CHANGES IN THE EXPRESSION OF INTERMEDIATE FILAMENTS AND DESMOPLAKINS DURING DEVELOPMENT OF HUMAN NOTOCHORD

Indirect immunofluorescence was used to study the expression of desmos omal and intermediate filament (IF) proteins in the human notochord be tween the 4th and 12th weeks of embryonic development. Towards the end of this period, the development of the notochord is characterized by its gradual physiological atrophy and disappearance inside the vertebr al bodies. In all of our embryos, the notochord cells expressed cytoke ratin and vimentin but not desmin, neurofilament protein or glial fibr illary acidic protein. Throughout the stages studied, the expression o f cytokeratin was strong. Vimentin expression, on the other hand, chan ged during the stages studied. In our youngest embryos, vimentin could be detected only in the peripheral cells of the notochord, During dev elopment, a distinct increase occurred in vimentin expression, and in the oldest embryos, all notochord cells showed bright vimentin-specifi c fluorescence. Simultaneously with this modification, a change occure d in the expression of desmosomal proteins: The notochord cells expres sed desmoplakins abundantly during early stages, but weakly or not at all during later stages. Correspondingly, electron microscopy of the s ame stages showed a striking decrease in the number of desmosomes betw een notochord cells. Our results confirm that, during early developmen t, the notochord displays features specific for epithelial cells. This accords with the view that notochord is of epithelial origin. The mod ifications observed in the expression of IF and desmosomal proteins we re temporally correlated with developmentally regulated atrophy of the notochord, The programmed regression of the notochord cells is thus a ssociated with a switch from a predominantly epithelial phenotype to a more mesenchymal one.