Sickled RBCs are characterized by their intracellular, oriented bundles of deoxyHb S polymer fibers. Under deoxygenated (or partially deoxygenated) conditions these polymers form parallel-oriented fiber bundles, or "fascicles"  or needle-like "tactoids" . These fiber bundles either initiate or facilitate changes in the cytoskeleton of certain subpopulations of RBCs. Note however, deoxygenation results in only a reversible distortion in the shape of most RBCs.
addition to reversible sickling, a subpopulation of RBCs acquires permanent Hb S
polymer-induced morphological changes. This population, referred to as
irreversibly sickled cells (ISCs)[65,66], form after repetitive
deoygenation-reoxygenation cycles and retain a classic 'sickle' shape even under
oxygenated conditions in the circulation.
From morphological studies, besides the ISC population, a number of other classic 'sickle' shapes have also been described, including "elongated", "holly leaf" (spiculated), "POSC" (partially oxygenated), and "granular" [63,64].