Critical kernels constitute a general framework settled in the category of abstract complexes for the study of parallel thinning in any dimension. The most fundamental result in this framework is that, if a subset Y of X contains the critical kernel of X, then Y is guaranteed to have "the same topology as X". Here, we focus on 2D structures in spaces of two and three dimensions. We introduce the notion of crucial pixel which permits to make a link with the framework of digital topology. Thanks to simple local characterizations, we are able to express thinning algorithms by the way of sets of masks. We propose several new parallel algorithms, which are both fast and simple to implement, to obtain symmetrical or non-symmetrical skeletons of 2D objects in 2D or 3D grids. We prove some properties of these skeletons, related to topology preservation, to minimality and to the inclusion of the topological axis which may be seen as a generalization of the medial axis. We also show how to use critical kernels in order to prove very simply the topological soundness of existing thinning schemes. At last, we make clear the link between critical kernels, minimal non-simple sets, and P-simple points.