Preservation of functioning human thyroid "organoids" in the severe combined immunodeficient mouse. III. Thyrotropin independence of thyroid follicle formation.

The severe combined immunodeficient (scid) mouse allows the in vivo reconstitution of thyroid follicles from thyroid monolayer cells when transplanted sc within an extracellular basement membrane matrix. After 2-3 weeks, these human thyroid organoids show an active microfollicular histology and secrete human thyroglobulin into the murine serum in response to the administration of recombinant human TSH. Furthermore, such 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 T3-induced hyperthyroid scid mice, in which endogenous murine TSH was presumed to be totally suppressed. By providing water with 12 micrograms/ml T3, we increased the murine serum T3 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 extracellular basement membrane matrix, and the suspension was transplanted sc into scid mice (with or without T3-induced hyperthyroidism) and allowed to reconstitute. Histological examination 4 weeks later showed a similar degree of thyroid follicle formation in mice treated with or without T3, indicating that the hyperthyroid state had caused no interference with thyroid follicle reconstitution. This was further confirmed by transmission electron microscopy, which demonstrated normal human thyroid follicle cell polarity in the organoids of both euthyroid and hyperthyroid mice. In addition, using an extracellular basement membrane preparation with reduced growth factor (epidermal growth factor, insulin-like growth factor-I, and platelet-derived growth factor) levels also allowed normal thyroid follicle formation in T3-fed mice. These data demonstrate that in vivo thyroid follicle formation is TSH independent and that extrathyroidal epidermal growth factor, insulin-like growth factor-I, and platelet-derived growth factor may be relatively unimportant. The factors and molecular mechanisms leading to follicular reorganization of adult human thyroid cells remain to be determined, but are likely to depend largely on intrathyroidal growth factor secretion and cell-cell interaction.