Identification of living oligodendrocyte developmental stages by fractal analysis of cell morphology

The Mandelbrot's fractal dimension (D), a measure of shape complexity, has been used to quantify the complex morphology of living cells. Previous studies on glial cells have shown that as cells increase in morphological complexity, their "D" value increases, suggesting that "D" could be used to estimate their stage of differentiation. In the present study the box-counting method was used to calculate the "D" values of rat cerebellar oligodendrocytes during their differentiation in primary culture. These values were correlated with the immunoreactivity of cells to antigenic markers commonly used for assessing their stages of differentiation: A2B5, O4 and anti-galactocerebroside (Gal-C). Our results show that changes of the fractal dimension during differentiation follow the well known pattern of markers expression by these cells. These results demonstrate that A2B5-, O4-, and Gal-C-expressing oligodendrocytes can be confidently estimated from their respective fractal dimension values. Based on this immunocytochemical calibration, the calculation of "D" allows an easy and fast determination of the developmental stage of living (unstained) oligodendrocytes before the study of their physiological characteristics. Using this method we precisely identified living oligodendrocyte progenitors and early pro-oligodendrocytes expressing voltage-activated sodium currents that is a common characteristic of these two immature developmental stages (Sontheimer et al. [1989b] Neuron 2:1135-1145).