The marginal band (MB) of nucleated erythrocytes (thos of nonmammalian vertebrates) is a continuous peripheral bundle of microtubules normally obscured by hemoglobin. Treatment of these elliptical cells with modified microtubule polymerization media containing Triton X-100 yields a semilysed system in which MB, nucleus, and trans-MB material (TBM) are visible under phase contrast. The TBM apparently interconnects structural components, passing around opposite sides of the nucleus and suspending it in native position. In uranyl acetatestained whole whole mounts (goldfish) examined by transmission electron microscopy, the TBM appears as a network. MBs of semilysed cells are relatively planar initially, but twist subsequently into a range of "figure-8" shapes with one of the two possible mirror-image configurations predominant. Nuclei and MBs can be released using proteolytic enzymes, to which the TBM seems most rapidly vulnerable. MBs thus freed are birefringent, generally untwisted, and much more circular than they are in situ. As a working hypothesis, it is prosposed that the flattened, elliptical shape of nucleated erythrocytes is a result of TBM tension applied asymmetrically across an otherwise more circular MB, and that the firure-8 configuration occurs when there is extreme TBM shrinkage or contraction.

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