Abstract
Since the 1980s, cancer cells have been considered to be generated from well-differentiated benign cells by transformation caused by accumulating damage in their genomes. However, recent progress in gene expression analysis in thyroid malignancies has raised the possibility of another model of thyroid carcinogenesis. We propose a novel hypothesis of thyroid carcinogenesis, the fetal cell carcinogenesis hypothesis, in which cancer cells are derived from the remnants of fetal thyroid cells, instead of from normal thyroid follicular cells. This hypothesis explains well the clinical and biological features and recent molecular evidence of thyroid carcinoma. It suggests the importance of clarifying the molecular mechanism of thyroid development and the identification of fetal thyroid cells such as thyroid stem cells (TSCs), since such data will lead to a better understanding of thyroid carcinogenesis and thyroid regeneration.
