mixed tumors

Digital case

 JRC:5375 : mixed tumor of the breast (in a dog)

Definition: In benign and in differentiated malignant neoplasms, the parenchymal cells bear a close resemblance to each other, as though all were derived from a single cell, as we know to be the case with cancers.

Infrequently, divergent differentiation of a single line of parenchymal cells into another tissue creates what are called mixed tumors. The best example of this is the mixed tumor of salivary gland origin.

These tumors contain epithelial components scattered within a myxoid stroma that sometimes contains islands of apparent cartilage or even bone.

All these elements, it is believed, arise from epithelial and myoepithelial cells of salivary gland origin; thus, the preferred designation of these neoplasms is pleomorphic adenoma.

The great majority of neoplasms, even mixed tumors, are composed of cells representative of a single germ layer. Teratomas, in contrast, are made up of a variety of parenchymal cell types representative of more than one germ layer, usually all three. They arise from totipotent cells and so are principally encountered in the gonads; they occur rarely in sequestered primitive cell rests elsewhere.

These totipotent cells differentiate along various germ lines, producing tissues that can be identified, for example, as skin, muscle, fat, gut epithelium, tooth structures-indeed, any tissue of the body.

A particularly common pattern is seen in the ovarian cystic teratoma (dermoid cyst), which differentiates principally along ectodermal lines to create a cystic tumor lined by skin replete with hair, sebaceous glands, and tooth structures.

Localization

 cutaneous mixed tumor
 salivary mixed tumor
 mammary mixed tumor