Rheological properties of echinoderm eggs during cell division

Multi-scale mechanics from molecules to morphogenesis

Dynamic mechanical processes shape the embryo and organs during development. Little is understood about the basic physics of these processes, what forces are generated, or how tissues resist or guide those forces during morphogenesis. This review offers an outline of some of the basic principles of biomechanics, provides working examples of biomechanical analyses of developing embryos, and reviews the role of structural proteins in establishing and maintaining the mechanical properties of embryonic tissues. Drawing on examples we highlight the importance of investigating mechanics at multiple scales from milliseconds to hours and from individual molecules to whole embryos. Lastly, we pose a series of questions that will need to be addressed if we are to understand the larger integration of molecular and physical mechanical processes during morphogenesis and organogenesis.

Unequal cell division regulated by the contents of germinal vesicles

Fertilization occurs during meiosis in many animals, when maternal centrosomes participate in the formation of spindles at the animal pole, which results in polar body formation. Paternal centrosomes do not participate in cell division during oocyte maturation. After meiosis, they form the spindles while the maternal centrosomes are discarded. It is unknown why paternal centrosomes do not form spindles during meiosis. Here, we show that the artificial incorporation of sperm at the animal pole of immature starfish oocytes causes unequal cell division and the formation of polar body-like fragments. The removal of germinal vesicles from the animal pole blocks the formation of polar body-like fragments. Furthermore, translocation of germinal vesicles to the vegetal pole by centrifugation induces the extrusion of polar body-like fragments from the vegetal pole, where sperm penetration is prerequisite. After germinal vesicle breakdown, cyclin B is localized in the maternal and paternal asters and spindles near the germinal vesicle. These results suggest that germinal vesicle components such as the cdc2-cyclin B complex interact with asters and spindles and can induce unequal cell division. During normal fertilization, paternal centrosomes are likely kept away from the germinal vesicle components, resulting in the inhibition of unequal paternal centrosome-dependent cell division.

Mechanical basis of cell shape: investigations with the scanning acoustic microscope

The shape of cells during interphase in sparse cultures often resembles that of fried eggs. XTH-2 cells, which have been derived from tadpole heart endothelia, provide a typical example of this type of shape. To understand the physical basis of this shape, the cytoskeleton of these cells has been investigated in detail. Subcellular elasticity data have been achieved by scanning acoustic microscopy (SAM). Their changes were observed during treatment of the cells with microtubule-disrupting agents (colcemid and low temperature), and shape generation in giant cells produced by electro-fusion was observed with SAM, revealing the role of the nucleus as a force centering organelle. From these observations combined with well-documented observations on cellular dynamics described in the literature, a model is developed explaining the fried-egg shape of cells by means of interacting forces and fluxes (cortical flow, bulk flow of cytoplasm, microtubule-mediated transport of cytoplasm) of cytoplasm. The model also allows the comprehension of the increase of tension in cells treated with colcemid.

Scanning acoustic microscopy visualizes cytomechanical responses to cytochalasin D

The reactions of XTH-2 cells (line derived from Xenopus laevis tadpole hearts) to cytochalasin D (CD) were followed using scanning acoustic microscopy (SAM) at 0.9 GHz, fluorescence and electron microscopy. The first reaction to CD which can be detected by SAM is a loss of image contrast, indicating a decrease in acoustic impedance of about 30%. Based on structural changes revealed by staining of actin with TRITC-phalloidin, and taking theoretical considerations into account, a relationship between impedance decrease and tension in the actin fibrillar system is deduced.