Figure 2. Schematic illustration of zebrafish thyroid development (modified from 7).

The thyroid primordium rises from the pharyngeal endoderm around 24 hours post-fertilization (hpf) and expresses the early thyroid genes nkx2.4, pax2a, pax8, hhex, and foxe1. The differentiation of thyroid precursors continues up to 48hpf, at which they begin to express the late thyroid genes tg and slc5a5. The development of the zebrafish thyroid gland is complete at 72hpf. From this time, the thyroid follicles proliferate and localize along the dorsal aorta and start to produce T4. Intrinsic and extrinsic factors (Jag1-Notch, bone morphogenetic protein [BMP], and fibroblast growth factor [FGF] signaling) are also reported to play a pivotal role during thyroid organogenesis. For a detailed description of the molecular mechanisms underlying zebrafish thyroid morphogenesis, we refer the reader to the review by Fagman and Nilsson ¹¹.