It has been long recognized that cell spreading acts as a mitogenic signal to endothelial cells (ECs) and other cell types in culture. However, the specific biophysical signaling by which changes in cell shape regulate the cell cycle remains poorly understood. Our aim was to assess the roles of cytoskeletal mechanics and nuclear volume as biophysical mechanisms underlying cell shape control of proliferation in single ECs. To this end, cells were cultured on micropatterned adhesive islands with different levels of spreading and elongation. Cell spreading induced parallel increases in cytoskeletal stiffness, nuclear volume, and DNA synthesis, and chromatin decondensation. In contrast, cell elongation induced a marked cytoskeletal softening without any changes in nuclear volume, chromatin condensation or proliferation. The results from this work strongly suggest that nuclear swelling and the associated chromatin decondensation are necessary for the process of G1-S phase transition. Importantly, our data also show that cytoskeletal stiffening alone cannot explain this process, although cytoskeletal tension could play a role, among other mechanisms, by transmitting mechanical stresses to the nucleus. These results shed light to the geometrical control of proliferation in ECs, a crucial driving mechanism of angiogenesis during embryonic development, wound healing and tumor growth.