An ultrastructural study of normal human thymic epithelium in primary cultures

The ultrastructural study of a large series of samples of normal human thymic epithelial cells (TECs) in primary cultures is here presented. Cultured TECs are heterogeneous due to marked differences in their cytoplasmic organization. In particular, a spectrum of ultrastructural features have been observed, ranging from poorly-differentiated cells to secretory and epidermoid-like ones, as well as intermediate transitional forms and cells co-expressing both epidermoid-like and secretory features. Cells with peculiar secretory (neuroendocrine-like) characteristics have also been identified in the culture obtained from one of the donors. These data suggest the existence of a thymic epithelial cell type which may act as a dynamic unit, changing its cytoplasmic organization in response to microenvironmental signals. The types and interrelationships of cytoplasmic organelles observed in the secretory cells suggest that the production of thymic peptides involves the classical pathway of protein synthesis and secretion.