[Comparative ultrastructural characteristics of epithelial cells of the parenchyma of the thymus gland and thymoma].

Comparative ultrastructural investigation of the parenchyma was performed for human thymus (7 specimens) and thymoma (34 specimens). For thymic parenchyma eight types of epithelial cells were identified. These may be represented by three different histogenetic lines: 1) spindle-shaped appendiceal cortical cells (type 1-4) 2) medullary cells showing signs of squamous-epithelium differentiation (type 5-6) 3) medullary cells with signs of glandular differentiation (type 7-8). Thymomas demonstrated cells ultrastructurally similar to thymic parenchyma and exhibiting features of either cortical or medullary epithelial cells. These findings served the basis for dividing thymomas into 3 major groups: 1) with cortical cell differentiation 2) with cortical and medullary cell differentiation 3) with medullary cell differentiation.