The studies described in this thesis sought to identify mechanisms responsible for thyroid follicular morphogenesis in vitro. As reviewed in Chapter 1, the thyroid gland is an epithelial organ that is specialized for the synthesis, storage and secretion of thyroid hormones. As such thyroid cells possess many structural and functional features common to epithelia in general, including a polarized phenotype and specialized intercellular junctions and adhesive mechanisms which maintain the cohesion and paracellular permselectivity of the epithelial layer. In addition, thyroid cells are organized in a characteristic three-dimensional follicular morphology which contributes to thyroidal function by participating in hormone synthesis as well as providing a store of preformed hormone. Therefore the mechanisms which generate and maintain the three-dimensional architecture of the thyroid are fundamental aspects of its biology.
To study these problems, primary cultures of porcine thyroid cells have been used which can be grown either as follicles (in three-dimensional cell aggregates) or as a confluent, two-dimensional epithelial monolayer. The general methods used to generate these cultures are described in Chapter 2, but descriptively the association of isolated cells into follicles involves at least three processes: (1) the formation of three-dimensional cell aggregates; (2) establishment of a polarized epithelial phenotype and assembly of specialized intercellular junctions; and (3) development and growth of a follicular lumen. This framework has been used as a guide throughout these studies.
The experiments in Chapter 3 sought to test the hypothesis that thyrotrophin (thyroid-stimulating hormone, TSH) might support thyroid cell aggregation by regulating the balance of adhesive forces acting upon cells grown in conventional substrate-adherent cultures (i.e. cell-cell and cell-substrate interactions). In suspension cultures it was observed that freshly isolated cells formed aggregates even in the absence of TSH, demonstrating that they possessed a constitutive cell-cell adhesion mechanism. However, this was potentiated by the addition of TSH or cyclic AMP analogues, indicating that cellcell adhesion could be modulated by this hormone, presumably acting through intracellular cAMP. Cellular adhesion was dependent on extracellular calcium and potentiated by a particulate cell-free membrane fraction, suggesting the presence of a Ca2+ -dependent adhesion mechanism on the cell surface. ....................................