Cellular diversity in sponges illuminates the evolution of the nervous system

Specialized cells located in the digestive chambers of sponges – our most distant animal relatives – may represent a starting point for the evolution of the nervous system, according to a new study. “Our work … puts sponges center stage in elucidating nervous system evolution,” the authors write. Their findings point to cells involved in regulating feeding and controlling the sponge microbial environment as precursors to more advanced cellular communication in animals. The evolutionary origin of complex and specialized metazoan cell types capable of directed communication with one another is not known. While Porifera, or sponges, are multicellular, they lack true tissues and organs and do not have nervous, digestive, or circulatory systems. Instead, they rely on water flowing through their porous body to obtain food, oxygen and to remove wastes. However, despite their simple organization, sponges have genes usually expressed in neurons or muscles of more complex animals, as well as genes used by our brains to build synaptic connections. What’s more, they can perform whole body contractions to flush debris from their bodies. Cells capable of such coordinated communication within these nervous system-less creatures have not yet been identified, in part because investigating the role of genes in these sponge cells has been a technological hurdle. Using whole-body single-cell RNA sequencing, Jacob Musser and colleagues conducted a comprehensive survey of cell types in the freshwater sponge, Spongilla lacustris. Musser et al. identified 18 distinct cell types, including several that were previously unknown, unrecognized or poorly understood. The authors describe contractile pinacocytes, amoeboid phagocytes and secretory neuroid cells closely associated with digestive choanocytes that express genes important in synaptic communication, all residing around the sponges’ digestive chambers. Correlative x-ray and electron microscopy of the neuroid cells revealed specialized cells that support cellular communication around the digestive chambers and are used to regulate feeding. Musser et al. argue that these modules may have become conserved and incorporated into the pre- and postsynapse in the nervous system of later animals.