RESTRICTED EXPRESSION OF THE ACTIN-REGULATORY PROTEIN, TROPOMYOSIN, DEFINES DISTINCT BOUNDARIES, EVAGINATING NEUROEPITHELIUM, AND CHOROID-PLEXUS FORERUNNERS DURING EARLY CNS DEVELOPMENT

In the hindbrain, rhombomeres represent morphological units that devel op characteristic, segment-specific structures. Similar segments, know n as prosomeres, have been proposed to exist in the forebrain. The neu roepithelial cells of the sharp boundary regions that form the borders between many segments often exhibit distinct shapes, reflecting uniqu e cytoskeletal organization, The present investigation examined the ex pression of one family of actin-binding, regulatory proteins, the trop omyosins (TM), in boundaries, We found that high molecular weight TMs selectively concentrate in boundary cells and other neuroepithelial zo nes that exhibit unique cell shapes and movements. Specific TM express ion is found at hindbrain boundaries as early as embryonic day 10 in t he rat, whereas rhombomeres themselves were TM-negative. Highly restri cted TM localization also defined some prosomere boundaries in the ear ly forebrain, particularly those exhibiting unique cell shapes. Furthe rmore, several regions of the neuroepithelium that evaginate are TM-im munoreactive, including tuberal and preoptic neuroepithelium. Most str iking, a subpopulation of neuroepithelial cells in the medial telencep halic wall expresses TM, apparently marking the neuroepithelial region that gives rise to the choroid plexus at least 2 d before its formati on. This suggests that the medial cerebral wall is not entirely dedica ted to generating cells that comprise allocortex. TM expression in the choroid plexus is maintained through initial evagination and appearan ce in all ventricles. The spatially restricted expression of TMs impli cates that this actin-binding protein is involved in the dynamic regul ation of cell shape or motility associated with boundary formation and morphogenesis of the neuroepithelium during critical stages of brain development.