  Actin cytoskeletal protrusions in crawling cells, or lamellipodia, exhibit
various morphological properties such as two characteristic peaks in the
distribution of filament orientation with respect to the leading edge. To
understand these properties, using the dendritic nucleation model as a basis
for cytoskeletal restructuring, a kinetic-population model with
orientational-dependent branching (birth) and capping (death) is constructed
and analyzed. Optimizing for growth yields a relation between the branch angle
and filament orientation that explains the two characteristic peaks. The model
also exhibits a subdominant population that allows for more accurate modeling
of recent measurements of filamentous actin density along the leading edge of
lamellipodia in keratocytes. Finally, we explore the relationship between
orientational and spatial organization of filamentous actin in lamellipodia and
address recent observations of a prevalence of overlapping filaments to
branched filaments---a finding that is claimed to be in contradiction with the
dendritic nucleation model.
