Hydra extracellular matrix moves

The body wall of hydra, a primitive metazoan animal, consists of two epithelial layers with an intervening extracellular matrix (ECM). Epithelial cells in the body constantly divide, resulting in a tissue movement towards the oral and the aboral ends of the animal. Until today it was generally assumed, that this tissue dynamics is based on migration of epithelial cells, which use the underlying ECM as a substrate. We established a new technique of stably labeling ECM in living animals by injecting fluorescently tagged monoclonal antibodies against hydra Collagen I and Laminin. Co‐labelling of epithelial cells and grafting of labeled tissue fragments into unlabelled animals allow us to study the dynamics of ECM and associated epithelial cells for a period of time up to one month. Our results show that ‐ unlike earlier assumptions ‐ hydra ECM is a dynamic structural element that moves along the body column towards the aboral end of the animal and towards the tips of the tentacles. The associated epithelial cells showed similar movement. However, cells and matrix show different dynamics in tissue evagination processes, regeneration, and in certain body regions of hydra. Our study in the cnidarian hydra emphasizes a new view of ECM as a dynamic tissue element.