Insect intercellular junctions: rapid freezing by jet propane.

Abstract

The ventral nerve cord of the cockroach, Periplaneta americana, and that of the locust, Schistocerca gregaria, have been studied after rapid freezing by cryo-jet, using liquid propane. Such tissues, unfixed and uncryoprotected, have been compared with unfixed cryoprotected tissues, as well as with material fixed with glutaraldehyde and cryoprotected with glycerol or polyvinylpyrrolydone. The perineurial tight junctions in the cryo-jet-frozen tissues exhibit characteristic intramembranous P-face ridges, but frequently these are composed of smooth-surfaced strands, comparable to those seen with fast-freezing in mammalian tissues, rather than of bead-like fibres. The intramembranous PF ridges, characteristic of axonal and glial processes in the insect central nervous system, also display a smooth surface in rapidly frozen preparations. Prior fixation and/or cryoprotection produces a bead-like appearance in the ridges. The interglial gap junctions, after fast-freezing, exhibit both clustered connexon arrays in the E-face and loosely aggregated ones; hence the coupled state cannot be unequivocally associated with the latter configuration. The septate junctions between glial cells are unchanged after rapid freezing, exhibiting the typical rows of P-face intramembranous particles with complementary E-face pits that are found in replicas from fixed and cryoprotected tissues. The surfaces of the axons and glial processes exhibit pleiomorphic depressions and associated particles as well as PF pits with complementary EF mounds, both with associated IMPs. These structures are not usually seen after fixation or cryoprotection and may represent some kind of receptor structure, or axo-glial specialization.