Cell contact during early morphogenesis in the chick embryo

Abstract

The primitive epithelial layers of the chick blastoderm, the epiblast and hypoblast, are underlain by discontinuous basement laminae. Primary mesenchymal cells, which comprise the mesoblast that migrates laterally from the primitive streak, make close junctions (intercellular space, 25–100 Å) and tight junctions (no apparent extracellular space) with the basal surfaces of the epiblast and hypoblast. Close and tight junctions are also prominent between mesenchymal cells, especially along their trailing edges. Filopodia are most numerous on the leading edges of the migrating cells as would be expected if the close and tight junctions played a role in contact inhibition, perhaps by mediating electrical coupling between the cells. The tight junctions within mesoblast, epiblast, and hypoblast are at first focal in nature (maculae occludentes), but subsequently those between cells in the same tissue become more extensive while those between cells in unlike tissues are disrupted. When the primary mesenchyme organizes itself into an epithelium (somite, lateral, and intermediate mesoderm), the trailing ends of the cells, which contain the Golgi organelles, are directed toward the middle of the mass where zonulae occludentes appear next to the newly created free surface. Subsequently, secondary mesenchyme derives from the mesodermal epithelium. The migrating cells are connected by broad tight junctions (fasciae occludentes) which seem to persist as maculae occludentes after the presumptive connective tissue cells differentiate. Within the epithelia, maculae adhaerentes (desmosomes) are particularly well developed at later stages and probably contribute to tissue-specific intercellular adhesion.