Resolving thyroid lineage cell trajectories merging into a dual endocrine gland in mammals

The thyroid has a remarkable evolution, first appearing in invertebrate chordates as an integral exocrine constituent of the pharyngeal endostyle that is transformed into an endocrine gland during metamorphosis in basal vertebrates. In mammals, the thyroid acquires a second endocrine cell type, calcitonin-producing C-cells, which for long were inferred a neural crest origin, shuttled to the embryonic thyroid by the ultimobranchial bodies. However, recent lineage tracing experiments firmly establish these neuroendocrine cells also derive from foregut endoderm. Key questions remaining unanswered are how thyroid primordia independently develop and, unlike in all non-mammalian vertebrates, merge into a dual endocrine organ. Here, by leveraging a single-cell transcriptome atlas derived from mouse pharyngeal endoderm and its subsequent cell fates, we characterize the global gene expression profile of thyroid- and ultimobranchial-derived progenitor cells and identify comprehensive gene regulatory networks of lineage-specific transcription factors and novel target genes predicted to differentially regulate cell proliferation, plasticity and differentiation during development. Spatiotemporal analyses reveal C-cell precursors are triggered to undergo epithelial-mesenchymal transition (EMT) and cell-autonomously down-regulate collagen IV and degrade laminin that delineates the ultimobranchial body epithelium. However, the EMT program is not fully deployed until both cell lineages are mixed and propagate conjointly thus forming the typical thyroid histoarchitecture of follicles and parafollicular C-cells, every follicle/C-cell unit being enveloped by a renewed basement membrane. Mixed-type thyroid carcinoma recapitulates a synchronous lineage growth pattern but only the neuroendocrine tumor cells are able to escape the compound follicle boundaries and become invasive adopting a C-cell precursor-like migratory phenotype.