<P> Many different types of eukaryotic cells undergo size - dependent transitions during the cell cycle . These transitions are controlled by the cyclin - dependent kinase Cdk1 . Though the proteins that control Cdk1 are well understood, their connection to mechanisms monitoring cell size remains elusive . A postulated model for mammalian size control situates mass as the driving force of the cell cycle . A cell is unable to grow to an abnormally large size because at a certain cell size or cell mass, the S phase is initiated . The S phase starts the sequence of events leading to mitosis and cytokinesis . A cell is unable to get too small because the later cell cycle events, such as S, G2, and M, are delayed until mass increases sufficiently to begin S phase . </P> <P> Many of the signal molecules that convey information to cells during the control of cellular differentiation or growth are called growth factors . The protein mTOR is a serine / threonine kinase that regulates translation and cell division . Nutrient availability influences mTOR so that when cells are not able to grow to normal size they will not undergo cell division . The details of the molecular mechanisms of mammalian cell size control are currently being investigated . The size of post-mitotic neurons depends on the size of the cell body, axon and dendrites . In vertebrates, neuron size is often a reflection of the number of synaptic contacts onto the neuron or from a neuron onto other cells . For example, the size of motoneurons usually reflects the size of the motor unit that is controlled by the motoneuron . Invertebrates often have giant neurons and axons that provide special functions such as rapid action potential propagation . Mammals also use this trick for increasing the speed of signals in the nervous system, but they can also use myelin to accomplish this, so most human neurons are relatively small cells . </P> <P> One common means to produce very large cells is by cell fusion to form syncytia . For example, very long (several inches) skeletal muscle cells are formed by fusion of thousands of myocytes . Genetic studies of the fruit fly Drosophila have revealed several genes that are required for the formation of multinucleated muscle cells by fusion of myoblasts . Some of the key proteins are important for cell adhesion between myocytes and some are involved in adhesion - dependent cell - to - cell signal transduction that allows for a cascade of cell fusion events . </P> <P> Oocytes can be unusually large cells in species for which embryonic development takes place away from the mother's body . Their large size can be achieved either by pumping in cytosolic components from adjacent cells through cytoplasmic bridges (Drosophila) or by internalization of nutrient storage granules (yolk granules) by endocytosis (frogs). </P>

Genes direct the growth development and maintenance of cells