New Cartilage Discovery Links Human Ears and Fish Gills Through Shared Genes

Recently Maksim Plikus, a developmental and stem cell biologist at the University of California, Irvine, and his colleagues discovered the new cartilage by accident while prepping mouse ear tissue for microscopic inspection.

Telltale gaps, which often indicate a sample contains lipids, developed when the tissue was treated with chemicals that dry it out. Upon closer inspection, they discovered that the ear cartilage differed from normal cartilage. It looked like bubble wrap, packed with cells filled with lipids. Their discovery defied the widely accepted belief that chondrocytes, the cells that make cartilage, have a low lipid content. ^[2]

More of this lipid-rich cartilage was found by the team in the sternum, larynx, and nose of mouse. They also found it in the ears of humans, bats, and opossums, among other species. But birds, reptiles, and amphibians—all of which lack external ears—do not have it.

The researchers discovered that the new cartilage was more pliable than regular cartilage from the knee and ribs but stiffer and more durable than fat after putting samples of it through a machine that stretches materials to determine their strength. It became more resilient and stiff after the lipids were chemically removed, resembling normal cartilage, suggesting that the lipids are responsible for its unique characteristics.

According to Markéta Kaucká, an evolutionary developmental biologist at the Max Planck Institute for Evolutionary Biology, the study "definitely checks all the boxes" required for a novel kind of cartilage cell. According to her, "it may have contributed to the exceptional hearing of mammals" because it is found in structures like the ears. According to Mary Goldring, a cartilage biologist at the Hospital for Special Surgery, the discovery complements earlier studies conducted in the last few decades that have broadened scientists' understanding of cartilage “We’ve seen an explosion of complexity in cartilage that was not expected because it looks like a very simple tissue.”

Since cartilage typically doesn't fossilize, the complete origin of mammals' external ears is still unknown. The cartilage of the external ear of mammals and the cartilage of fish gills grow from the same cells during embryonic development. Developmental biologist J. Gauge Crump of the University of Southern California and his colleagues describe in the Nature paper that there is a deep-rooted genetic explanation for that. They found that the patterns in human ear cartilage were most comparable to those in zebrafish gill tissue when they compared the gene activity of zebrafish and human cartilage.

The researchers also found that human ear's unique cartilage and fish gills share important DNA regions that increase the activity of specific genes. Zebrafish gill cartilage had five of the fourteen sequences they found in human ear cartilage. However, only one was present in non-gill cartilage from the fish. The scientists depicted that some of the sequences are interchangeable between fish and mammals. When they constructed the human versions into a zebrafish, the sequences turned on genes mainly in the gills of the fish. And after adding one of the gill gene activators from zebrafish, it stimulated genes in the ears of rodents. ^[2]

Astonishing facts of different mammals’ ears:

• A dog's ears can be pinned back or perked up to indicate its behaviour.

• A bat locates delectable flying insects with the help of its antenna-like hearing.

• On hot days, elephants' enormous ears act as fans! ^[1]

Future prospects: (Benefits in healthcare)

As Plikus and colleagues showed by cultivating human embryonic stem cells into structures that contained cartilage resembling face and neck cartilage, complete with lipid-loaded chondrocytes, the new knowledge regarding cartilage may also aid researchers in producing replacement tissue for damaged or worn-out body parts. Future face and neck cartilage restorations may be possible with such lab-grown human cartilages, he says.

References:

[1] Raul Ramos et al., Superstable lipid vacuoles endow cartilage with its shape and biomechanics. Science 387,eads9960(2025).DOI: 10.1126/science.ads9960

[2] Thiruppathy, M., Teubner, L., Roberts, R.R. et al. Repurposing of a gill gene regulatory program for outer ear evolution. Nature (2025). https://doi.org/10.1038/s41586-024-08577-5

(Input from various sources)

(Rehash/Sanika Dongre/MSM)