Benign follicular thyroid nodule composed of signet-ring-like cells with PAS-negative thyroglobulin accumulation in dilatated rough endoplasmic reticulums.

A benign follicular thyroid nodule composed of signet-ring-like cells in a 46-year-old Japanese euthyroid woman is reported. Thyroglobulin was positive, and periodic acid-Schiff (PAS) and other mucin stainings were negative in the cytoplasm of signet-ring-like cells. Ultrastructurally their cytoplasm contained many dilatated rough endoplasmic reticulums (rERs), the largest of which compressed the nucleus toward the cellular apical portion. Functional disturbance of rER or Golgi apparatus might be the cause of this characteristic cellular change.