The fine structure of myoid cells in the human thymus.

Abstract

In human thymuses obtained from thoracic surgery myoid cells were studied by light and electron microscopy. The myoid cells were found, though not abundantly, in 19 out of 21 thymuses examined. They should thus probably be considered as common elements in the human thymus. In light microscopy the myoid cells appeared as a large, round or somewhat elongate cells with markedly eosinophilic cytoplasm and a vesicular, irregularly-shaped nucleus. The cells were usually found singly in the medulla, especially near the cortico-medullary boundary. Electron microscopy revealed that the myoid cells were characterized by the presence of myofibrils and triad-like membranous structures. These characteristic cytoplasmic components were the same in ultrastructural features as those known in striated muscle cells, although they were less developed and far less regularly disposed in the myoid cells than in muscle cells. In other cytological features, the myoid cells were similar to the usual epithelial reticular cells. In addition, small poorly-developed desmosomes were occasionally encountered between the myoid cells and adjacent epithelial reticular cells. From these findings the myoid cells may be considered a form of epithelial reticular cell.