Profiling cellular diversity in sponges informs animal cell type and nervous system evolution.

Area of Science:

• Cell biology
• Evolutionary biology
• Zoology

Background:

• The evolutionary origins of complex metazoan cell types, such as neurons and muscles, remain largely unknown.
• Sponges, as early branching animals lacking nervous and muscular systems, provide a unique model for studying primitive cell differentiation and communication.

Purpose of the Study:

• To investigate the cell types and intercellular communication systems present in sponges.
• To identify potential precursors to nervous system components in early animals.

Main Methods:

• Whole-body single-cell RNA sequencing was employed to identify and classify distinct cell types within a sponge.
• Correlative X-ray and electron microscopy were used to visualize the ultrastructure and interactions of identified cell types, particularly neuroid cells.

Main Results:

• Eighteen distinct cell types were identified in the sponge, including nitric oxide–sensitive contractile pinacocytes, phagocytes, and secretory neuroid cells.
• Neuroid cells were observed in close proximity to digestive choanocytes, featuring secretory vesicles and projections that enwrap choanocyte structures.
• A communication system organized around digestive chambers was revealed, utilizing conserved molecular modules.

Conclusions:

• The study identified a primitive communication system in sponges, involving neuroid cells and choanocytes.
• These findings suggest that fundamental components of intercellular communication, conserved across metazoans, originated before the evolution of nervous systems.
• The identified sponge cell types and communication mechanisms offer insights into the evolutionary trajectory towards neuronal and muscular systems in animals.