In order to discuss the phenomenon of neoplasia, it is important that certain aspects of currently accepted nomenclature be understood. It should be noted, however, that there is no one system of nomenclature of neoplasms used worldwide; in this country, nomenclature has revolved around the use of the suffix -oma, which literally means “tumor.” With some exceptions, words with this suffix do refer to neoplasms. An exception is the term granuloma, which is a nonneo- plastic tumor of inflammatory tissue.
In the behavioristic classification, benign tumors may be named with a prefix that refers to the tissue from which the neoplasm arose and with the suffix -oma. For example, a benign neo- plasm of fibrous tissue is called a fibroma; a benign neoplasm of cartilage, a chondroma; a be- nign neoplasm of glandular tissue, an adenoma; and so on. When one considers the malignant tumors, however, some other aspects of classification apply. Malignant neoplasms are divided into two general categories, depending on their embryonic origin. Figure 2.5 outlines some of the steps in the early development of a fertilized egg of higher vertebrate animals. After fertiliza- tion, the egg divides a number of times, giving rise to 2, 4, 8, 16, 32 cells, and so on; these then form a small spherical structure, termed the blastula, with a central cavity. Continued develop- ment of this structure to the gastrula involves an invagination of the cells of one part of the surface, giving rise to a small “ball within a ball,” as diagrammed in a sectional view in Figure 2.5. At this stage it is possible for the embryologist to distinguish three different layers of cells. The outermost layer is termed the ectoderm and develops to give rise to the skin and its associ- ated structures in the adult. The layer of invaginated cells is termed the endoderm and ultimately gives rise to the gastrointestinal tract and its associated structures. Between these two layers, a mass of cells forms in the gastrula, termed the mesoderm, giving rise in the adult to supporting structures such as bone, fat, muscle, blood, and so on. As can be seen from the figure, a malig- nant neoplasm arising from derivatives of the mesodermal (mesenchymal) embryonic germ layer is termed a sarcoma. If the neoplasm arises from tissues derived from embryonic ectoderm or endoderm, the term carcinoma applies to malignant neoplasms of such tissues. Thus, the term adenocarcinoma—of the stomach, pancreas, or breast—is appropriate for malignant neoplasms of the glandular epithelium of these organs. On the other hand, a liposarcoma may arise from the fat tissue of the breast, a chondrosarcoma from cartilage of the ribs, or an osteogenic sarcoma from the bony rib itself.
Figure 2.5 Developmental biology of the early fertilized egg and embryo of the vertebrate. At the devel- opmental stage of gastrulation, the three germ layers (ectoderm, mesoderm, and endoderm) are clearly dif- ferentiated. Tissues derived from these different layers may give rise to malignant neoplasms, with the terminology based on their germ layer of derivation. See text for further details.
Several terms do not fit strictly into this type of nomenclature. The suffix -blastoma is used to denote certain types of neoplasms to indicate that the tissue has a primitive appearance that resembles embryonic structures. Examples of this situation are the neuroblastoma and the myoblastoma. In other examples the terminology is rather confusing. A highly malignant tumor that has the appearance of both a carcinoma and a sarcoma is termed a carcinosarcoma. This would indicate that the neoplasm was derived from two germ layers. Another condition, the “mixed” tumor of the salivary gland, which is definitely not a carcinosarcoma, was once thought to have this same embryonic derivation. It is now felt that the mixed tumor is probably a low- grade carcinoma, and the term carcinosarcoma should be reserved only for highly malignant, quite primitive tumors with the histological characteristics mentioned above.
The most common neoplasm of multiple-tissue origin is the teratoma, which is derived from all three germ layers. These neoplasms may be either benign or malignant in the behavior- istic sense.
Table 2.2 lists a number of neoplasms classified according to the behavioristic and histo- genetic methods. Although the rules of classification mentioned in this outline are reasonably inclusive, there are some exceptions in the designation of specific neoplasms, as can be seen from the table. For example, the melanoma is not a benign neoplasm but rather a highly malig- nant tumor of melanocytes. Although the term hepatoma suggests a benign neoplasm, it is ma- lignant in almost all instances. As one’s knowledge of experimental oncology increases, the specific exceptions to the rules become more familiar.