Permeability of gap junctions at the segmental border in insect epidermis.

We have examined cell-cell communication between epidermal cells of fifth-instar larvae of the milkweed bug Oncopeltus fasciatus and those of maggots of the blowfly Calliphora erythrocephala. Ionic coupling and the transfer of injected Lucifer Yellow (molecular weight 450) and lead-EDTA (molecular weight 374) were used to map the pattern of communication. All epidermal cells, regardless of their position with respect to the segmental border, were ionically coupled. In both species Lucifer yellow was transferred freely between cells lying in the same segment--that is, in the same developmental compartment as defined by cell lineage. Dye injections close to the segmental border showed that Lucifer Yellow was not transferred between cells in adjacent segments--that is, across the compartmental border. In Calliphora failure of Lucifer Yellow transfer at the segmental border was always observed; in Oncopeltus Lucifer Yellow was not transferred in 90% of preparations examined. Injections of PbEDTA2- in Calliphora showed that this anion was transferred freely from cell to cell and did not respect the segmental boundary. Previous studies of the distribution of gap junctions at and away from the segmental border make it unlikely that the failure of Lucifer Yellow to cross from segment to segment is due to reduced number of gap-junctional channels at the border. We conclude that gap junctions at the segmental borders may have different permeability properties from those between cells in the same segment.