Scientists have discovered a new type of cartilage that looks like fat-filled “bubble wrap”

Scientists say they have identified a new type of cartilage – one that was discovered in the 19th century, forgotten, rediscovered, and then forgotten again.

Medical textbooks describe three types of cartilage: hyaline, elastic and fibrocartilage. Hyaline cartilage helps bones slide smoothly over each other in joints; elastic cartilage is very flexible and is found in the outer ear, the voice box, and the tube between the ear and the throat; and fibrocartilage is durable and absorbs impacts in the joints and spine. The cells in these tissues are surrounded by a large amount collagen and elastic fibers, the proportions of which give each type of cartilage distinct characteristics.

However, scientists say there is a fourth type of cartilage that looks very different from the others.

The tissue they call “adipose cartilage” looks very similar to adipose tissue – better known as fat. It contains bulbous, balloon-shaped cells filled with oils, and the cells are surrounded by a thin matrix of fibers rather than the thick matrix typical of other cartilage. The cells are also very uniform and may be packed close together like bricks. Together, the cells form a resilient, spongy tissue that is somewhat pliable but still resistant to deformation and tearing; this tissue is found in structures such as the outer ear and nose.

Some experts were impressed by the new analysis of lipocartilage. For example, Viviana Hermosilla Aguayo AND Doctor Licia Selleri from the University of California, San Francisco wrote in: comment that this “long-ignored type of cartilage” may “warrant an update to histology and anatomy textbooks.”

Others said that researchers have presented strong evidence for the existence of this tissue, but are not sure whether fatty cartilage deserves its own classification.

“The authors presented evidence that this lipid-containing cartilage tissue occurs in many mammals, including humans,” he added. Shouan Zhudirector of the Osteoarthritis Research Laboratory at Ohio University, who was not involved in the study, told Live Science in an email. But “I’m not sure whether it should be considered a distinct new tissue type or just a new feature of existing tissue,” namely elastic cartilage.

What’s old is new again

“This was an unexpected finding,” the study’s senior author Maksym Plikusprofessor in the Department of Developmental and Cell Biology at the University of California, Irvine, told Live Science in an email. The team was examining the skin of mice’s ears when they came across fat-filled cartilage cells, which Plikus compares to “bubble wrap.”

But as they dug deeper, the team learned that their discovery wasn’t entirely novel. It turns out that this unique tissue had already been noticed by other scientists, Plikus noted.

microscopic image of tissue resembling bubble wrap

Image of fatty cartilage from a mouse ear. (Photo: Plikus Laboratory, University of California, Irvine)

In the 1850s histologist Franz von Leydig wrote about his observations of rat ear cartilage under a microscope. “At first glance, it looks like adipose tissue,” but it still has a distinct matrix like cartilage, he noted. Leydig’s observations were forgotten for over a hundred years. Then, in the 1960s, scattered reports described similar fatty tissue in the ears of rodents. In 1976a pair of scientists coined the term “lipochondrocyte” to describe cells found in fatty cartilage. But again, these discoveries were quickly forgotten.

Now the results of their study were published Thursday (Jan. 9) in the journal SciencePlikus and colleagues offer a very detailed analysis of adipose cartilage, revealing its stages of development, genetic characteristics and molecular properties. The study highlighted how the tissue differs from its similar appearance – fat – and how it is similar to other types of cartilage.

Using mice, the team showed that the fat-holding structures in adipose cartilage are “super-stable.” Unlike fat cells, which become larger or smaller depending on food intake, lipocartilage cells do not shrink during starvation or swell in response to excess. The team found that this stability is partly due to the tissue’s lack of enzymes that break down fats, as well as the lack of transporters that move fats from food into the tissue.

“This feature may provide an evolutionary advantage to the auricle” — the outer ear — “by increasing its ability to collect and focus acoustic waves,” Zhu said. Sound waves they break through fat very effectivelyso perhaps keeping this high-fat cartilage in the outer ear is useful for hearing, the authors also suggested.

Because the fatty cartilage in the ear does not swell or shrink based on caloric intake, the acoustics it supports should also remain constant over time.

Where is lipocartilage found?

The study authors initially identified fatty cartilage in the outer ears, noses and throats of mice. “It’s located in the nasal cartilage, at the very tip of the nose,” Plikus said. “It forms all of the ear cartilage,” as well as the vast majority of the larynx, or voice box, he said.

“A high degree of flexibility is required in all of these areas, and unlike other cartilage in the body, such as articular cartilage, these structures do not bear weight,” he added. For example, a structure in the throat called epiglottis Flexibly flaps back and forth while swallowing to prevent food from entering the respiratory tract.

After examining the mice, the researchers also identified fatty cartilage in human fetal tissues, namely the ear, nose, epiglottis and thyroid cartilage, which sits above the thyroid gland. They also found that lipocartilage appeared in human cartilage models grown in the lab from stem cells.

To find out how common fatty cartilage is in the animal kingdom, the team examined museum specimens of dozens of species. They noticed this tissue in several mammals, such as the Cairo spiny mouse (Acomys cahirinus), squirrel glider (Petaurus norfolcensis) and Pallas’s long-tongued bat (Glossophaga soricina) – but it has not been found in any non-mammalian animals such as frogs, birds or alligators.

The authors say questions remain about when lipocartilage first appeared and what evolutionary advantages it might offer to the animals in which it occurs. The team hopes to study the evolution of the tissue, as well as its ability to regenerate after injury and whether lipocartilage contains different cell subtypes. They also want to better understand how cells deal with so much fat, “which can be toxic to many other cell types,” Plikus said.

“We believe that discoveries about a new type of cell and a new type of tissue are of fundamental importance and change the paradigm,” he said of the work so far.