ABSTRACT
In considering the various forms of disk-sphere transformation which mammalian red cells undergo, reference has repeatedly been made to the ideas of Norris, Gough, Teitel-Bernard and others who have suggested that the discoidal form of the cell is the result of an equilibrium between two sets of forces, one expansive, and the other (which includes surface tension) conducing to contraction of the surface (Norris, 1882; Gough, 1924; Teitel-Bernard, 1932; Ponder, 1929, 1934, 1936, 1937, 1939, 1941, 1942 a, b, c;Furchgott, 1940a, 19406; Furchgott & Ponder, 1940, 1941). When the disk changes into the sphere, we are dealing with a diminution or disappearance of the ‘expansive force’ which is probably due to a modification or even a destruction of a surface ultrastructure, and in the absence of anything to oppose it, the tension at the cell-fluid interface determines that the cell takes up the spherical form in which the surface is a minimum for the volume. As will be seen below, other forces are probably operative in the same direction. A re-establishment of the ultrastructure, with the ‘expansive forces’ inherent in its molecular arrangement, leads to the transformation of the sphere into the original disk, and the various conditions under which this reversal can be brought about have already been described. We have now to consider a situation in which the ‘expansive forces’ are so great, or the forces resisting them so small, that the red cell assumes a series of shapes which are characterized by extension, just as the spherical form is characterized by contraction.
Wintrobe (1942) gives an extensive bibliography (forty-five references) of papers on sickle-cell anaemia and the sickle-cell trait.
Sickling proceeds more rapidly at higher temperatures, and is almost completely inhibited in the cold (4°c.). This may be partly due to the reduction in red and white cell metabolism at low temperatures, so that the O2 tension falls more slowly, and partly to the viscosity and rigidity of the red cells being increased.
Through the kindness of Dr E. J. Cohn, Dr B. H. Vickery, and Dr Hans Neurath, I have been able to obtain a number of plasma protein fractions in a state of known purity, and have been able to confirm the conclusion (Furchgott & Ponder, 1940) that only the albumin fraction produces reversal of the slide-cover-glass shape transformation. Cholesterol, in the form of the sol prepared by the method of Lee & Tsai (1942), does not reverse the disk-sphere transformation produced by lecithin.
The total surface area of the sickle cell is probably greater than that of the disk from which it is derived, but it is impossible to measure it with any degree of accuracy. It is certainly much ‘straighter’, i.e. the mean curvature is less. The red cell in sickle-cell anaemia is more resistant to haemolysis by hypotonic saline than is the normal human red cell. In spite of a prevalent idea that red-cell shape can be inferred from measurements of resistance to hypotonic haemolysis, this is not to be interpreted as evidence that the surface of either the promeniscocyte or the meniscocyte is greater than that of the normal cell. Because of the observation that red cells of different degrees of flatness haemolyse at different critical volumes, attained in different tonicities (Gansslen, 1922; Haden, 1934; Castle & Daland, 1937; Ponder, 1937), the inference that increased resistance means increased flatness can sometimes be made, but only when factors such as the critical volume at which the cells haemolyse, the way in which volume change is related to tonicity change, etc., remain constant. That they do not always do so is shown by the fact that the critical volume in hypotonic NaCl is smaller than that in hypotonic plasma (Ponder, 1937), and the observation that red cells do not always swell as ‘perfect osmometers’ (Ponder, 1940; Guest & Wing, 1942).
The essential factor for the development of the expansive forces seems to be the passage of the haemoglobin from the combined to the reduced state, for the sickle cells become disks when not only oxygen, but also CO, is admitted to the preparation. The forces are thus peculiarly associated with reduced haemoglobin.
Both Gough and Teitel-Bernard have suggested that the flattened form of the mammalian red cell is due to the haemoglobin existing as a liquid crystal with expansive forces in the plane of the equatorial axis. That this is not the whole explanation for the discoidal shape is shown by the fact that the watery ghost is discoidal, but it may be that a biconcave discoidal ultrastructure accommodates haemoglobin arranged in a liquid crystal of the same general shape, and giving rise to forces of type III because of a tendency to expand in the equatorial plane.
A number of instances area known in which the haemoglobin is not uniformly distributed throughout the red cell, at least when it is fixed and stained. The best examples are the erythroblast in the stage of haemoglobinization, the target cell, and the cell of Mediterranean anaemia (Cooley).
