Bioprinting can be defined as additive biofabrication of three-dimensional (3D) tissues and organ constructs using tissue spheroids, capable of self-assembly, as building blocks. The thyroid gland, a relatively simple endocrine organ, is suitable for testing the proposed bioprinting technology. Here we report the bioprinting of a functional vascularized mouse thyroid gland construct from embryonic tissue spheroids as a proof of concept. Based on the self-assembly principle, we generated thyroid tissue starting from thyroid spheroids (TS) and allantoic spheroids (AS) as a source of thyrocytes and endothelial cells (EC), respectively. Inspired by mathematical modeling of spheroid fusion, we used an original 3D bioprinter to print TS in close association with AS within a collagen hydrogel. During the culture, closely placed embryonic tissue spheroids fused into a single integral construct, EC from AS invaded and vascularized TS, and epithelial cells from the TS progressively formed follicles. In this experimental setting, we observed formation of a capillary network around follicular cells, as observed during in utero thyroid development when thyroid epithelium controls the recruitment, invasion and expansion of EC around follicles. To prove that EC from AS are responsible for vascularization of the thyroid gland construct, we depleted endogenous EC from TS before bioprinting. EC from AS completely revascularized depleted thyroid tissue. The cultured bioprinted construct was functional as it could normalize blood thyroxine levels and body temperature after grafting under the kidney capsule of hypothyroid mice. Bioprinting of functional vascularized mouse thyroid gland construct represents a further advance in bioprinting technology, exploring the self-assembling properties of tissue spheroids.
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