Fig. 1

Confocal microscopy images of thyroid budding, migration, and expansion from 48 to 72 hpf with 3D simulated images. All embryos shown are oriented anterior to the top in the ventral views. (A, B) at 48 h postfertilization (hpf). The thyroid primordium is located near the OFT, is surrounded by aortic arch artery 1 (aa1), and possesses a globular appearance (A). 3D simulated images reveal that the thyroid is sphere‐shaped with a small number of small protrusions (B). (C‐D) From ~ 55 hpf, mature thyroid follicles begin to appear, and the morphology of the thyroid gland undergoes severe transformation with numerous protrusions extending in all directions, thus suggesting that the thyroid has begun to migrate. The morphology was easily recognized using the simulated 3D image. In the early development of the thyroid, an increased number of long protrusions can be observed more clearly in 3D images than they can in images produced using confocal microscopy stacking imaging techniques (B, D). (E‐F) Thyroid expansion and proliferation begin at 72 hpf along with the extension of the HAs. The thyroid extends along the HAs, and the shape of the thyroid becomes an inverted ‘Y’ when viewed in the ‘head‐up’ position (E). Three‐dimensional microscopy and image fusion reconstructions were conducted to visualize the thyroid volume (F). Magnified views of the yellow box are presented below the corresponding images. * aa1: aortic arch artery 1; ** as: aortic sac. Arrowhead: protrusions of the thyroid in 3D view. Scale bar: 50 μm.