The elliptical, anucleate erythrocytes of camels have been examined for the presence of marginal bands and their constituent microtubules. Lysis of erythrocytes under microtubulestabilizing conditions readily revealed marginal bands in at least 3 % of the cells, as observed by phase-contrast and darkfield light microscopy. Microtubules plus a marginal bandencompassing network of material are visible in lysed cell whole mounts with transmission electron microscopy. Marginal band microtubules are also evident in electron micrographs of thin-sectioned camel erythrocytes identifiable as reticulocytes on the basis of submaximal electron density (reduced haemoglobin iron content) and presence of polysomes. The results suggest that marginal bands may be involved in morphogenesis of camel erythrocytes but are not required for maintenance of their ellipticity after cells are fully differentiated.

The marginal band (MB) is a discrete circumferential bundle of microtubules with a probable role in alteration and perhaps maintenance of cell shape. MBs occur in the elliptical, nucleated erythrocytes of non-mammalian vertebrates (Dehler, 1895; Meves, 1911; Fawcett, 1959), in the thrombocytes of both mammalian and nonmammalian vertebrates (Fawcett & Witebsky, 1964; Behnke, 1965; Sandborn, LeBuis & Bois, 1966), and in blood cells of certain invertebrates (Cohen, Nemhauser & Jaeger, 1977). MBs have not been observed in the mature anucleate, diskoidal erythrocytes of mammals.

Among the mammals, members of the family Camelidae (camels, vicunas, guanacos, llamas, alpacas) are unique in that their erythrocytes, though anucleate, are elliptical (Andrew, 1965). The question thus arises as to whether MBs play a role in cell shape generation and/or maintenance in these species. Barclay (1966), in an abstract, reported the occurrence of MB microtubules in thin-sectioned camel erythrocytes. Recently, however, Goniakowska-Witaliúska & Witaliñski (1976) were unable to verify Barclay’s observation after examining more than 2000 thin-sectioned camel and llama cells. In order to help resolve this issue a somewhat different approach appeared desirable, one which would avoid potential fixation and sampling problems associated with thin-sectioning. In the work reported here, camel erythrocytes have been lysed in a microtubule-stabilizing medium, permitting direct visualization of MBs in large numbers of lysed cells by means of phase-contrast and darkfield light microscopy, with subsequent TEM on whole mounts (Cohen et al. 1977). These results thus support Barclay’s (1966) report. Thin-section ultrastructural data on intact cells, upon which Barclay’s abstract was based, are presented for further documentation of the presence of MB microtubules in camel erythrocytes.

Lysed erythrocytes

Observations were made on blood samples from 2 camels (Camelos dromedarios), 2 guanacos (Lanta goanicoe), and 1 llama (Lama glama). Whole blood was drawn by syringe from the jugular veins of donor animals at the Bronx Zoological Park by Dr Emil P. Dolensek of the New York Zoological Society. The blood was collected in either citrated or heparinized tubes and kept warm in the hand until samples were lysed (within 10–30 min of collection).

In order to observe and photograph individual erythrocytes, whole blood was diluted approximately 1:100 with mammalian Ringer solution (Krebs formula, Cavanaugh, 1975). The lytic medium, based upon microtubule-polymerization conditions (Weisenberg, 1972; Rebhun, Rosenbaum, LeFebre & Smith, 1974), consisted of 100 mM PIPES (piperazine-N-N’-bis [2-ethanesulphonic acid]), 1 mM MgCl,, 5 mM EGTA, 10 mM TAME (p-tosyl arginine methylester HC1), and 0·4% (w/v) Triton X-100 brought to pH 6·8 with KOH. The TAME protects against proteolysis, while the EGTA reduces Ca2+ below polymerization-inhibiting levels (Rebhun et al. 1974).

Whole blood samples were lysed at a ratio of 1 vol. blood to 9 vol. medium (generally 0·1 ml + 0·9 ml). Some samples were diluted further with the same medium to facilitate counts of MBs versus cell number in non-overlapping fields in phase contrast. Light-microscopic observations were made in a Zeiss phase-contrast microscope. To achieve a darkfield effect, the 100 × condenser annulus was used in conjunction with the 16 × phase objective. Slides and coverslips were detergent-cleaned and washed to remove oily films, and petroleum jelly was used to seal coverslips for long-term observation. For TEM of whole mounts one drop of lysed cell suspension was placed on each Formvar-coated grid for 2 min, drawn off with filter paper, and replaced by a drop of deionized, distilled water. The water was immediately drawn off and replaced by a drop of 2 % aqueous uranyl acetate for 15-30 s. After removal of excess stain, the grid was allowed to air dry. Grids were examined in an Hitachi HS-8 transmission electron microscope operating at 50 kV.

