Change of form of septate and gap junctions during development of the insect midgut

In insects, smooth septate junctions join cells derived from the embryonic midgut, and pleated septate junctions are found in all other tissues. Relatively little is known about either type of septate junction or the relationship between them, but they have been treated as two different junctions in the literature. The gap junctions which are associated with these septate junctions also differ. Crystalline gap junctions are found in the midgut, associated with smooth septate junctions, and irregular gap junctions are found in tissues where pleated septate junctions are located. We have examined the development of smooth septate junctions and crystalline gap junctions and the relationship between them, by studying the embryogenesis of the midgut in Manduca sexta (tobacco hornworm). At 56 hr of development (hatching is at 104 hr) pleated septate junctions and irregular gap junctions joined the midgut epithelial cells. At 65 hr, the septate junctions had disappeared, but gap junctions persisted. At 70 hr, smooth septate junctions had replaced the earlier pleated septate junctions and gap junctions associated with these smooth septate junctions were often of the crystalline form. In later embryos, the smooth septate junctions matured and enlarged, while all gap junctions became crystalline in form.