The discovery of the growth cone and its influence on the study of axon guidance

Abstract

For over a century, there has been a great deal of interest in understanding how neural connectivity is established during development and regeneration. Interest in the latter arises from the possibility that knowledge of this process can be used to re-establish lost connections after lesion or neurodegeneration. At the end of the XIX century, Santiago Ramón y Cajal discovered that the distal tip of growing axons contained a structure that he called the growth cone. He proposed that this structure enabled the axon's oriented growth in response to attractants, now known as chemotropic molecules. He further proposed that the physical properties of the surrounding tissues could influence the growth cone and the direction of growth. This seminal discovery afforded a plausible explanation for directed axonal growth and has led to the discovery of axon guidance mechanisms that include diffusible attractants and repellants and guidance cues anchored to cell membranes or extracellular matrix. In this review the major events in the development of this field are discussed.

Keywords: brain; chemotropic; development; neurotropic.

Figure 1

Drawing by Cajal of a spinal cord section of a three-day old chick embryo. (A) Ventral nerve root; (B) Posterior nerve root; (a), young nerve cells; (b), more developed cells that probably correspond to commissural neurons; (c) piriform cell of the ventral nerve root; (d) growth cone of a commissural axon; (e) radicular cell with rudiments of protoplasmic branches; (h,i) growth cones of ventral nerve root axons; (o) dorsal root ganglion cells. Figure taken and legend translated from Cajal (1929b).

Figure 2

Single neuron electroporated with a green fluorescent protein (GFP) vector in an embryonic chick hindbrain. (A) Growth cone; (B) Neuronal soma; (C) Filopodia (digitally enhanced for better visualization).

Figure 3

Dorsal root ganglion neuron in culture immunostained for cytoskeletal components. β-III tubulin microtubules (green) are located mainly in the axon shaft and in the central part of the growth cone. F-actin filaments (red) are located at the leading edge and in the lamellipodia and filopodia.

Figure 4

Aspects involved in the response of growth cones to guidance cues. Growth cones respond to several external factors including chemotropic proteins and signals anchored to cell or axon membranes and to the extracellular matrix. Growth cones integrate the information conveyed by the receptors to the various guidance signals inducing the changes in the cytoskeleton associated to axon navigation. The cell-cell and cell-substrate interactions involved, make axon pathfinding a complex and multifactorial event. In the figure, a pioneer axon is shown (red) that interacts with its substrate (yellow) and responds to a chemotropic signal (green gradient). A follower axon (purple) interacts additionally with the pioneer axon. The differential distribution of microtubules and actin are indicated in the open view of the red axon.

Figure 5

Fasciculated projection of GABA⁺ and dopaminergic (TH⁺) axons in culture. E13.5 rat embryo pretectum (P1) explants exert an attractive effect upon axon GABA/TH fascicles growing from ventral midbrain explants (VM) in collagen gel cultures. Cultured gels were immunostained for GABA (red) and TH (green) (Garcia-Peña et al., 2014).