Thin sections

Whole blood was obtained as above from the jugular veins of 2 mature camels at the Vilas Park Zoo, Madison, Wisconsin. The blood was collected in tubes containing acid-citrate-dextrose anticoagulant and fixed within 30 min in 5 % glutaraldehyde, 0·075 M phosphate buffer, pH 7·4, for 30 min. The cells were rinsed in 0·075 M phosphate buffer and then postfixed in similarly buffered 1 % osmium tetroxide for 1 h. The cells were centrifuged into a pellet before each solution change and then resuspended. After postfixation, the pellets were cut into small pieces, dehydrated in an ethanol series, and embedded in Epon 812. Thin sections were transferred to carbon-reinforced parlodion-coated grids and stained with uranyl acetate followed by lead hydroxide. The grids were examined in an RCA EMU 3E electron microscope.

Lysed cells

Large numbers of elliptical MBs were visible under phase-contrast (Fig. 1) in preparations of lysed erythrocytes from a young camel (10 months). The majority of lysed cells, however, did not display MBs but appeared as partially collapsed, roughly elliptical ghosts. The MBs were of approximately the same axial dimensions as intact cells (Figs. 2–5). Many MBs had dense, thickened regions along their inner surface, often but not always near the ends of the ellipse (Figs. 4, 5). Some MBs were twisted about their long axis so as to form ‘figure-eights’ in certain views (Fig. 6A, B). Within the regions circumscribed by most MBs there was no material visible, giving the initial impression that the MBs were completely free of other cellular material. Large multilobed nuclei, presumably derived from leucocytes, formed small clumps in the medium (Fig. 1). Counts made in non-overlapping fields chosen at random showed that there was approximately one clearly visible MB per 35 cells for this animal (total count: 19 MBs, 664 cells). This is probably a minimum estimate as some relatively thin MBs may not be observable in phase-contrast.

Fig. 1.

Low-magnification view of lysed cell preparation from a young camel. Many elliptical MBs are present (arrows), but most cells lack MBs and appear as smaller ghosts. Some MBs (fainter, bright contrast) are slightly out of plane of focus. Clumped nuclei (n) are apparent remnants of leucocytes. Phase-contrast, × 510.

Fig. 1.

Low-magnification view of lysed cell preparation from a young camel. Many elliptical MBs are present (arrows), but most cells lack MBs and appear as smaller ghosts. Some MBs (fainter, bright contrast) are slightly out of plane of focus. Clumped nuclei (n) are apparent remnants of leucocytes. Phase-contrast, × 510.

Fig. 2.

Camel erythrocytes in mammalian Ringer’s solution (dilution of whole blood required for observation of individual cells). Typical cells are flattened, anucleate, and elliptical, with the long axis of the ellipse in the range of 7-9 micrometres. Phase-contrast, × 1420.

Fig. 2.

Camel erythrocytes in mammalian Ringer’s solution (dilution of whole blood required for observation of individual cells). Typical cells are flattened, anucleate, and elliptical, with the long axis of the ellipse in the range of 7-9 micrometres. Phase-contrast, × 1420.

Fig. 3.

Lysed camel cell preparation at same magnification as Fig. 2, showing MB with axial dimensions similar to intact cells, and smaller ghosts lacking MBs (arrows). Phase-contrast, × 1420 (mag. bar as in Fig. 2).

Fig. 3.

Lysed camel cell preparation at same magnification as Fig. 2, showing MB with axial dimensions similar to intact cells, and smaller ghosts lacking MBs (arrows). Phase-contrast, × 1420 (mag. bar as in Fig. 2).

Fig. 4.

Examples of MBs with associated dense, thickened regions typical of preparations from this young camel. Different MBs contained from 1 to 4 such enlarged areas always protruding inward. Most ghosts lacking MBs appear empty (arrow) but some contain dense granules (g). Phase-contrast, × 1420 (mag. bar as in Fig. 2).

Fig. 4.

