Pseudopod behavior in hyperplastic thyroid follicles in vivo

As an early step in the secretion of thyroid hormones, colloid droplets are formed by pseudopods of thyroid epithelial cells. However, such droplets are generally absent from thyroid stimulating hormone (TSH)-stimulated thyroid follicles in places where luminal colloid has been largely resorbed. To investigate the reason for this absence, stimulated thyroid follicles were examined. Even in markedly colloid-depleted lumens, pseudopods were frequently observed. These were often short and flat, abutting against the opposing epithelial surface and following its contour. No room appeared available for these pseudopods to roll up to form colloid droplets. In addition, ultrastructural changes were often visible in the opposing surface just subjacent to regions of pseudopod contact. These changes consisted of the loss of apical microvilli and the formation of an organelle-excluding microfilamentous meshwork, suggestive of early psuedopod formation. The possibility that contact with a pseudopod might be capable of inducing the formation of a second pseudopod was also suggested by the frequent finding of pairs of pseudopods, one abutting against or partially enveloping another.