Patterns of cellular proliferation during thyroid organogenesis.

The changes in rate and location of cellular proliferation were analyzed to determine if localized areas of cell division were influencing shape changes in the chick thyroid. Pulse labeling with tritiated thymidine indicates that the gland's labeling index declines throughout its development. Initially, the thyroid placode has a lower labeling index than the neighboring pharyngeal epithelium. An evaluation of the positions of pulse-labeled cells reveals that the evaginating thyroid grows by annexing cells from the pharyngeal epithelium. The older evaginated regions of the gland exhibit the lowest labeling indices. The newly acquired regions still maintain higher labeling indices.