Scientists from the University of Virginia School of Medicine and the U.S. National Institutes of Health have identified a previously unknown cellular structure. This discovery could reshape our understanding of cellular maintenance and disease. Named the “hemifusome,” this transient organelle appears to play a crucial role in how cells manage internal cargo. The research, detailed in a new study published in Nature Communications, utilized advanced imaging to capture this structure in action, revealing a new pathway for sorting and processing materials within the cell.

The discovery was made possible by a powerful imaging technique called in situ cryo-electron tomography (cryo-ET). This method involves flash-freezing living cells, preserving their internal components in a near-native state without the need for chemical fixatives that can alter delicate structures. By examining four different types of mammalian cells, the team observed that hemifusomes are formed when two distinct vesicles—small, membrane-bound sacs that transport substances—partially merge. A stable membrane bridge maintains this connection, known as a hemifusion diaphragm, which is consistently associated with a tiny, 42-nanometer structure that the team identified as a proteolipid nanodroplet (PND).

According to the researchers’ model, the hemifusome acts as a dynamic platform for creating new vesicles. “You can think of vesicles like little delivery trucks inside the cell,” said Seham Ebrahim, a researcher at UVA’s Department of Molecular Physiology and Biological Physics, in a university press release. “The hemifusome is like a loading dock where they connect and transfer cargo.” This process is fundamental to the formation of cellular sorting centers known as multivesicular bodies (MVBs). Notably, this new mechanism for creating MVBs operates independently of the well-studied Endosomal Sorting Complexes Required for Transport (ESCRT) protein complex, which was previously considered the primary machinery for this task, suggesting that cells have a previously unknown system for managing their internal logistics.

This fundamental discovery has significant implications for understanding and potentially treating a range of inherited disorders linked to faulty cellular transport, such as Hermansky-Pudlak syndrome. This rare genetic condition can cause albinism, lung disease, and bleeding problems, all stemming from defects in how cells handle internal cargo. By providing a new blueprint for these essential processes, the identification of the hemifusome offers a fresh avenue for research. “It’s exciting because finding something truly new inside cells is rare—and it gives us a whole new path to explore,” Ebrahim said.

References

• Tavakoli, A., Hu, S., Ebrahim, S., & Kachar, B. (2025). Hemifusomes and interacting proteolipid nanodroplets mediate multi-vesicular body formation. Nature Communications, 16(1), 4609. https://doi.org/10.1038/s41467-025-59887-9
• University of Virginia. (2025, June 25). Scientists discover unknown organelle inside our cells. Phys.Org; University of Virginia. https://phys.org/news/2025-06-scientists-unknown-organelle-cells.html