CADHERIN-MEDIATED ADHESION AND APICAL MEMBRANE ASSEMBLY DEFINE DISTINCT STEPS DURING THYROID EPITHELIAL POLARIZATION AND LUMEN FORMATION

The biogenesis of follicles from aggregates of precursor cells is an i mportant morphogenetic process in thyroid embryology. It necessitates the creation of a polarized cell phenotype, assembly of specialized ce ll-cell junctions, and generation of follicular lumena. In this study we sought to investigate the relationship between cell polarization an d lumen formation by studying the cell surface events that occurred wh en freshly isolated adult porcine thyroid cells reorganized to form fo llicles in primary culture. Follicular reorganization entailed the ini tial formation of solid three-dimensional cell aggregates and the subs equent appearance of lumena within aggregates. During the initial stag e of cell aggregation, the adhesion molecule, E-cadherin, became expre ssed at all surfaces involved in cell-cell contact. Aggregation was in hibited by monoclonal antibodies that block cadherin function, indicat ing directly that E-cadherin is a dominant initial cell-cell adhesion molecule. Cell aggregation was also associated with the recruitment to the cell surface of ZO-1, a tight junction-associated protein, and Na +/K+-adenosine triphosphatase. These proteins were initially found thr oughout regions of cell-cell contact and only subsequently redistribut ed to their mature locations in tight junctions and the basolateral ce ll surface, respectively. In contrast, components associated with the apical membrane were first detected within large intracellular vacuole s, which subsequently fused with the cell surface between maturing tig ht junctions to yield the apical membrane domain and nascent follicula r humena. Follicle formation occurred independently of basal lamina as sembly and TSH, although maintenance of follicular architecture requir ed the presence of this hormone. These findings indicate that cultured follicles form in two distinct stages: 1) initial aggregation mediate d by E-cadherin and associated with recruitment of components of both tight junctions and the basolateral membrane domain, and 2) subsequent formation of a specialized apical membrane domain by coordinated fusi on of intracellular vacuoles at sites of the cell surface where tight junctions are maturing. We propose that follicular morphogenesis may a rise as a consequence of epithelial cell polarization within coherent three-dimensional cell aggregates.