F-actin in early echinoderm development

Richard Anthony Heil-Chapdelaine, Purdue University

Abstract

There are morphological and mechanical changes during maturation of starfish oocytes and fertilization of sea urchin eggs which are mediated by the remodeling of the F-actin cytoskeleton. I characterized these changes by quantifying changes in the relative levels of F-actin during maturation of starfish oocytes and during fertilization of sea urchin eggs by quantitative fluorescence microscopy. In response to 1-methyladenine, transient F-actin filled spikes project from the surface of the starfish oocyte via de novo polymerization of actin. After spike retraction, at the time that the germinal vesicle breaks down, the level of F-actin within the whole oocyte decreases to below that of the immature oocyte. The timing of this decrease in the level of F-actin correlates with the decrease in oocyte stiffness that occurs at this time and may contribute to the change in stiffness. Although the oocyte increases in stiffness during the formation of polar bodies, there is no change in the level of F-actin during polar body formation. Therefore, there may be a secondary F-actin based mechanism for increasing the stiffness of the oocyte during the formation of polar bodies. Many of the changes in F-actin levels can be uncoupled from maturation. Spike formation can be bypassed by inducing the oocytes to resume meiosis with dithiothreitol, and germinal vesicle breakdown can occur in the absence of spike retraction if oocytes are pretreated with forskolin. A decrease in cortical F-actin to below the levels in the immature oocyte can be induced before or in the absence of GVBD. Finally, germinal vesicle breakdown occurs in oocytes microinjected with phalloidin which prevents depolymerization of F-actin. During fertilization of sea urchin eggs, an increase in cortical F-actin occurs which correlates with the elongation of F-actin filled microvilli. The increase in cortical F-actin occurs in the presence of a cellular decrease in F-actin. These observations suggest that a small shift in F-actin from an unstable pool to a stable pool may be responsible for the increase in cortical F-actin during fertilization.

Degree

Ph.D.

Advisors

Otto, Purdue University.

Subject Area

Zoology|Anatomy & physiology|Animals

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