Mutable collagenous tissues in sea cucumbers

Echinoderms possess connective tissues with remarkable abilities to change their mechanical properties rapidly and reversibly under the control of the nervous system, a feature unique across the Metazoa. These tissues are called mutable collagenous tissues (MCTs) and are involved in the energy-sparing maintenance of posture for feeding, defense, and other functions. The sea cucumber body-wall dermis is a typical MCT. Upon stimulation (by touching the animal with the hand, for instance), the dermis hardens while it becomes soft again when the stimulation stops. In a few species, intense stimulation of the animal can lead to the irreversible disintegration of the body wall. Sea cucumbers are at the forefront of MCT research and much of our knowledge of the molecular mechanisms driving MCT mechanical adaptation has been derived from holothuroid models. In this chapter, we review the types of MCTs found in sea cucumbers, their functions, the molecular mechanisms involved, and their potential biomimetic applications. These unique tissues are a source of inspiration for the development of biomaterials with potential applications in regenerative medicine.