The mechanical consequences of cytokinesis
For a mulitcellular organism the only time they under go a mechanical change that spans their entire being is during their first cell division. Cytokineses, the process of splitting a cell in two is driven by tiny pico newton forces generated by motor proteins that slide cytoskeletal filaments. (Fig. 1.2) This mechanical process has fascinated scientists for over a century and we have unraveled many secrets of cytokinesis \cite{Pollard_2010}. The classic model for how eukaryotic cells generate the force required to pinch themselves in two is through sliding filaments of actin with the motor protein Myosin II. Many eukaryotes from amoebas, yeast, and humans, use this method to pinch themselves in two, however, many organism have found other methods such as plants which rely on membrane vesicle fusion and the addition of extracellular cell wall material \cite{Assaad_2001}, or the slime mold which in addition to filament sliding can also use traction forces to drive cytokinesis \cite{Reichl2008,Neujahr1997}. Yeast can also divide with several disabled myosin motors \cite{Lord2004}. Surprisingly, successful cytokinesis can occur in vertebrates with Myosin II mutants that produce tension within the ring but cannot translocate actin filaments \cite{Ma_2012}. In addition to actin and Myosin II there are many additional genes involved in cytokinesis. The most robustly characterized eukaryotic cytokinesis is the tiny fission yeast where over 150 genes are involved \cite{Pollard_2010a}. Through studies in yeast and many other organism we have found that in order for a cell to successfully divide it must overcome several obstacles. First the cell must properly position the contractile ring. Second the cell must assemble the ring and connect it to the membrane. Third, contraction of the ring must be established, tuned, and maintained while ring disassembly occurs as the ring grows smaller and smaller. Finally, the membrane must be fused to separate the daughter cells. Most of the research in this field has focused on this first division of multicellular creatures or division in single cell organisms \cite{Pollard_2010,Rappaport_1996,Green_2012}. While this research has been extremely useful there is a finite amount we can learn about cytokinesis in isolated cells. For a multicellular being the moments after the first division provide a new challenge, a neighboring cell! Most of the human cells that divide in our body have many neighboring cells, not just one, this posses all sorts of obstacles for a dividing cell. How does the dividing cell overcome the resistive forces of neighboring cells? Do neighboring cells assist with cell division? How does the tissue maintain its barrier functions as cells within it divide? Chapter two of my dissertation aims to answer several of these questions.