Arthropod immune system. III. Septate junctions in the hemocytic capsule of the german cockroach, Blattella germanica.

Granulocytes (GRs) and/or plasmatocytes (PLs), the two major immunocytes in arthropods, participate in cellular encapsulation of foreign tissue. Although gap and desmosome junctions have been reported in insect capsules, smooth septate junctions are being reported for the first time by both thin section and freeze-fracture techniques in Blattella germanica. In 7-day-old capsules, the septa are 23 nm thick, faintly 'scalloped' and slightly curved in appearance; the interseptal space has a periodicity of about 5 nm. In freeze-fractured capsules, the septa are associated on both sides with the corresponding intramembranous structures, belonging to the plasma membranes of the two junction-forming GRs. The intercellular space is 27 nm wide. There are 36-40 septa/1 microm junctional length. The junctions show furrows on the extracellular fracture face (E) and the complementary regular rows of intramembranous particles on the cytoplasmic face (P). The septate junctions often occur in the region of the capsule that also shows the presence of gap junctions, but only rarely that of desmosomes. The septate junctions are in close proximity with mitochondria. It is suggested that the function of these junctions is to produce compact capsules.