Examples of MBs with associated dense, thickened regions typical of preparations from this young camel. Different MBs contained from 1 to 4 such enlarged areas always protruding inward. Most ghosts lacking MBs appear empty (arrow) but some contain dense granules (g). Phase-contrast, × 1420 (mag. bar as in Fig. 2).

Fig. 5.

Examples of MBs with associated dense, thickened regions typical of preparations from this young camel. Different MBs contained from 1 to 4 such enlarged areas always protruding inward. Most ghosts lacking MBs appear empty (arrow) but some contain dense granules (g). Phase-contrast, × 1420 (mag. bar as in Fig. 2).

Fig. 5.

Examples of MBs with associated dense, thickened regions typical of preparations from this young camel. Different MBs contained from 1 to 4 such enlarged areas always protruding inward. Most ghosts lacking MBs appear empty (arrow) but some contain dense granules (g). Phase-contrast, × 1420 (mag. bar as in Fig. 2).

Fig. 6.

One of many camel MBs twisted into ‘figure-8’ shape, A, FOCUS at upper surface of MB, showing cross-over from upper right to lower left; B, focus at lower surface, with cross-over reversed. Phase-contrast, × 1420 (mag. bar as in Fig. 2).

Fig. 6.

One of many camel MBs twisted into ‘figure-8’ shape, A, FOCUS at upper surface of MB, showing cross-over from upper right to lower left; B, focus at lower surface, with cross-over reversed. Phase-contrast, × 1420 (mag. bar as in Fig. 2).

To test the possibility that this concentration of MBs might be typical only of young animals, comparison was made with preparations of erythrocytes from a much older camel (27 years; Fig. 7). In general there was little difference, with approximately one clearly visible MB per 33 cells (total count: 22 MBs, 717 cells). On these MBs, however, the dense, thickened regions seen in the lysed cells of the younger camel were not observed. The MBs varied to some extent in size and in apparent thickness within a given preparation, and on rare occasions large MBs, almost twice the length of most others (major axis of ellipse), were noted (Fig. 8). In a single instance an elliptical MB was observed surrounding an elliptical nucleus. This nucleus and MB were apparently attached to one another by material not visible in phase-contrast (Fig. 9), as verified by observing their conjoined movement in medium flowing under the coverslip. Several other MBs contained within their boundaries large granules or droplets (Fig. 10) which remained trapped in position as material moved in a flow.

Fig. 7.

Low-magnification view of lysed cell preparation from older camel, as seen in darkfield. MBs appear as bright ellipses. Note bright granules in some of the ghosts in background, × 510 (mag. bar as in Fig. 1).

Fig. 7.

Low-magnification view of lysed cell preparation from older camel, as seen in darkfield. MBs appear as bright ellipses. Note bright granules in some of the ghosts in background, × 510 (mag. bar as in Fig. 1).

Fig. 8.

One of several extra large MBs observed in camel blood preparations; compare with typical MB nearby (arrow). Phase-contrast, × 960.

Fig. 8.

One of several extra large MBs observed in camel blood preparations; compare with typical MB nearby (arrow). Phase-contrast, × 960.

Fig. 9.

An elliptical nucleus (n) surrounded by large, elliptical MB. A typical camel MB (arrow) and presumed leucocyte nuclei (ln) are present. Phase-contrast, × 960 (mag. bar as in Fig. 8).

Fig. 9.

An elliptical nucleus (n) surrounded by large, elliptical MB. A typical camel MB (arrow) and presumed leucocyte nuclei (ln) are present. Phase-contrast, × 960 (mag. bar as in Fig. 8).

Fig. 10.

Lysed camel erythrocytes with MBs; one contains a dense droplet, apparently trapped between transparent sheets of material traversing MB. Phase-contrast, × 960 (mag. bar as in Fig. 8).

Fig. 10.

Lysed camel erythrocytes with MBs; one contains a dense droplet, apparently trapped between transparent sheets of material traversing MB. Phase-contrast, × 960 (mag. bar as in Fig. 8).

Fig. 11.

MB of lysed guanaco erythrocyte. Phase-contrast, × 960 (mag. bar as in Fig 8).

Fig. 11.

MB of lysed guanaco erythrocyte. Phase-contrast, × 960 (mag. bar as in Fig 8).

