Structural macromolecules of the cell membranes and the extracellular matrices of the insect midgut

Abstract

Insect midgut cells are joined to each other on their lateral borders by junctional complexes whose function can be mechanical (smooth septate junctions, fasciae adhaerentes, hemi-adhering junctions) or for intercellular communication (gap junctions). The most prominent intercellular associations are the apical smooth septate junctions, which form a circumferential belt around the luminal borders of the midgut cells. Beneath this region, gap junctions are usually found. In the deep part of the lateral border the two adjacent cell membranes may form strip or spot-like adhering junctions, fasciae adhaerentes, whereas the basal plasma membrane that is in contact with the underlying extracellular matrix of the basal lamella may be modified to form hemi-adhering junctions. The general appearance of intercellular junctions in the insect midgut epithelium is thus markedly different from that of the lower chordates and higher vertebrates. The latter have a junctional complex consisting of zonulae occludentes (tight junctions (TJ)) at the luminal edge, zonulae adhaerentes (ZA or intermediate junctions) beneath, with maculae adhaerentes (MA, desmosomes) and gap junctions on the more basal part of the lateral borders (Stevenson and Paul, 1989).