Confined filaments in soft vesicles- the case of sickle red blood cells.*

Abnormal shapes of red blood cells (RBC) have been associated with various diseases. Here, we focus on sickle-shaped RBC, which form due to the abnormal growth of semi-rigid hemoglobin (HbS) fibers confined in RBC. Using the area difference elasticity (ADE) model for RBC and worm-like chain model for the confined HbS fibers, we explore shape deformations at equilibrium using Monte-Carlo simulations. We show that while a single HbS fiber is not rigid enough to produce sickle-like deformation, a fiber bundle can do so. We also consider multiple disjoint filaments and find that confinement can generate multipolar RBC shapes and even promote helical filament conformations, which have not been discussed before. When applied to microtubules confined in phospholipid vesicles, we show that the same model predicts vesicle tubulation. Also, we reproduce the tube collapse transition and tennis racket type vesicle shapes, as reported in experiments.
The highlight of this work is several important non-axisymmetric RBC and vesicle shapes, which have never been explored in simulations.


References:
1. Lim et al., Proc. Natl. Acad. Sci., 99, 16766 (2002).
2. Fygenson et al., Phys. Rev. Lett., 79, 4497 (1997).