Cortical differentiation of the animal pole during maturation division in fertilized eggs of Tubifex (Annelida,Oligochaeta): II. Polar body formation

Changes in the cortical organization at the animal pole are examined by scanning and transmission electron microscopy in the Tubifex egg undergoing second polar body formation. At very early anaphase of the second meiosis, the egg surface overlying the meiotic apparatus is undulated, but its neighboring surface appears to be smooth. Although a microfilamentous cortical layer is found in the smooth area, the cortical layer of the undulating area is thin and devoid of filamentous structures except for its central part where some filaments are observed. This local differentiation takes place normally in colchicine-treated eggs where the meiotic apparatus is destroyed. Along with the progression of the anaphase movement, the egg surface of the undulating area is, first, uplifted into a cone-shaped cytoplasmic bulge (presumptive polar body); then the height and surface area of the bulge gradually increase. The distal surface of the growing bulge appears to be undulated whereas the sides of the bulge are relatively smooth. Transmission electron microscopy reveals that a thick microfilamentous cortical layer is always localized at the proximal region of this bulge; other regions of the bulge are characterized by a thin cortical layer which is devoid of filamentous structure except for the apical portion of the bulge. Microfilaments at the base of the bulge are perpendicular or oblique to the egg surface. The cortical layer of the egg which is continuous to that of the proximal region of the bulge comprises microfilaments running parallel to the surface. The attainment of the bulge to its full size is followed by the development of the cleavage furrow along its base. The cleavage furrow appears to bisect the spindle midway between its poles. In cytochalasin B-treated eggs, where some cortical microfilaments are detected at the animal pole, a cytoplasmic bulge lower in height and wider in the diameter of its base than the normal one forms at the animal pole; however, it is subsequently resorbed into the egg. The formation of a cleavage furrow is not observed in these eggs. The mechanism of the polar body formation is discussed in the light of the present observations.