Gravity Affects Cellular Organization During Growth in Xenopus laevis Oocytes

Abstract

Intracellular organization is controlled in part by physical properties of cells, from the cytoskeleton determining cell shape to the cellular membrane serving as a selective barrier between the cell and its surroundings. In Xenopus laevis oocytes, an extensive nuclear viscoelastic actin network regulates the organization of nucleolar bodies throughout the growth of the oocyte, and gravitational creep results in a loss of nuclear organization. The spatial and size distributions of these nucleoli impact their function, and are well characterized for late stage oocytes, namely stage VI. However, the distributions within early stage oocytes are still poorly understood. Here we examine stage I and stage IV oocytes, in order to quantify the spatial and size distributions of their nucleoli. We use fluorescent microscopy to assess the spatial distribution, and find measures of the centrality and spread of nucleoli within the nucleus. We find that the distribution becomes more compact and shifts from the center to the edges of the nucleus with increasing centrifugation, due to the actin network’s ability to withstand only a certain amount of force, indicative of the gravitational creep it undergoes in the frog under natural conditions. We also examine the size distribution of nucleoli in stage IV oocytes, and find that it is much narrower than in stage VI oocytes due to the fact that fewer nucleoli have fused together. Physical parameters of the nuclei and nucleoli are determined, representing the first thorough investigation of early stage oocytes.