For comparative purposes, observations were also made on llama and guanaco erythrocytes. The llama blood contained many cells with readily identifiable MBs, but with fewer MBs per total number of cells than in the camel blood. Blood from one guanaco also had many MB-containing cells, though again in lower percentage than the camels, while in that of another guanaco scarcely any MBs were observed.

TEM observation of uranyl acetate-stained whole mounts confirmed the presence of MBs in some lysed camel cells and their absence in the majority. TEM revealed that the MBs were not really cell-free structures as suggested by phase-contrast microscopy, but rather were associated with a network of stained material (Fig. 12). This material appeared to extend just beyond the boundary of the MB, as if the MB were trapped within a collapsed sac (Fig. 13). In some lysed cells there were regions in which the material coating the MB was less dense and the MB somewhat flattened, generally near or within the highly curved ends of the ellipse. In such cases it was possible to resolve the longitudinal array of microtubules constituting the MB bundle (Figs. 14, 15). Where individual microtubules could be distinguished, their diameter was in the range of 22·0-24·0 nm (Fig. 14). Material comprising those lysed cells which lacked MBs was qualitatively different from that encompassing MBs in that it typically exhibited denser, more uniform staining (Fig. 16). Thickened regions were observed along many MBs of the young camel’s erythrocytes in the uranyl acetate-stained whole mounts (Fig. 12), probably corresponding to those observed under phase contrast in the same preparation.

Fig. 12.

Uranyl-acetate-stained whole mount of lysed camel erythrocyte, as seen in TEM. The MB is a continuous, densely staining peripheral band, traversed by a network of irregularly stained material in which there is a fold (f). Dense enlargement on inner side of MB (b) is distinguished from stained contaminant (X). TEM, × 13000.

Fig. 12.

Uranyl-acetate-stained whole mount of lysed camel erythrocyte, as seen in TEM. The MB is a continuous, densely staining peripheral band, traversed by a network of irregularly stained material in which there is a fold (f). Dense enlargement on inner side of MB (b) is distinguished from stained contaminant (X). TEM, × 13000.

Fig. 13.

Higher-magnification view of the area within the rectangle in Fig. 12. Outer surface of the MB is coated with network material (arrow). TEM, × 92000.

Fig. 13.

Higher-magnification view of the area within the rectangle in Fig. 12. Outer surface of the MB is coated with network material (arrow). TEM, × 92000.

Fig. 14.

Region at one end of MB in which overlying material is less dense and the MB flattened, permitting view of individual microtubules (mt). The MB here is about 10 microtubule diameters in width. Uranyl acetate staining; TEM, × 92000.

Fig. 14.

Region at one end of MB in which overlying material is less dense and the MB flattened, permitting view of individual microtubules (mt). The MB here is about 10 microtubule diameters in width. Uranyl acetate staining; TEM, × 92000.

Fig. 15.

Comparable views of lysed cells with and without MBs. A rough network of material appears to encompass and coat the MB (Fig. 15); material comprising ghosts which lack MBs is more electron-dense and uniform (Fig. 16). Uranyl acetate staining; TEM, × 58000.

Fig. 15.

Comparable views of lysed cells with and without MBs. A rough network of material appears to encompass and coat the MB (Fig. 15); material comprising ghosts which lack MBs is more electron-dense and uniform (Fig. 16). Uranyl acetate staining; TEM, × 58000.

Fig. 16.

Comparable views of lysed cells with and without MBs. A rough network of material appears to encompass and coat the MB (Fig. 15); material comprising ghosts which lack MBs is more electron-dense and uniform (Fig. 16). Uranyl acetate staining; TEM, × 58000.

Fig. 16.

Comparable views of lysed cells with and without MBs. A rough network of material appears to encompass and coat the MB (Fig. 15); material comprising ghosts which lack MBs is more electron-dense and uniform (Fig. 16). Uranyl acetate staining; TEM, × 58000.

