Introduction

Fibres in development

The skeletal systems of animals and plants contain extracellular fibres. Throughout development these fibres are somehow manipulated into precise directions within the supporting tissues, so as to be best adapted to their mechanical and optical functions. How this is done constitutes the subject of this book. The problem, which is unsolved, is a facet of developmental biology which is usually neglected by authors of textbooks. This control of fibre orientation occurs in plants as well as animals: in plant cell walls (from algae to timber), in the cornea in birds and other vertebrates, in human bones, in basement lamellae, in arthropod exoskeletons, and in many other systems. It is therefore a truly interdisciplinary topic in biology, relevant to researchers in all of these fields and many more. Without accurate control of fibre architecture, many animals and plants would collapse.

Those macromolecules whose functions are skeletal, such as the polysaccharides cellulose and chitin and the protein collagen, are very long. They assemble in parallel to form microfibrils, fibrils, or fibres, depending upon the particular system involved. Their intrachain backbone bonds are covalent, whereas the lateral interactions between chains are by interchain hydrogen bonds. Such assemblies have a tendency to split in a fibrous manner, like wood, if pulled in the most vulnerable direction. This is because both the strength and stiffness of fibrous assemblies are about ten times weaker when stretched across rather than along the length of the molecular backbone. Furthermore, the continuous matrices which glue the individual fibrous components together are weaker still.