Animals don't come much simpler than sponges. Nestled against yeast in the tree of life, sponges lack a mouth, heart and brain, yet they digest food, reproduce sexually like all other animals and heal when they get hurt. Marina Pozzolini from the University of Genova, Italy, explains that scientists have observed the sponge's healing process in fine detail using a scanning electron microscope, but no one knew which cellular molecules drive healing. As transforming growth factor-β (TGF-β) is a key player in mammalian wound healing, Pozzolini and colleagues from the University of Genova and Institute for Sustainable Plant Protection – Turin Unit, Italy, began investigating the role that the growth factor plays in healing sponge's wounds.
Focusing on the marine sponge Chondrosia reniformis, Pozzolini first discovered that the sponge produces seven versions of the TGF signalling molecule, in addition to producing three receptor molecules that can be activated by the TGF molecules to trigger healing. The team then cut cylinder-shaped pieces from sponges and allowed them to begin healing in fresh seawater, monitoring the process for up to 6 days to find out which TGF molecules were involved as the sponge healed.
Initially, during the first day of recovery, the sponge fragments increased production of TGF5 and TGF6 as they mobilised a specialised group of cells called archaeocytes, which are key components of the healing process and have the ability to morph into any cell type required. On the second day, the cylinders began to swell into barrel shapes while maintaining the increase in TGF5 production. However, TGF3 production fell as the sponges continued producing archaeocyte cells to heal their wounds. The team noticed that the archaeocytes were migrating to different regions of the sponge before transforming into exopinacyte cells, the specialised cells that cover the sponge's external surface. Over the final 4 days (days 3 to 6), the sponges maintained the increase in TGF5 production, in addition to producing more TGF4. However, the levels of TGF1, 3 and 6 all fell as more of the archaeocyte cells transformed into wound-sealing exopinacyte cells.
So, TGF growth factor is a key player in sponge wound healing, which means that this mechanism, which we also used for wound healing, first developed in one of our most ancient ancestors. However, Pozzolini explains that sponge TGF molecules reduce the production of collagen – a key component of skin and bones – in contrast to triggering it in mammals.
