Sickled RBCs are characterized by their intracellular, oriented bundles of deoxyHb S polymer fibers[55]. Under deoxygenated (or partially deoxygenated) conditions these polymers form parallel-oriented fiber bundles[55], or "fascicles" [62] or needle-like "tactoids" [29]. These fiber bundles either initiate or facilitate changes in the cytoskeleton of certain subpopulations of RBCs[55]. Note however, deoxygenation results in only a reversible distortion in the shape of most RBCs.
In
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[55].
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].