The cytoskeletal system of mammalian primitive erythrocytes: Studies in developing marsupials

Abstract

Seeking to resolve conflicting literature on cytoskeletal structure in mammalian "primitive" generation erythrocytes, we have utilized the circulating blood of developing marsupials. In young of the Tammar Wallaby (Macropus eugenii) and the Gray Short-tailed Opossum (Monodelphis domestica), relatively large, nucleated primitive erythrocytes constituted nearly 100% of the circulating population at birth (= day 0) and in fetuses (Tammar) several days before birth. These cells were discoidal or elliptical, and flattened except for a nuclear bulge. Their cytoskeletal system, consisting of a marginal band of microtubules enclosed within a cell surface-associated network (membrane skeleton), closely resembled that of non-mammalian vertebrate erythrocytes. By day 2 or 3, much smaller anucleate erythrocytes of "definitive" morphology, lacking marginal bands, appeared in abundance. These accounted for greater than 90% of the circulating population of both species by day 6-8. Non-nucleated erythrocytes of a different type, constituting 1-6% of the cells in most blood samples up to day 7, were identified as anucleate primitives on the basis of size, shape, and presence of a marginal band. Thus, loss of erythrocyte nuclei in mammals appears to begin earlier than generally recognized, i.e., in the primitive generation. Counts of these anucleate primitives in young of various ages implicated nucleated primitives as their probable source. Pointed erythrocytes, occasionally found in younger neonates of both species, occurred in greatest number in fetuses (Tammar) prior to birth. This is in accord with previous work on non-mammalian vertebrates suggesting that such cells are morphogenetic intermediates. The results confirm the long-suspected similarity between mammalian primitive erythrocytes and the nucleated erythrocytes of all non-mammalian vertebrates.

Yolk sac–derived primitive erythroblasts enucleate during mammalian embryogenesis

“…In contrast, definitive erythrocytes of almost all mammals, with the exception of the immature red cells of camels, 42 lack these structural features. Although marginal bands have been detected in the primitive red cells of marsupials, 23 it is not clear whether true circumferential marginal bands exist in the primitive erythroblasts of mice. 43,44 Consistent with their enucleation, murine primitive erythroblasts lose intermediate filaments during their maturation in the bloodstream.…”

“…14 Furthermore, large enucleated red cells have been noted in the bloodstream of mouse embryos by several investigators, 14,[18][19][20] raising the possibility that primitive erythroblasts might ultimately enucleate. Studies in the Syrian hamster and in marsupials indicate that primitive erythroblasts can enucleate, [21][22][23] To determine whether primitive erythroblasts enucleate in the mouse embryo and to examine the transition from primitive to definitive erythropoiesis, we raised antibodies to embryonic ␤H1-globin and optimized the immunohistochemical identification of primitive (␤H1-globin-positive) red cells. We report here that, contrary to widely held opinion, murine primitive erythroblasts enucleate and continue to circulate throughout late gestation and even into the postnatal period.…”

Tubulin pools in human erythrocytes: altered distribution in hypertensive patients affects Na+, K+-ATPase activity

“…In addition, the erythrocyte membrane in vertebrates (except mammals) is reinforced by a bundle of microtubules, termed the marginal band, which circumscribes the cortical membrane in the horizontal plane of the cell. The marginal band is thought to provide structural support for the cell membrane and the elliptical cell morphology typical of vertebrate erythrocytes [2,3]. Electron microscopy studies indicated that human erythrocytes contain neither microtubules nor marginal band [4].…”

“…Electron microscopy studies indicated that human erythrocytes contain neither microtubules nor marginal band [4]. In view of this finding, further studies on tubulins of human erythrocytes were not performed, and many researchers presumed that they do not exist [2,3,5]. However, recent proteomics studies [6] showed the presence of various isoforms of tubulin in human erythrocytes.…”

Characterization of mutations affecting embryonic hematopoiesis in the medaka, Oryzias latipes

“…The ICM is similar to the yolk sac hematopoietic tissue of higher vertebrates, since many hematopoietic transcription factors, including GATA-1 and GATA-2, are expressed in both the ICM of teleosts and the yolk sac of higher vertebrates (Detrich et al, 1995). The embryonic blood cells in teleosts are nucleated as are those in mammals, whereas the adult erythrocytes of teleosts are also nucleated, unlike the anucleated erythrocytes in mammals (Cohen et al, 1990).…”