The perineurium of the adult housefly: ultrastructure and permeability to lanthanum.

The ultrastructure of the perineurial cells of Musca overlying the first optic neuropile was examined by transmission electron microscopy. These cells are somewhat similar to those of other insects but cytoplasmic flanges seem to be absent, and mitochondria are relatively large and sinuous. The intercellular channel system on the lateral border of the cells is relatively spacious and highly meandering. Perineurial cells are joined by septate, gap, and tight junctions, hemidesmosomes, and desmosomes. Tight and septate junctions bond perineurial cells and glial cells. These data are evaluated on the basis of tracer studies with lanthanum. This material penetrates the extracellular space between perineurium and underlying glial and nerve cells, between epithelial glial cells and retinular axon terminals (capitate projections), and between the alpha-beta fiber pair in the optic cartridge (gnarls). If no damage occurs to the perineurial cells during tissue preparation, this passage of lanthanum to neuronal surfaces indicates that the blood brain barrier is incomplete in this restricted area. Supportive evidence for such permeance is based on electrophysiological data, considerations of membrane specializations in the optic neuropile, and Na+/K+ ratios of dipteran hemolymph.