Very little is known about how the size of an organism, or a specific tissue in an organism, is regulated. Coordinating and regulating the size of tissues is necessary for proper development, wound healing, and regeneration. Defects in a tissue-size regulation mechanism could lead to birth defects or cancer. In addition, there is a strong psychological aspect to some areas of tissue size regulation, as many cosmetic surgery procedures involve enlarging or reducing the size of some body parts. This review addresses the little bit that we know about size regulation. A key concept is that the size of a tissue is the size of the component cells multiplied by the number of those cells. This breaks the size regulation problem down to two parts. The size of cells can be regulated by nutrient sensing and secreted factors, and may have an upper limit due to an upper limit of a genome's ability to produce mRNA's and thus proteins. To regulate the number of cells in a tissue, there are several simple theoretical models involving secreted factors. In one case, the cells can secrete a characteristic factor and the concentration of the factor will increase with the number of cells secreting it, allowing the tissue to sense its own size. In another scenario, a specific cell secretes a limited amount of a factor necessary for the survival of a target population, and this then limits the size of the target population. There are currently several examples of secreted factors that regulate tissue size, including myostatin, which regulates the amount of muscles, leptin, which regulates adipose tissue, and growth hormone and insulin-like growth factors which regulate total mass. In addition, there are factors such as the <> found in Dictyostelium that regulate the breakup of a tissue into sub-groups. A better understanding of how these factors regulate size will hopefully allow us to develop new therapeutic procedures to treat birth defects or diseases that affect tissue size.
