Biomechanical Properties Of Glial Cells And Neurons In The CNS

The biomechanical properties of animal cells were shown to play important roles in the development and functioning of the body, and also in the pathological mechanisms of many diseases. However, only in recent years there are a few studies on mechanics in the central nervous system (CNS), whereas other tissues have been extensively studied. It became clear that such mechanical phenomena may be very important also for the CNS, where, for instance, neuronal cells are being stretched during ontogenetic development (the formation of the cauda equina may serve as an example to illustrate the problem). The mechanical properties of neural cells may contribute to neural tissue structure and even to neuron-glia cross talk. Thus, in the present study we investigated the mechanical properties of glial cells and neurons in the CNS.We measured the viscoelasticity of neural cells by using scanning force microscopy (SFM). Since the cells are viscoleastic, dynamic measurements wereperformed by introducing a high-frequency, low-amplitude oscillation to the cantilever. All experiments were performed on acutely dissociated cells from two different CNS regions: (i) pyramidal neurons and astrocytes (glial cells) of the hippocampus, and (ii) bipolar and amacrine cells (i.e., interneurons) and Müller (glial) cells of the retina. There are two components of the complex viscoelastic constant K~*, of which K' reflects the elastic storage response of the cells and K" reflects the viscous loss response of the cells. These constants, as well as the Piosson's ratio v, were obtained for single cells at three different frequencies (30, 100, and 200 Hz).By applying this method, we found that the elastic constant of neurons is only about 1000 Pa. This is less than that of other cells; for instance, fibroblasts are two times suffer than neurons. Furthermore, glial cells were found to be about two times softer than neurons; thus, they behave as a type of very soft and compliant spring. For both types of cells, K' is higher than K"; this means that, compared to the viscous property, the elastic property is dominant. Thus, CNS-cells display the features of a very soft elastic rubber. Parallel biophysical and electron microscopal studies in our laboratory confirmed the data on the levels of tissue sections and of single cells.To figure out the viscoelastic properties of different cellular compartments, retinal Müller cells - as prototypical glial cells - were subjected to measurements along their length, comprising various parts from endfoot, inner process, soma, and outer process up to the sclerad end. Müller cells not only posses a complex cytoskeleton but contain different cellular organelles in these different cellular compartments. In addition to actin filaments which form a "cortex" of the entire cell, the endfeet of Müller cells are densely packed with smooth endoplasmic reticulum, the inner processes contain intermediate filaments, the somata are packed with the nucleus, and the outer processes contain microtubules. The results showed that both Müller cell processes were significantly softer than the soma and the endfoot. A similar situation was found in pyramidal cells, whose soma contains the nucleus and the processes are packed with microtubules. In these cells, the apical dendrite was significantly softer than the soma. However, it remains to be elucidated why the local mechanical properties of CNS cells vary depending on their subcellular structure(s).In conclusion, the results suggest that in general, both neurons and glial cells are very soft ('rubber elastic') and considerably viscous ('dissipative'). Glial cells are even softer than neurons. These results allow the conclusion that glial cells do neither serve as support cells nor as glue. Rather, they may act as a shock absorbing material embedding the neurons and diminish the damage of the neurons in case of mechanical trauma. Furthermore, they constitute a very soft (and, thus, optimal) substrate for the growth of neuronal cells and their processes. Therefore, this mechanical feature of the glial cells may support neuronal plasticity.