Morphogenetic movement of the thyroid primordium in the rat: a scanning and transmission electron microscopic study.

This study deals with the fine structural and three dimensional aspects of the early morphogenetic movements of the thyroid primordium in rat embryos. By scanning electron microscopy, the cells of the presumptive thyroid area show peculiar cauliflower-like structures at day 10 of gestation prior to the starting of its invagination. These structures increase in number as the invagination proceeds. Each cauliflower-like structure consists of a large apical cytoplasmic process with many microvilli and a few small bleb-like protrusions. This phenomenon is then followed by the decrease and ultimate disappearance of the microvilli and bleb-like protrusions, while many large spherical bodies, possibly derived from the bleb-like protrusions and cell debris, appear on the epithelium of the thyroid primordium, especially around the marginal zone of the primordial invagination which soon closes. These observations suggest that the gross morphogenetic movement of the thyroid primordium is due partly to the changes of cell shapes in this area. Transmission electron microscopy reveals conspicuous bundles of microfilaments about 6 nm in diameter located in the apical cytoplasm and converging on well-developed junctional complexes during the invagination process. The appearance of the cytoplasmic processes and bleb-like protrusions appearing in the cells of this region is believed to indicate a role in the invagination of the thyroid primordium and the closure of the starting point of the invagination.