Diameter of haemoglobin S fibres in sickled cells.

Abstract

SICKLE-CELL disease is a manifestation of the assembly of the Hb S variant of human haemoglobin into fibres that alter erythrocyte morphology. Recent studies have focused on two distinct forms of the fibres—a 6-stranded form composed of stacked rings of six Hb S molecules1,2 and a more complex form3–5 which we have recently studied by three-dimensional image reconstruction6: this revealed a 14-stranded structure composed of 4 inner and 10 outer strands in an arrangement with hexagonal packing that produces an elliptical cross-section. A critical issue is the question of the predominant form of the fibres in sickled cells. In commenting on our paper describing the 14-stranded structure, Finch7 focused on the diameter of the fibres observed in sectioned sickled cells and the relationship of this parameter to the diameters observed in negatively stained preparations of isolated 6-stranded and 14-stranded fibres. He concluded that the 6-stranded fibres, with diameters of 16–18 nm (refs 1, 2), were more consistent in size with the diameter of fibres found in sectioned sickled cells. However, this conclusion was based on limited data for sectioned sickled cells and an apparent over-estimation of the diameter for the 14-stranded fibres. We have recently completed extensive measurements on the diameter of fibres in sectioned sickled cells based on centre-to-centre distances between fibres in bundles as well as measurements on the diameter of the 14-stranded fibres in negatively stained preparations that are corrected for flattening. Measurements have also been made on the centre-to-centre distance of fibres in bundles from stirred haemolysates5,8. The results of these measurements are reported here and indicate an agreement in the average diameter observed for the fibres in sickled cells and negatively stained 14-stranded fibres, with values in the range of 20–22 nm obtained in both cases. An average diameter in this range is also found for the fibres in the bundles from stirred haemolysates.