  Detailed knowledge of mechanical parameters such as cell elasticity,
stiffness of the growth substrate, or traction stresses generated during axonal
extensions is essential for understanding the mechanisms that control neuronal
growth. Here we combine Atomic Force Microscopy based force spectroscopy with
Fluorescence Microscopy to produce systematic, high-resolution elasticity maps
for three different types of live neuronal cells: cortical (embryonic rat),
embryonic chick dorsal root ganglion, and P-19 (mouse embryonic carcinoma stem
cells) neurons. We measure how the stiffness of neurons changes both during
neurite outgrowth and upon disruption of microtubules of the cell. We find
reversible local stiffening of the cell during growth, and show that the
increase in local elastic modulus is primarily due to the formation of
microtubules. We also report that cortical and P-19 neurons have similar
elasticity maps, with elastic moduli in the range 0.1-2 kPa, with typical
average values of 0.4 kPa (P-19) and 0.2 kPa (cortical). In contrast, DRG
neurons are stiffer than P-19 and cortical cells, yielding elastic moduli in
the range 0.1-8 kPa, with typical average values of 0.9 kPa. Finally, we report
no measurable influence of substrate protein coating on cell body elasticity
for the three types of neurons.
