ABSTRACT A fundamental problem in cell and organism biology is to understand how intracellular structures are properly scaled to carry out their essential functions. I propose to explore the phenomenon of organelle size, documenting the changes that occur during development and tumorigenesis, and investigating the underlying molecular mechanisms. My laboratory will undertake a systematic analysis of cell and organelle scaling during embryonic development, evaluating nuclei, spindles and other compartments in Xenopus laevis, as the ~1 millimeter diameter egg rapidly cleaves to form smaller blastomeres, which by the 15th division are reduced to 40 microns across. Which structures have constant dimensions, and which change their size as cells become smaller? Other model organisms and cancer cells will be compared to generate a survey of organelle scaling in normal and abnormal cell growth states. Cytoplasmic egg and embryo extracts of X. laevis and the related, smaller frog X. tropicalis will be used to monitor nuclear, spindle and cellular compartment scaling in vitro. This approach is prompted by our observation that meiotic extracts prepared from X. tropicalis eggs generate spindles that are ~30% shorter than those in X. laevis reactions using the same chromosome source, and mixing experiments have revealed a dynamic, dose-dependent regulation of spindle size by cytoplasmic factors. We will determine which organelles in addition to the spindle are scaled in X. laevis and X. tropicalis extracts, and use activity-based assays to identify the factors responsible for the observed differences. Candidate factors will be tested for their roles in organelle scaling during development and cancer progression, and computational approaches applied to model our observations. These studies will provide novel insight into how cell/organelle scaling contributes to intracellular morphogenesis and cell division, processes essential for viability and development, and defective in human diseases including cancer.