Large clear cell adenoma of the parathyroid in a patient with MEN-1 syndrome. Ultrastructural study of the tumour exhibiting unusual RER formations.

A 48 year old man presented with hyperparathyroidism and acromegaly. Although pancreatic tumour was not demonstrated, the diagnosis of MEN-1 syndrome was made. The parathyroid adenoma was removed surgically and investigated by histology and transmission electron microscopy and was diagnosed as large clear cell adenoma with extensive deposition of glycogen. Much of the endoplasmic reticulum was present in two special forms: annulate lamellae and the rare rail-like configurations both of which showed continuity with the conventional RER as well as with each other. No obvious correlation was apparent between presence of special RER formations and growth rate or endocrine activity of tumour; they may be regarded as signs of cellular dedifferentiation.