Bipolar segregation of mitochondria, actin network, and surface in the Tubifex egg: Role of cortical polarity

Abstract

The role of the cortex in ooplasmic segregation of the yolky eggs of Tubifex has been studied by epifluorescence microscopy. Living eggs labeled with rhodamine 123 and fine carbon particles placed on the surface showed that, following the second polar body formation, the egg surface cosegregates with subcortical mitochondria in a bipolar fashion, viz. toward the animal and vegetal poles in the animal and vegetal hemispheres, respectively. The egg surface of each pole moves spirally while the equatorial surface appears to remain stationary during this process. The rhodamine-phalloidin staining of whole eggs reveals that actin networks cosegregate with mitochondria. Isolated cortices which were stained with rhodamine-phalloidin demonstrated that cortical actin is organized bipolarly and that, during ooplasmic segregation, it undergoes reorganization directed toward both poles of the egg. The cortical polarity expressed as actin organization is not disrupted by centrifugal force sufficient to stratify the egg cytoplasm into five layers. The surface of a centrifuged egg moves according to the original cortical polarity. This surface movement is accompanied by the reorganization of cortical actin which appears to be identical to that in intact eggs. Other centrifugation experiments have demonstrated that the connection of the subcortical cytoplasm to the cortex is resistant to a centrifugal force of up to 650 g. The nature of cortical polarity and its role in ooplasmic segregation are discussed in the light of the present results.