Membrane structure of vesiculotubular complexes in developing axons in rat optic nerve: freeze-fracture evidence for sequential membrane assembly.

Intra-axonal vesiculotubular complexes, located within developing axons in the optic nerve of eight-day-old rats, were examined by freeze-fracture electron microscopy. The clusters usually fill most of the cross section of the axon and extend for approximately 1 micron along the fibre axis. As seen in freeze-fracture, the E- and P-faces of the membranes comprising these clusters exhibit a paucity of intramembranous particles (i.m.ps). This i.m.p.-poor membrane structure is different from that of the axolemma per se, which contains i.m.p. densities of ca. 120 micron-2 on the E-face and ca. 400 micron-2 on the P-face. Since earlier studies indicate that the vesiculotubular complexes fuse with the axon membrane so as to contribute to membrane growth, it is suggested that axonal differentiation involves a sequential mode of membrane development, in which an initial growth of a relatively undifferentiated membrane bilayer is followed by in situ insertion of specialized proteins within specific membrane domains.