These figures are formed by scaled B-spline curves passing through the polygon vertices of a Voronoi diagram on a set of random points. Such figures often form in irregular porous structures and random aggregates [1].

[1] X. Y. Kou and S. T. Tan, "A Simple and Effective Geometric Representation for Irregular Porous Structure Modeling", Computer-Aided Design, 42(10), 2010, pp. 930–941. doi:10.1016/j.cad.2010.06.006.