Membrane specializations of neuritic growth cones in vivo: a quantitative IMP analysis

The internal structure of the membranes of axonal and Schwann cell growth processes was examined by freeze-fracture in the growing olfactory nerve, a simple in vivo system consisting of a homogeneous neuronal population. Excision of the mature nerve of adult bullfrogs provides well-synchronized primary neuritic outgrowth that is highly enriched in growth cones at its distalmost segment. The extreme uniformity of olfactory axons in terms of their diameter and their intramembrane particle (IMP) composition permits clear identification of the cellular origin of the growth cone structures observed in replicas. In vivo, growth cones of the olfactory nerve appear as irregularly shaped enlargements of the distal tip; filopodia are only infrequently exposed by the fracture plane. Axonal and Schwann cell growth cones are distinguished by 1) the larger size of the Schwann cell growth cone and the smaller diameter of its attached processes, and 2) the distinct differences in IMP composition of Schwann cell and axonal growth cones and cell processes. Schwann cell growth cones display a uniformly high IMP density on their P-face leaflet, with the exception of circumscribed moundlike protrusions that are relatively free of IMPs. In contrast, axonal growth cones display sharp regional variation in IMP density on their P-face: broad regions almost devoid of IMPs are interspersed with zones of high IMP density. Cytotic profiles occur within high IMP density zones located, most often, at the base of the axonal growth cone. A comparison of IMP size histograms of both high and low-density regions of axonal growth cones and that of the neighboring distal shaft of the axon indicates a strict partitioning of membrane components between these two regions. The IMP profile of the axonal growth cone, notable for its relative enrichment in large-diameter particles, suggests that IMP components of the growth cone are delivered to the distal tip by a mechanism that is distinct from the lateral diffusion process described for particles of the growing axon's shaft [cf. Small and Pfenninger, 1984]. The IMP profile of the concave P-face leaflet of the internal vesicles found clustered at the base of the growth cone is more similar in composition to the profile of the neuronal shaft than that of the growth cone.