Myoid cell differentiation in true thymic hyperplasia and lymphoid hyperplasia.

Four cases of thymic hyperplasia (TH) and/or lymphoid hyperplasia (LH) were investigated immunohistochemically for evidence of myoid cell (MC) and epithelial cell (EC) differentiation using antibodies to myoglobin, desmin, and cytokeratin. Although MCs were identified in all the cases, the most impressive perifollicular proliferation of ECs and MCs was seen in the hyperthyroid patient with TH/LH, suggesting that these cells have a role in immune function. The unusual combination of LH and TH in this patient demonstrates that exceptional cases can show overlapping features of what are otherwise considered to be separate thymic lesions. The observations that MCs occur in close proximity to ECs and ECs are depleted along with MCs in myasthenia gravis provide strong evidence supporting closely related histogeneses for ECs and MCs within the thymus.