The smooth functioning of the body’s joints, the flexibility of the ears and nose, and the shaping of bones are all made possible by the skeletal tissue known as cartilage.1 According to popular medical textbooks, cartilage is made up of only one type of specialized cell called a chondrocyte, which is small and secretes large quantities of extracellular matrix, giving cartilage its biomechanical properties.2 But now, new research makes these textbooks outdated.
More than a decade ago, while studying fat cells in the mouse ear skin, Maksim Plikus and his colleagues observed a puzzling pattern in dye uptake. “There were some fat cells that stained, which were the true adipocytes,” said Plikus, a developmental biologist at the University of California, Irvine. “Then there was another group of fat cells that didn’t [stain], no matter which marker.” Initially, he thought these odd adipocytes could simply be a type that stubbornly resisted dyes. However, upon digging deeper, Plikus realized that they were a completely new type of fat-laden cartilage cells that formed the pliable lipo-cartilage in body parts like the nose, ear and throat. “At first we really had to pinch ourselves because it made no sense,” Plikus exclaimed.
They called these newly-identified cells lipochondrocytes (LCs), and after a decade of investigation into their structure and function, the team published their report today in Science.3 These findings will expand the current understanding of skeletal biomechanics and open new avenues in regenerative medicine.
“This is ground-breaking research,” said Markéta Kaucká, a developmental biologist at the Max Planck Institute for Evolutionary Biology, who was not involved in the study. “They found that the cells have different morphology, gene expression profiles, functions, and biomechanical properties. It really is a new cell type.”
Researchers have discovered a new type of cartilage cell called lipo-chondrocyte, that is filled with large fat droplets (shown here in green) and is found in many mammalian species.
Maksim Plikus, University of California, Irvine
Despite its prevalence in the body, researchers have overlooked LCs and lipo-cartilage for many decades. Lipochondrocytes were first observed in 1854 by Franz Leydig in rat ear cartilage and then largely forgotten for nearly a century.4 Even research groups that stumbled upon the cells later did not study them or the tissue in detail.5 So, when Plikus and his team came across these cells that looked like fat cells but were present in the cartilage, they knew they had to examine them from every possible angle.
The authors started by cataloging the presence of LCs in the mouse body. Using different genetic drivers, they identified LCs in the cartilage of the nose, larynx, sternum, and ear. Upon tracking the growth of LCs through development, the team found that these cells live for a long time and have limited turnover. LCs were even present in the nose and ears of aged mice.
Depending on the location, the stiffness and pliability of cartilage varies. The team wanted to understand how the lipid vacuoles inside LCs modified these biomechanical pro
