Organization of actin filaments during polar body formation in eggs of Tubifex (Annelida, Oligochaeta).

Microfilaments in the Tubifex egg undergoing polar body formation can be decorated with heavy meromyosin (HMM). Since the HMM-arrowheads on filaments are dissociated by rinsing with ATP, they are identified as actin filaments. From metaphase through telophase of the second meiotic division, actin filaments are present in the cortex and in the meiotic apparatus. Consistent with the previous observations [26, 27], cortical filaments show stage-specific localization patterns. At metaphase the thick filamentous layer is localized at the animal pole, that means the region overlying the peripheral aster of the meiotic apparatus; filaments are organized in a meshwork. Upon the onset of anaphase chromosome movement, the filament meshwork mostly disappears and the remaining filaments are found to be solitary. Surrounding the animal pole region, but not extending over the animal pole, more and more filaments reappear during anaphase. As the polar body forms at the animal pole, filaments surrounding its base are organized into small bundles which are oriented randomly. By early telophase these filaments come to run circumferentially in the cortical layer (i.e. they form the contractile ring) engirding the cleavage furrow which develops at the base of the polar body. In the remaining cortex, filaments run along the surface and are oriented randomly. At all stages, adjacent filaments are often found to have opposite polarities; in favorable sections, HMM-decorated filaments appear to attach to the oolemma. The mechanism underlying the filament alignment of the contractile ring is discussed in relation to formation of the polar body.