UBB and columnar cell hyperplasia of the breast: We review the embryology of the UBB and previous literature postulating that UBB (or other pharyngeal endodermal precursors) may be the cell of origin participating in neoplastic progression from UBB to CCH/MMC to MTC and may be the origin for some follicular epithelial and mixed tumors of follicular epithelial/medullary cells.