Tumors and Specialized Cell Types

There are 4 major categories of tumors. The first category and most common tumors are epithelial in origin. The remainder of tumors categories arise from non-epithelial tissues throughout the body. A second category is mesenchymal, which arises from connective tissue.  A third category is blood forming (hematopoietic) tissues and cell s of the immune system. The fourth category is small consisting of tumors arising from cells that form various components of the central and peripheral nervous system, and the reproductive system.

Epithelial Tumors

Epithelial cells are sheets of cells that line walls of cavities, channels, and serve as a protective covering the most notable is skin. This happens to be the largest organ in a patient’s body. Many epithelia also contain specialized cells that secrete substances into the ducts or cavities they line. Benign tumors of epithelial origin are termed adenoma, papilloma, or epithelioma. Malignant tumors of epithelial origin are termed carcinoma and adenocarcinoma if they form glands or ducts. Below is a table that names the common epithelial tumor types in Veterinary Medicine.

Hematopoietic and Lymphoreticular Tumors

Blood forming and immune cells are derived from a number of different hematopoietic tissues. These cells include erythrocytes that form red blood cells, plasma cells that secrete antibodies used by the immune system to bind antigens, and lymphocytes (B and T) used in the fight against infection by the immune system. Neutrophils, basophils, and eosinophils also known as white blood cells because of their lack of pigment, and are used by the immune system. Leukemia refers to the abnormal proliferation of cells in the blood or bone marrow originating from any of the above cell lines. Lymphoma refers to the malignant disease of lymphoid lineages B and T lymphocytes. Below is a table that names the common hematopoietic and lymphoreticular tumor types in Veterinary Medicine.

Nervous System and Reproductive System Tumors

The last categories of tumors arise from cells that form various components of the central and peripheral nervous system along with tumors arising from cells of the reproductive system. These cells line nerves, function as part of the nervous and reproductive systems. Below is a table that names the other tumor types that include tumors of the central/peripheral nervous system and reproductive system in Veterinary Medicine.

Tumor Names that "Break the Rules"

In some cases the suffix "oma" is used when the tumor is malignant such as malignant melanoma and lymphoma. In other cases a suffix used to indicate a malignancy isn't used such a leukemia which is a malignant neoplasia of white blood cells in the hematopoietic tissues and usually in the blood.

Benign versus Malignant

In most cases, a pathologist makes a diagnosis based on histological features what they see under the microscope that distinguish malignant from benign neoplasia. There are a number of tumors that can be definitively diagnosed on histological features if the tumor is sampled correctly. These tumors include osteosarcomas, mast cell tumors, some soft tissue sarcomas, and squamous cell carcinomas.  Additional features of malignancy include destruction of normal tissue and its architecture, lymph node involvement, and widespread metastasis. Below is table that lists the histological features of benign and malignant neoplasia.

Tumor Histology You Can’t Trust

In some tumors, histologic features do not correlate with the tumor behavior. Canine histiocytoma and plasmacytoma both have features of malignancy but are clinically benign. Certain types of canine fibrosarcomas are histologically low grade, but biologically high grade so have benign features under the microscope, but tend to spread. In these few cases histologic behavior cannot be trusted and clinical history and tumor behavior distinguishes benign from malignant.