The thyroid gland is composed of spherical units called the follicles. Follicles are delineated by a monolayer of polarized thyrocytes surrounding the colloidal space, and resting on a basement membrane. Folliculogenesis occurs during embryonic development by reorganization of an unstructured mass of non-polarized cells, but regulatory mechanisms remain largely unknown. Using thyroid-specific VegfaKO and Smad1/5dKO, we show that the absence of endothelial cells or of epithelial BMP signaling prevent follicle formation. Furthermore, these KO thyroids displayed impaired basement membrane assembly. Interestingly, medium conditioned by embryonic endothelial progenitor cells (eEPC) rescues the folliculogenic defects of both KO thyroids. Laminin α1β1γ1, abundantly released by eEPC into the culture medium, is critically required for folliculogenesis. These results indicate that endothelial cells and epithelial BMP signaling promote folliculogenesis via assembly of the basement membrane.(BIFA - Sciences biomédicales et pharmaceutiques) -- UCL, 201
