Junctional structures in the midgut cells of lepidopteran caterpillars

The junctional structures present between the midgut cells of 3 lepidopteran caterpillars have been examined using freeze-etching, conventional staining and lanthanum tracer techniques. The bonding junction present in this type of tissue is the so-called continuous junction. Septa have only occassionally been reported in conventionally strained cross-sections of these junctions. During the present study septa have been observed in such sections but were more readily located in tissue treated with lanthanum tracer. Tangential sections of lanthanum-impregnated tissue show that these septa are parallel-sided, in contrast to the honeycomb appearance of the septa in septate junctions. The septa in freeze-etch replicas of glutaraldehyde-fixed tissue often show continuous rods on one membrane face, suggesting that the continuous junction may be more akin to the tight junction than to the normal septate junction. However, freeze-etch replicas of unfixed tissue appear much more like replicas of normal septate junctions. The main differences between septate and continuous junctions appear to be that the inter-membrane septa of the continuous junction are parallel-sided in tangential section, as against the honeycomb appearance of the septate junction, and that the particles which delineate the septa in freeze-etched preparations appear to be both somewhat differentl bonded and closer together in the continuous junction. A diagram is presented showing the internal and inter-membrane structures of the 2 types of junction based on the present study, and suggesting a possible explanation of the fact that septa are more readily seen in cross-sections of septate junctions than in continuous junctions. As septa are present in both types of junction, and because their freeze-etch appearances are not very different, it is suggested that the two types of septate junction be called 'smooth septate' and 'pleated septate' junctions, to indicate their characteristic appearances in tangential section.