Fixation-induced intramembrane particle movement demonstrated in freeze-fracture replicas of a new type of septate junction in echinoderm epithelia

Abstract

Conventional thin-section, lanthanum tracer, and freeze-fracture techniques have revealed a previously undescribed type of septate junction in echinoderm epidermal tissue. This new junction type is of particular interest because experiments with it demonstrate that a large degree of intramembrane particle translocation is possible during glutaraldehyde fixation. This translocation apparently results in disconnection of intermembrane junctional structures from their associated intramembrane components. The junction has the 15- to 18-nm intercellular spacing common to all invertebrate septate junctions. In tangential view, septa appear double, straight, and unbranched. In freeze-fracture of fixed tissue the junction appears as a broad band around the apical circumference of cells, but the particles in the band do not line up in rows, as would be expected after viewing tangential sections. In contrast replicas of unfixed tissue reveal junctional structures aligned in rows. It is proposed that this junction be called the echinoderm double-septum septate junction.