While the epithelia of the body are derived from all three germ cell layers, malignant neoplasms of epithelial cells are called carcinomas regardless of the tissue of origin. Thus, malignant neoplasms arising in the renal tubular epithelium (mesoderm), the skin (ectoderm), and lining epithelium of the gut (endoderm) are all considered carcinomas. Furthermore, mesoderm may give rise to carcinomas (epithelial), sarcomas (mesenchymal), and hematolymphoid tumors (leukemias and lymphomas).
Carcinomas are subdivided further. Carcinomas that grow in a glandular pattern are called adenocarcinomas, and those that produce squamous cells are called squamous cell carcinomas. Sometimes the tissue or organ of origin can be identified, as in the designation of renal cell adenocarcinoma, but it is not uncommon for tumors http://ebooksmedicine.net
Fig. 6.1 Colonicpolyp.Thisglandulartumorisseenprojectingintothecoloniclumen.Thepolypisattachedtothemucosabyadistinctstalk.
to show little or no differentiation. Such tumors are referred to as poorly differentiated or undifferentiated carcinoma.
The transformed cells in a neoplasm, whether benign or malignant, usually resemble each other, consistent with their origin from a single transformed progenitor cell. In some unusual instances, however, the tumor cells undergo divergent differentiation, creating so-called “mixed tumors”. Mixed tumors are still of monoclonal origin, but the progenitor cell in such tumors has the capacity to differentiate down more than one lineage. The best example is mixed tumor of salivary gland. These tumors have obvious epithelial components dispersed throughout a fibromyxoid stroma, sometimes harboring islands of cartilage or bone (
Fig. 6.2 ). All of these diverse elements are thought to derive from a single transformed epithelial progenitor cell, and the preferred designation for these neoplasms is pleomorphic adenoma. Fibroadenoma of the female breast is another common mixed tumor. This benign tumor contains a mixture of proliferating ductal elements (adenoma) embedded in loose fibrous tissue (fibroma). Unlike pleomorphic adenoma, only the fibrous component is neoplastic, but the term fibroadenoma remains in common usage.
Teratoma is a special type of mixed tumor that contains recognizable mature or immature cells or tissues derived from more than one germ cell layer, and sometimes all three. Teratomas originate from totipotential germ cells such as those that normally reside in the ovary and testis and that are sometimes abnormally present in midline embryonic rests. Germ cells have the capacity to differentiate into any of the cell types found in the adult body; not surprisingly, therefore, they may give rise to neoplasms that contain elements resembling bone, epithelium, muscle, fat, nerve, and other tissues, all thrown together in a helterskelter fashion.
The specific names of the more common neoplasms are presented in
Table 6.1 . Some glaring inconsistencies may be noted. For example, the terms lymphoma, mesothelioma, melanoma, and seminoma are used for malignant neoplasms. Unfortunately for students, these exceptions are firmly entrenched in medical terminology.
There also are other instances of confusing terminology:
Hamartoma is a mass of disorganized tissue indigenous to the particular site, such as the lung or the liver. While traditionally considered developmental malformations, many hamartomas have clonal chromosomal aberrations that are acquired through somatic mutations and on this basis are now considered to be neoplastic.