Septate and Gap Junctions in Molluscan Gill Epithelium

Previous studies on the electrically-coupled gill epithelium of the mussel Elliptio complanatus failed to reveal the presence of gap junctions . Therefore, the septate junction, which is present as a prominent belt between all the epithelial cells in this tissue, seemed to be an attractive candidate for providing the low-resistance pathway necessary for electrical coupling (7, 19), as also was the case in invertebrate salivary gland epithelium (20) . Recently, there have been several reports that macular gap junctions can be found along with septate junctions in various invertebrate epithelia (8, 9, 10, 13, 16) . Since the gap junction is the membrane specialization found in the electrotonic synapse of the crayfish giant septate axon (14) and has been implicated in low-resistance coupling of vertebrate tissues (1, 5, 6, 17, 18), we thought it necessary to reexamine mussel gill material more extensively for gap junctions as part of our attempt to define the role of junctions in intercellular communication . In this paper, utilizing conventional thin-section and freezeetch electron microscopy, we report the coexistence of septate and gap junctions in mussel gill epithelium .