6 Growth Regulation by Insulin and TOR Signaling in Drosophila

Abstract

Growth, defined as increase in mass, is a fundamental biological process that influences organism, tissue, and cell size. Thus, growth must be regulated to ensure that animals grow to optimal sizes and that their composite tissues are both appropriately sized and correctly proportioned. For example, if the heart were too small or the limbs too long, the circulatory system would fail. Likewise, although cell size can vary considerably, there are both minimum and maximum sizes at which individual cells will become nonviable or unable to perform their biological function. For example, neurons that are too small may fail to make the required connections, and differentiating cells that are too large may contact too many other cells and therefore receive multiple inductive signals. The observation that individuals of the same species and genotype tend to grow to a highly predictable size suggests that growth is under genetic control. However, growth and final body size are also influenced by external factors, including nutrient levels and temperature. Thus, growth is an essential process that is under genetic control and can be modulated in response to environmental influences.

Tissue growth is accompanied by increases in cell size and/or cell number and/or the accumulation of extracellular matrix (e.g., accretion during bone growth). The term cell growth has been defined as an increase in cell mass and can be used to discuss the growth of unicellular organisms or cells in culture. However, during metazoan tissue growth, individual cells within growing tissues can actually become progressively smaller...