PRESERVATION OF FUNCTIONING HUMAN THYROID ORGANOIDS IN THE SEVERE COMBINED IMMUNODEFICIENT MOUSE .3. THYROTROPIN INDEPENDENCE OF THYROID-FOLLICLE FORMATION

The severe combined immunodeficient (scid) mouse allows the in vivo re constitution of thyroid follicles from thyroid monolayer cells when tr ansplanted sc within an extracellular basement membrane matrix. After 2-3 weeks, these human thyroid organoids show an active microfollicula r histology and secrete human thyroglobulin into the murine serum in r esponse to the administration of recombinant human TSH. Furthermore, s uch organoids survive for more than 3 months in this functional state. To assess whether thyroid follicular reconstitution was TSH dependent , we examined organoid follicular reconstruction in TB-induced hyperth yroid scid mice, in which endogenous murine TSH was presumed to be tot ally suppressed. By providing water with 12 mu g/ml T-3, we increased the murine serum T-3 levels from a mean of 1.9 nmol/liter in controls to greater than 12.0 nmol/liter. After 3 weeks, thyroid cells derived from normal human thyroid monolayers were suspended in an extracellula r basement membrane matrix, and the suspension was transplanted sc int o scid mice (with or without T-3-induced hyperthyroidism) and allowed to reconstitute. Histological examination 4 weeks later showed a simil ar degree of thyroid follicle formation in mice treated with or withou t T-3, indicating that the hyperthyroid state had caused no interferen ce with thyroid follicle reconstitution. This was further confirmed by transmission electron microscopy, which demonstrated normal human thy roid follicle cell polarity in the organoids of both euthyroid and hyp erthyroid mice. In addition, using an extracellular basement membrane preparation with reduced growth factor (epidermal growth factor, insul inlike growth factor-I, and platelet-derived growth factor) levels als o allowed normal thyroid follicle formation in T-3-fed mice. These dat a demonstrate that in vivo thyroid follicle formation is TSH independe nt and that extrathyroidal epidermal growth factor, insulin-like growt h factor-I, and platelet-derived growth factor may be relatively unimp ortant. The factors and molecular mechanisms leading to follicular reo rganization of adult human thyroid cells remain to be determined, but are likely to depend largely on intrathyroidal growth factor secretion and cell-cell interaction.