We report here four additional cases of hyalinizing trabecular adenoma of t
he thyroid and describe the findings of the accumulated basement membrane (
BM) material. Focal lumpy depositions of BM material were noted in the stro
ma of all four tumors and were particularly prominent in two. Antibodies ag
ainst type TV collagen and laminin strongly immunoreacted with this materia
l, which also showed diastase-resistant periodic acid-Schiff positivity. Th
e ultrastructural findings of accumulated basement membrane was examined in
detail. The most striking ultrastructural findings were seen at the periph
ery of the cell clusters, where the basal aspect of the tumor cells rested
on a highly accumulated BM materials displaying mesh-like, fir-branching, o
r frame-like appearances. Intracytoplasmic BM islands were frequently obser
ved in neoplastic cells. These BM islands were membrane-bounded and focally
continuous with the cell surface membrane, suggesting pseudointracytoplasm
ic depositions of BM material due to marked cytoplasmic infoldings. From th
e findings, electron microscopic features of accumulated BM material are ve
ry characteristic in hyalinizing trabecular adenomas and can be discernible
from other histologic types of BM-producing tumors.