X-RAY-MICROANALYSIS WITH TRANSMISSION ELECTRON-MICROSCOPY DETERMINED PRESENCE AND MOVEMENT OF TRACER (LANTHANUM CHLORIDE) AT BLOOD-NEURON BARRIER OF DROSOPHILA-MELANOGASTER (DIPTERA, DROSOPHILIDAE) LARVA

In studying the larval Drosophila (Diptera: Drosophilidae) blood-brain barrier, it was important to determine if even minute amounts of trac er ultimately seeped through the septate junctions between perineurial cells to reach the neuronal region. Concurrent TEM with X-ray microan alysis was undertaken to resolve that issue. Ultrathin sections of Dro sophila nervous tissue in LR White embedment were exposed to ionic tra cer (lanthanum chloride) and assayed for presence or absence of lantha num extracellular to the perineurium and glia making up the nerve shea th. Tracer filled the distal interseptal lattice of pleated sheet-sept ate junctions, but was contained prior to reaching the proximal parace llular space. No detectable tracer passed through septate junctions to enter the glial-neuronal domain. Based on our present data and the re search of others, septate junctions in immature Drosophila are multifu nctional structures that enforce spatial relationships between cells, seal intercellular spaces, and control cell proliferation in the epith elia. Septate junctions in Drosophila with the (dig) gene also exhibit protein homologies to the Z0-1 human tight junction component.