circulation, fibrous tissue and other immune cells. They are therefore important in allergic responses, tissue remodelling, wound healing and non-allergic skin diseases. Mast cells in hair follicles also help to regulate the cyclical activity of those follicles.

Mast cells originate in the bone marrow but only mature in peripheral tissues. They are found in all tissues of the body but most are near surfaces in contact with the external environment such as the skin, respiratory and digestive tracts. Mast cells produce many chemicals with differing effects on the body (histamine, proteoglycans, neutral proteases and chemotactic growth factors). These chemicals are present in granules in the cytoplasm of mast cells. They are discharged from the cells in response to various stimuli, and they induce inflammatory reactions. Mast cells also interact with cells of the immune system producing antibodies involved with allergic reactions (immunoglobulin E, IgE), presenting foreign molecules (antigens) and recruiting cells (phagocytes) to engulf foreign material. As well as being a cellular barrier to external agents, they have a regulatory function on nerves, blood circulation, fibrous tissue and other immune cells.

Melanocytes are cells that produce a pigment called melanin. They are found in many parts of the body where there is pigment, particularly skin, hair and eyes.

Squamous cell carcinoma in situ is a disordered growth of the skin epidermis that may extend to include epithelium of the upper part of the hair follicles.