5 Introduction to Morphogenesis
Cells collectively organize into higher-order structures, forming tissues and organs. The process by which tissues are given their shape is called morphogenesis, which is Greek for the generation of form. Gene regulation, (sub-)cellular effectors, and tissue-scale mechanics are the three ingredients of morphogenesis (Collinet and Lecuit (2021) Gilmour, Rembold, and Leptin (2017)). Regulation between these players goes both ways: Genes may determine expression of cytoskeletal regulators, which in turn exert forces resulting in tissue reshaping. Vice-versa, both tissue shape and forces can feed back on cytoskeletal proteins, which feed back to gene expression. Earlier in this course, you learned about patterning. In this part we’ll pick up from there to learn how cells and tissue mechanics interplay to generate ordered structures.
Despite their great internal molecular complexity, cells do a few basic things: Grow, divide, die, move around, change their shape, adhere to each other, and secrete or absorb material. This repertoire of cell behaviors is used to build higher-order structures. Additionally, non-cellular components which are generated and regulated by cells also contribute to shaping tissues and organs: Osmotic pressure and extracellular matrix proteins. For instance, bones are shaped by a specialized, stiff extracellular matrix, while the inside of eyes is filled with an osmotically active, jelly-like fluid that exerts pressure that maintains the shape of light-sensitive tissues, much like an inflated balloon.
The relatively small number of processes occurring at the cellular level has inspired the development of various computational models that treat cells as the central unit. In this chapter of the course, you’ll learn about these cell-based models.