ReviewForces controlling organ growth and size

- Epithelial cells are embedded in a biophysical microenvironment which influences cellular behavior.
- Mammalian bones adapt to mechanical loads in size and shape.
- Studies on epithelia revealed that mechanical constraints, resulting from tissue dynamics, feed-back on tissue growth.
- Growth models for the Drosophila wing disc incorporated mechanical feedback loops to modulate growth and final size.
- In vivo measurements and manipulations of tissue mechanics remain challenging.

One of the fundamental questions in developmental biology is what determines the final size and shape of an organ. Recent research strongly emphasizes that besides cell-cell communication, biophysical principals govern organ development. The architecture and mechanics of a tissue guide cellular processes such as movement, growth or differentiation. Furthermore, mechanical cues do not only regulate processes at a cellular level but also provide constant feedback about size and shape on a tissue scale. Here we review several models and experimental systems which are contributing to our understanding of the roles mechanical forces play during organ development. One of the best understood processes is how the remodeling of bones is driven by mechanical load. Culture systems of single cells and of cellular monolayers provide further insights into the growth promoting capacity of mechanical cues. We focus on the Drosophila wing imaginal disc, a well-established model system for growth regulation. We discuss theoretical models that invoke mechanical feedback loops for growth regulation and experimental studies providing empirical support. Future progress in this exciting field will require the development of new tools to precisely measure and modify forces in living tissue systems.