Defining Microglial Morphology: Form, Function, and Fractal Dimension

Objective: Expand knowledge of microglial form and function by developing, applying, and interpreting a method of objectively quantitating microglial morphology. Method: Microglial morphology's significance in health and disease was examined, and methods of objectively studying and measuring it were developed using fractal/chaos theory. The complexity of microglial morphology presumably changes with microglial function yet is not accounted for by traditional metrics. Accordingly, two computer programs were developed: MicroMod for modelling and FracLac for assessing microglial morphology, along with a method of extracting patterns from digital images. FracLac provides measures objectively describing microglial morphology including the box counting fractal dimension, lacunarity, multifractality, the number and density of foreground pixels in an extracted pattern, and the 2-dimensional size and shape (i.e., elongation and circularity) of the convex hull a cell covers. Results: Essential changes in microglial morphology were modelled using fractal principles and objectively quantitated by the fractal dimension delivered by FracLac. The fractal dimension was not redundant with any feature measured and, although some measures quantitated morphological changes in accordance with people's visual categorizations, the fractal dimension was especially sensitive to morphological changes with subtle differences in the levels of stimulation and presumably activation of cells. The fractal dimension was the most consistently useful measure of, and in some cases, the only measure refined enough to detect, the subtlest changes.