Ultrastructural features of the differentiating thyroid primordium in the sand lizard (Lacerta agilis L.) from the differentiation of the cellular cords to the formation of the follicular lumen

• The ultrastructural features of the developing thyroid primordium in Lacerta agilis are described.
• The cellular cords at the periphery of the thyroid anlage differentiate earlier than those in the central part.
• The follicular lumens at the periphery differentiate before the division of the cellular cords.
• The follicular lumens in the central part differentiate after the division of the cellular cords into presumptive follicles.
• The lumen of the follicles is formed by cavitation.

The differentiation of the thyroid primordium of lacertilian species is poorly understood. The present study reports on the ultrastructural analysis of the developing thyroid primordium in the sand lizard (Lacerta agilis) during the early stages of differentiation. The early thyroid primordium of sand lizard embryos was composed of cellular cords that contained single cells with a giant lipid droplet, which were eliminated by specific autophagy (lipophagy). The follicular lumens at the periphery of the primordium differentiated even before the division of the cellular cords. When the single cells within the cords started to die through paraptosis, the adjacent cells started to polarise and junctional complexes began to form around them. After polarisation and clearing up after the formation of the lumens, the cellular cords divided into definitive follicles. The cellular cords in the central part of the primordium started to differentiate later than those at the periphery. The cellular cords divided into presumptive follicles first and only later differentiated into definitive follicles. During this process, a population of centrally located cells was removed through apoptosis to form the lumen. Although the follicular lumen in sand lizard embryos is differentiated by cavitation similar to that in the grass snake, there were very important differences during the early stages of the differentiation of the cellular cords and the formation of the thyroid follicles.