Honeycomb structure of the apical surface of thyroid epithelium during involution of the hyperplastic thyroid gland.

A heretofore undescribed feature consisting of closely packed pits 0.5-1 micron in diameter was observed on the apical surface of thyroid epithelial cells. The walls of the pits were generally smooth, except at the base where there was a high incidence of irregularities looking like sites of recent fusion of apical vesicles with the pits. The matrix of the partition between pits was similar to the matrix of pseudopods in these cells in being free of membrane-bounded organelles and containing a low concentration of ribosomes. The pits were observed early in the involution of the hyperplastic gland, most prominently between 14 h and 8 days of involution. This is a time when thyroglobulin is accumulating in follicular lumens and the apical end of the epithelial cells usually contains a high concentration of apical vesicles ordinarily considered to be exocytic in character. It is important to recognize the existence of this feature to avoid confusing profiles of it with structures involved in macropinocytosis.