17.11: Cells Coordinate Growth and Proliferation
Cell size is a significant factor impacting cellular design, function, and fitness. There exists some internal coordination by which cells double their masses before division, thus, achieving homeostasis. Coordination between cell growth and proliferation depends on the checkpoints in between cell cycle phases. Loss of coordination or failure in the checkpoint mechanism can drive the cell to uncontrolled growth and loss of cellular function. Like dividing cells that coordinate cellular growth, non-dividing cells in adults regulate cell size depending on their metabolic states.
In adults, the size of non-dividing muscle cells varies depending on the environmental conditions and nutritional state. Regular physical workout causes adult muscle cells to enlarge as individual myocytes grow in size due to the absence of proliferation in the myocytes themselves or the muscle stem cell population. In contrast, nutrient deficiency can severely damage the muscle cells. The size of muscle cells depends on the balance between the anabolic pathway that increases the cell size and the catabolic pathway that degrades intracellular proteins causing the cell size to reduce.
The anabolic or IFF/PI3K/AKT/mTORC1 pathway involves mTORC1 signaling that leads to protein synthesis, giving rise to a condition called- muscle cell hypertrophy. However, this increase in cell size is temporary. For the cells to maintain their size, one must regularly exercise for continuous mTORC1 signaling. Lack of exercise, starvation, or certain muscle disease triggers a catabolic pathway or Myostatin/SMAD2/3 for protein degradation. The degradation of proteins mobilizes amino acids to other cells of the body, thus, reducing the size of the skeletal muscle cells.