Platelet-like anucleate cell fragments mediate wound healing and immune response in the sea star <i>Patiria pectinifera</i>

Blood coagulation and immune responses have long been considered as discrete processes. However, recent studies have revealed that mammalian platelets play a critical role in immune regulation in addition to their well-established role in hemostasis. This dual functionality suggests an evolutionary link between platelets and invertebrate blood cells that contributes to both immune responses and wound healing. However, megakaryocytes, the precursor cells of platelets, are unique to mammals. It is widely believed that invertebrates lack a mechanism for producing anucleate cell fragments similar to platelets. Here, we report the discovery of anucleate cell fragments in the coelomic fluid of the sea star Patiria pectinifera. Detailed scanning electron microscopy revealed that the anucleate cell fragments exhibit both morphological and functional similarities to mammalian platelets. These cell fragments, which are derived from immune cells (coelomocytes), accumulate at wound sites, aggregate with coelomocytes, and serve as major sources of extracellular vesicles. Furthermore, extracellular vesicles released by these fragments not only promote aggregate formation in response to foreign substances but also transfer their contents into coelomocytic cytoplasm. Our findings suggest that a mechanism for generating anucleate cell fragments with platelet-like functions already exists in invertebrate immune cells. This study provides new insights into the evolutionary origins and broader immunological significance of platelets.