Thin sections

Marginal band microtubules, 22·5-25·0 nm in diameter, are clearly visible in thin sections of some camel blood cells (Fig. 17). A clear zone or halo of lighter density than the rest of the cytoplasmic matrix surrounds each microtubule. Longitudinal sections of microtubules indicate that the zone of lighter density extends along the length of the microtubule. The combined diameter of the microtubule plus the clear zone as measured in cross-section is about 31·5-35·0 nm. The marginal band microtubules were readily apparent in cells identified as circulating reticulocytes, which are less electron-dense than the mature erythrocytes due to a lower haemoglobin concentration. The number of microtubules seen in cross-section generally ranged from 7-22, although clusters of as many as 35-55 were occasionally seen (Fig. 17). There did not seem to be any correlation between the number of polysomes and the number of MB microtubules visible in reticulocytes; cells with very few polysomes had as many microtubules as did less mature reticulocytes with many polysomes. MB microtubules were rarely observed in fully differentiated erythrocytes, and the contrast between microtubule and background haemoglobin was less striking than in the younger cells, perhaps due to a diminution of the halo or clear zone around the microtubules.

Fig. 17.

Thin section through whole, fixed camel erythrocytes. MB microtubules (mt) are evident in cross- and oblique section at opposite ends of central cell. A clear zone or halo of lighter density than the rest of the cytoplasmic matrix surrounds many of the microtubules. Note difference in haemoglobin concentration (electron density) between this cell and others, indicating that MB-containing cell is a reticulocyte. TEM, × 28000. Inset: Higher-magnification view of microtubules in upper right of Fig. 17. TEM, × 48000.

Fig. 17.

Thin section through whole, fixed camel erythrocytes. MB microtubules (mt) are evident in cross- and oblique section at opposite ends of central cell. A clear zone or halo of lighter density than the rest of the cytoplasmic matrix surrounds many of the microtubules. Note difference in haemoglobin concentration (electron density) between this cell and others, indicating that MB-containing cell is a reticulocyte. TEM, × 28000. Inset: Higher-magnification view of microtubules in upper right of Fig. 17. TEM, × 48000.

Direct light-microscopic observation of camel erythrocytes lysed under microtubule-stabilizing conditions shows that MBs are present in at least 3% of the cells. Correspondingly, MB microtubules are consistently seen in thin sections of those red blood cells identified as reticulocytes, but only rarely in those which appear fully mature as judged by high cytoplasmic electron density due to haemoglobin iron and by absence of ribosomes or other organelles. The possibility exists that the high background density, and/or lack of a clear zone, obscures the presence of MB microtubules in mature cells, and that all of the elliptical erythrocytes actually contain MBs. However, one would then have to assume that a majority of MBs solubilize in the lytic medium. This is unlikely, as experiments with a wide range of vertebrates show that the same medium stabilizes MBs in all of the erythrocytes in a given preparation (Cohen et al. 1977; Cohen, 1978).

The observed occurrence of MBs in both lysed cell preparations and thin sections of intact cells is consistent with a role of MBs in establishing the elliptical morphology during differentiation, with possible subsequent loss of MBs as the cells mature and age. This is supported by the observation that all of the lysed or thin-sectioned cells are initially elliptical whether or not MBs are visible, suggesting that the MB is not required for maintenance of ellipticity in this system. Behnke (1970) also reported conditions under which ellipticity of chick erythrocytes was retained in the apparent absence of MB microtubules.

The proposed interpretation is in agreement with that put forward by Barrett & Dawson (1974) for chicken erythrocytes. Here the chick cells will lose their elliptical shape shortly after differentiation in response to certain agents, but are resistant to shape change after a period of maturation. In addition, microtubule number diminishes after the differentiating cells attain an elliptical morphology. The number of MB microtubules is similarly reduced during differentiation of larval erythrocytes in the rainbow trout (Yammoto & Luchi, 1975). Although there does not seem to be any reduction in number of microtubules during maturation of the camel reticulocyte in peripheral blood, microtubules have rarely been identified in fully mature erythrocytes. Furthermore, a comparison has not been made with more immature cells which would be found in the bone marrow. However, based upon the results reported in other species, one would predict that erythropoietic cells in camel bone marrow contain MB microtubules, possibly in greater number per cell and in a greater percentage of cells than in the peripheral blood. Goniakowska-Witalinska & Witalinski (1976) state that microtubules occur temporarily in the course of erythropoiesis in the llama and suggest that ellipticity is induced by the temporary presence of these microtubules. One indication that camels may typically release immature erythrocytes into the peripheral blood is the occurrence of nucleated red cells in smears stained with Wright’s stain (Andrew, 1965). The nucleated MB-containing lysed cell observed in this study (Fig. 9) was probably the product of one such cell.

