Massive thymic hyperplasia with myoid cell differentiation.

A 12-year-old boy with massive true thymic hyperplasia presented with respiratory distress and dysphagia. The thymus weighed 245 g and demonstrated normal cortical and medullary components histologically. The findings in this case were compared with the clinical and pathologic features of seven previously published cases of massive hyperplasia and with cases of mild or "borderline" hyperplasia. By electron microscopy and immunoperoxidase techniques, myoid cell differentiation was demonstrated, the first documented example of myoid cells in thymic hyperplasia. These findings support the hypothesis that myoid cells are a normal component of thymic parenchyma.