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Microhemorrhage in the hyperplastic thyroid gland of the rat.

A microhemorrhagic process was consistently observed in association with the induction of thyroid hyperplasia by dietary thiouracil in the rat. This process appeared to involve the extravasation of erythrocytes (RBCs) through hyperplastic capillary walls. Those sites of extravasation which were directly visualized involved endothelial openings of less than 1 mu. These openings were surrounded by endothelial cytoplasm containing a dense fibrous material and were associated with RBC constriction during passage. Extravasated RBCs were most often noted singly or in small groups, either in columns between follicular epithelial cells or embedded amongst the basal epithelial infoldings. Occasionally, extravasated RBCs were also observed within follicular lumens. Extravasated RBCs were usually intact ultrastructurally, but occasionally an apparent hemolytic process was observed, both for RBCs embedded amongst epithelial cells and for those within follicular lumens. The nature and etiology of this microhemorrhagic process are considered in relation to the hypervascularity of the gland, the possibility of capillary wall alterations, the presence of endothelial cell mitoses, and the localization of the process.