One can only speculate as to why MBs were not observed by Goniakowska-Witalirtska & Witalihski (1976) in camel erythrocytes. Apart from the possibility of fixation or thin-section sampling artifacts, it may be that not all camels are identical with respect to the percentage of cells containing MBs or that there is variation within the same camel at different times. Although in the present study of lysed cells both camels exhibited similar numbers of MBs, one of the two guanacos examined had considerably greater numbers of MBs than the other.

The probable loss of MBs during final stages of erythrocyte differentiation implies that changes occur in surface-associated cellular material, perhaps by alteration of molecular constituents or molecular cross-linking, so as to maintain elliptical morphology once it is established by the MB system. Such a process could account for the difference in appearance between the network of MB-associated material in lysed cell whole mounts and that of more uniformly electron-dense cell ghosts or remnants lacking MBs, as observed with TEM. An alternative possibility for this difference in appearance, however, could be shrinkage or contraction of the cell surface material in lysed cells lacking MBs. This would also account for the reduced size of such ghosts as compared with intact erythrocytes.

The network of material enclosing MBs of lysed camel erythrocytes is morphologically similar to that observed in the semi-lysed, nucleated elliptical erythrocytes of fish and amphibians, and referred to previously as trans-band material or TBM (Cohen, 1978; Cohen et al. 1977). It has been postulated that a TBM network is normally under tension in such cells, applying force asymmetrically across the MB so as to deform an otherwise more circular MB into an ellipse. The presence of a TBM correlate in elliptical mammalian erythrocytes is consistent with this hypothesis. Similarly, the figure-8 camel MB configuration corresponds to figure-8 MBs observed in semilysed erythrocytes of many non-mammalian species and interpreted as excessive deformation of the MB due to extreme TBM shrinkage or contraction (Cohen, 1978).

The occurrence of MBs in camel erythrocytes is possibly correlated with ontogeny of distinctive physiological properties. Camels are adapted to survive extreme dehydration and rapid rehydration. Their erythrocytes can withstand considerable osmotic stress, responding in a manner more similar to the elliptical, nucleated erythrocytes of non-mammalian vertebrates than to the biconcave diskoidal cells typical of other mammals (Ponder, 1942; Trotter, 1956). Camel erythrocytes are highly resistant to hypotonic haemolysis (Perk, 1963; Yagil, Sod-Moriah & Meyerstein, 1974), exhibit a low rate of water transport (Naccache & Sha’afi, 1974), and are also relatively stable under hypertonic conditions, in which crenation was not observed (Yagil et al. 1974). In addition, very young camels (6 months or less) apparently possess 2 populations of erythrocytes with respect to osmotic resistance: one population with adult-type response, the other with still greater haemolytic resistance (Perk, 1966). Direct studies of the possible correlation between occurrence of MBs and osmotic resistance in camel erythrocytes would therefore appear to have potential value for understanding MB and erythrocyte function.

It is of interest to consider whether the family Camelidae is unique among the mammals in having MBs associated with erythrocyte structure, or whether the Camelidae have simply come under closer scrutiny because their mature erythrocytes are elliptical. Grasso (1966) reported that MBs were present in nucleated foetal erythroblasts of rabbits, a species in which mature adult erythrocytes have the anucleate biconcave diskoidal structure typical of most mammals. More extensive investigation of the possible role of MBs during mammalian erythrogenesis in general would therefore seem warranted.

W. D. Cohen wishes to express his gratitude to Dr Emil P. Dolensek, Veterinarian of the New York Zoological Society, for his interest and generous cooperation in providing blood samples from animals at the Bronx Zoological Park. The helpful discussion and assistance of I. Nemhauser, J. Hoffman, R. Mawe, and R. I. Sha’afi, as well as support of this work by grant 11619 of the PSC/BHE City University of N.Y. Research Award Program and by NIH grant HL 20902 from the National Heart, Lung, and Blood Institute to Dr Cohen, is gratefully acknowledged. Part of this work was supported by NIH Training Grant 5-707-GM-00723 to the University of Wisconsin Anatomy Department and N. Barclay Terwilliger is grateful to Dr David B. Slautterback for his encouragement.

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Terminology in the literature is often confusing with respect to erythrocyte shape, referring to ‘round disks’, ‘elliptical disks’, and ‘disk-shaped’ elliptical cells. The terms ‘disk’ and ‘diskoidal’ should be reserved for cells which are flattened and circular; a disk cannot be elliptical.