Cellular dynamics during evagination of the thyroid primordium in the chick embryo.

The thyroid forms as an outpouching of the ventral pharynx. Evidence supports the conclusion that formation of the thyroid pit is mediated by changes in the cytoskeleton that cause constriction of cell apices. However, it seems unlikely that a relatively flat epithelial sheet can be converted into a pit without either distortions of the surface or considerable rearrangement of cells to reduce surface area. Possible cellular rearrangements were investigated by tracing the movements of individual cells by using time-lapse video microscopy. Changes in shape of the primordium were investigated by marking with carbon and DiI and by scanning electron microscopy. Cell movements occurred only over short distances, mostly shifts relative to a neighbor, especially at the edge of the pit. Instead, cells rearranged into clusters that piled up at the edge of the pit and then tilted inside. Adjacent rings of pharyngeal cells were annexed by the growing thyroid, undergoing rearrangement into clusters, piling up at the edge, and moving inside the pit. The consequence was the formation of a series of shelf-like extensions within the cavity, representing successive generations of cell rings moving inside. These results have implications for the formation of other organs by evagination.