Elimination of Midbodies From Mitotic Erythroblasts and Their Contribution to Fetal Blood Plasma23

The formation and fate of attached and eliminated midbodies (Zwischenkörper) of mitotic erythroblasts in rat fetal liver were studied by electron microscopy. Actual and theoretical numbers of interzonal spindle fibers were determined. The actual number was about equal to the total number of fibers (x) for twice the number of chromosomes (2n) plus 9 for centriologenous fibers, or x = 4n + 9. Midbodies were flattened bands or ribbon-like and not cylindrical. Those with shorter telophase bridges remained attached to daughter erythroblasts in the late basophilic and early polychromatophilic stages of development. A pincer-like process produced by invaginations of the plasma membrane at the base of the shorter telophase bridges apparently caused the elimination of midbodies from the daughter cells. In both attached and free midbodies, dense amorphous material was distributed into a darker cortical and lighter medullary zone. Phagocytosis of midbodies was not observed. Some midbodies were carried by the flow of intercellular fluid into the perisinusoidal space of Disse. They seemed to disappear by extracellular dissolution and thereby contribute protein to the composition of fetal plasma. Studies are under way to determine the exact chemical nature of the dense amorphous material. These studies will be extended to human bone marrows with increased mitotic activity.