The growth cones of elongating nerve fibres* are in many ways similar to the ruffling membranes of migrating fibroblasts. They both move at the same rate and in a characteristic fashion in which surface structures appear to be carried rapidly backwards away from the leading edge. Many of their morphological features seen by light or electron microscopy are the same. It seems likely that these similarities have a common molecular basis and that the two structures have a closely related genetic specification. Ostensibly their functions are different. Whereas the ruffling membrane appears to be involved in the locomotion of the cell, there is evidence that the growth cone is concerned with neurite elongation. Observations on isolated neurons in culture show that branch points and fibre surface do not advance with growth, and suggest that these are formed at, or close to, the growth cone. The apparent paradox is resolved if, in both structures, surface assembly and forward migration are concomitant processes. The model suggested for fibroblast locomotion in which surface membrane is cycled from assembly at the leading edge to uptake at a more proximal region, requires only the modification that rates of assembly exceed rates of uptake to account for nerve growth. Electron microscopy of growth cones has shown structures potentially able to carry out these activities. They contain microfilament networks of the kind often associated with movement, cytoplasmic membrane which shows continuity with the surface membrane, and lysosomal vacuoles which could be involved in the uptake of surface membrane.
ASJC Scopus subject areas
- Biochemistry, Genetics and Molecular Biology(all)