Researchers proclaim they have recognized a novel type of cartilage — one that was initially found in the 19th century, forgotten, rediscovered, and then overlooked again.
Medical literature outlines three categories of cartilage: hyaline, elastic, and fibrocartilage. Hyaline cartilage facilitates smooth movement of bones across each other at the joints; elastic cartilage provides significant flexibility and is present in the external ear, larynx, and the conduit between the ear and throat; while fibrocartilage is robust and absorbs shock at the joints and along the spine. The cells within these structures are encased in a significant amount of collagen and elastic fibers, with the proportions determining the unique traits of each category of cartilage.
Currently, however, researchers claim there is a fourth type of cartilage that appears markedly different from the others.
This tissue, referred to as “lipocartilage,” superficially resembles adipose tissue — commonly known as fat. It consists of rounded, balloon-like cells filled with oils, and these cells are surrounded by a delicate fiber matrix, rather than the dense matrix typical of other cartilage. Additionally, the cells are highly uniform and can align closely packed like bricks. Collectively, the cells create a resilient, pliable tissue that can compress yet still resists deformation and tearing; this tissue is found in components such as the external ear and nose.
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Some specialists were impressed with the fresh analysis of lipocartilage. For example, Viviana Hermosilla Aguayo and Dr. Licia Selleri from the University of California, San Francisco stated in a commentary that this “long-overlooked cartilage type” may “merit updates to histology and anatomy textbooks.”
Others mentioned that the researchers presented substantial proof of the tissue’s existence, but they remain uncertain if lipocartilage deserves its separate classification.
“The authors demonstrated that this lipid-rich cartilage tissue exists in various mammals, including humans,” Shouan Zhu, director of the Osteoarthritis Research Laboratory at Ohio University, who was not associated with the study, informed Live Science via email. However, “what I am uncertain about is whether it should be recognized as a distinct new type of tissue or simply a new characteristic of an existing tissue,” specifically, elastic cartilage.
What’s old has become new once more
“This was an accidental discovery,” stated study senior author Maksim Plikus, a professor in the Department of Developmental and Cell Biology at the University of California, Irvine, in an email to Live Science. The team was examining the skin of mouse ears when they stumbled upon the fat-filled cartilage cells, which Plikus likens to “Bubble Wrap.”
However, upon further investigation, the team learned their finding was not entirely unprecedented. It appears that other researchers had previously documented this peculiar tissue, Plikus remarked.
In the 1850s, histologist Franz von Leydig reported his findings regarding the cartilage of rats’ ears through microscopy. “At first glance, it resembles adipose tissue,” but it still exhibits a unique matrix, akin to cartilage, he observed. Leydig’s findings would fade into obscurity for over a century. Then, in the 1960s, sporadic reports referenced similar fatty tissues within rodent ears. In 1976, a duo of researchers introduced the term “lipochondrocyte” for the cells discovered in lipocartilage. Yet again, these findings were…soon overlooked.
Recently, with their research released Thursday (Jan. 9) in the journal Science, Plikus and his associates provide a thorough examination of lipocartilage, showcasing its developmental phases, genetic properties, and molecular features. The research emphasizes how the tissue contrasts with its counterpart — fat — while also sharing similarities with other types of cartilage.
In experiments with mice, the researchers demonstrated that the structures that contain the fat nestled within lipocartilage are “superstable.” Unlike fat cells that enlarge or shrink based on dietary habits, lipocartilage cells remain consistent in size during periods of hunger and do not expand in reaction to excess. This resilience is partially due to the tissue’s lack of enzymes responsible for fat degradation, as well as a scarcity of transporters that facilitate the entry of fats from food into the tissue, they discovered.
“This trait may confer an evolutionary benefit for the ear pinna” — the outer ear — “by amplifying its capacity to collect and direct sound waves,” Zhu stated. Sound waves propagate through fat very effectively, suggesting that preserving this high-fat cartilage in the outer ear could be advantageous for auditory processes, the authors also indicated.
Since the lipocartilage in the ear is not subject to swelling or shrinking based on caloric consumption, the acoustics it supports are likely to remain stable over time.
Where is lipocartilage located?
The research team initially located lipocartilage in the outer ears, noses, and throats of mice. “It exists in the nasal cartilage at the nose’s very tip,” Plikus mentioned. “It constitutes the full ear cartilage,” along with the vast majority of the larynx, or voice box.
“In all these regions, a high level of elasticity is necessary, and unlike other cartilages in the body, like joint cartilage, these structures do not bear weight,” he continued. For instance, a structure in the throat known as the epiglottis flexibly moves back and forth during swallowing to prevent food from entering the respiratory passage.
After their studies on mice, the researchers also discovered lipocartilage in human fetal tissues, specifically from the ear, nose, epiglottis, and thyroid cartilage, which is positioned above the thyroid gland. They also noted that lipocartilage developed in their lab-generated human cartilage models created from stem cells.
To assess the prevalence of lipocartilage throughout the animal kingdom, the team analyzed museum specimens from numerous species. They identified the tissue in several mammals — including the Cairo spiny mouse (Acomys cahirinus), squirrel glider (Petaurus norfolcensis), and Pallas’s long-tongued bat (Glossophaga soricina) — but did not find it in any non-mammalian species, such as frogs, birds, or alligators.
The authors indicate that questions still exist regarding when lipocartilage first appeared and what evolutionary benefits it may provide to the animals in which it is found. The research team aspires to investigate the tissue’s evolution, explore its regenerative capabilities post-injury, and examine whether lipocartilage houses various cell subtypes. They also seek to gain insight into how these cells manage such high fat content, “which can be harmful for many other cell types,” Plikus stated.
“We consider the discovery of a new cell type and a new tissue type to be foundational and revolutionary,” he remarked about the ongoing research.