The fine-structure of the mitotic spindle in sea urchin eggs

The mitotic apparatus of dividing eggs of Arbacia punctulata has been observed with the electron microscope in ultrathin sections of the cells fixed in subzero ethanol, sea water-OsO 4, and buffered, isotonic OsO 4. Marked differences in appearance obtain among the different preparations. In general, ethanol tends to aggregate and coarsen fine-structure, and OsO 4 to disperse it. Features of spindle ultrastructure common to all preparations, and hence assumed to reflect conditions in vivo are: (1) very large numbers of small, dense particles, assumed to be ribonucleoprotein; (2) general fine-structure identical with that of the ground cytoplasm elsewhere in the cell; (3) exclusion from the forming and completed spindle of all cytoplasmic particulates larger than a few hundred Å; and (4) a fibrous (or finely tubular) component, probably connecting kinetochores with the centriolar region, the fibers being associated along their entire length with the RNP particles.