Culture Models for Studying Thyroid Biology and Disorders
Figure 6
Immunohistochemistry with the growth marker BrdU in the organotypic cultures of thyroid (a) and adipose (b) tissue fragments, and scheme of thyroid tissue regeneration theories (c). (a) and (b) Intranuclear BrdU uptakes of thyrocytes and adipose cell types at peripheral zone (Pe) of thyroid (a) and adipose tissue fragments (b) are extensively greater than those at the central zone (Ce). Pe: peripheral zone. Ce: central zone. (c) Two theories regarding the mechanism of thyroid follicle regeneration that occurs specifically at peripheral zone of the tissue fragments. In regard to “cell density theory”, central zone of thyroid tissue fragments is characterized by higher cell density, whereas the peripheral zone is characterized by lower cell density. In general, increased cell density in a microenvironment inhibits the regeneration and growth of cells that are subjected to contact the inhibition of cell growth. Namely, central zone is tissue-static area with cell growth inactivation, while peripheral zone is tissue-remodeling area with cell growth activation. Thus, lower cell density of the peripheral zone may contribute to active development of thyroid follicles. In regard to “niche theory”, central zone concentrated by mature thyroid follicles may be subjected to mature thyroid follicle-organized niche-like environment, whereas peripheral zone with sparse population of the mature follicles may lose the environment. In general, a niche environment for stem cell types maintains their resting state. Thus, the niche-like environment formed by the mature follicles may inhibit regeneration of the follicles at the center, while its loss at the peripheral zone may contribute to their regeneration.
