Imagine a small organ that works tirelessly—chewing, swallowing, speaking, and sensing. Despite its apparent simplicity, the tongue undergoes remarkably complex development. When something goes wrong during this process, the consequences can last a lifetime: a split tongue, abnormal tongue growth, or a frenulum that never forms.
A study published in eLife in October 2024 has revealed a hidden mechanism behind these abnormalities—and its central finding is quite surprising. Cells that should become tongue muscles can apparently “choose the wrong path” and transform into brown adipose tissue rather than muscle fibers because of a failure in intercellular communication during embryonic development.
drg. Finsa Tisna Sari, M. Biomed., Ph.D., a researcher from Niigata University who contributed to the study, was part of an international team led by Atsushi Ohazama. Together with colleagues from King’s College London, the University of Naples, and several other institutions, the researchers investigated how mutations in primary cilia proteins—tiny antenna-like organelles attached to nearly every cell in the body—can disrupt the entire architecture of tongue development.
Damaged Cellular Antennas, an Abnormal Tongue
Primary cilia are more than simple cellular appendages. These organelles function as signaling centers that receive various signals, including Hedgehog (Hh) signals, a cell-communication pathway crucial to embryonic development. When the cilia malfunction, Hh signaling is disrupted. This condition is known as a ciliopathy, a genetic disorder that can cause malformations in various vital organs, including the brain, kidneys, lungs, and tongue.
The researchers used a mouse model carrying a mutation in the Ofd1 gene, which encodes a protein located at the basal body of primary cilia. Mice in which Ofd1 was completely lost in cranial neural crest-derived cells (CNCCs)—a population of cells that migrates from the neural tube crest to the developing tongue primordium—developed aglossia, or complete absence of the tongue. Meanwhile, mice with partial mutations developed tongues with deep clefts, duplicated protrusions, and ectopic adipose tissue in areas that should have contained muscle.
The most surprising finding was that this abnormal tissue was neither a tumor nor a cyst. Gene-expression analysis and immunohistochemistry confirmed that the tissue was brown adipose tissue, or brown fat. It did not originate from misplaced fat cells, but rather from mesodermal cells that should have differentiated into tongue muscle myoblasts.
A Conversation That Failed Between Two Cell Populations
The major question was: why did the mesodermal cells take the wrong developmental path?
The answer lies in cell-to-cell interactions. During normal development, CNCCs and mesodermal cells meet within the pharyngeal arch around embryonic day 10 (E10) in mice. This meeting is more than simple proximity—CNCCs actively “instruct” mesodermal cells through Hh signaling to differentiate into myoblasts.
When Ofd1 is mutated, the cilia in CNCCs fail to function, Hh signaling is not activated, and this communication breaks down completely. Without the appropriate signal, mesodermal cells take an alternative differentiation pathway and become brown adipocytes.
The team demonstrated this through a series of elegant experiments. In cell culture, mesodermal cells separated from CNCCs showed expression of fat-cell markers. However, when Hh signaling was artificially activated using SAG, an Hh agonist, the same mesodermal cells successfully expressed MyoD, a muscle-cell marker, even in the absence of CNCCs.
“The first interaction between CNCCs and mesodermal cells is crucial for all subsequent events in tongue development. Failure of Hh signaling activation in CNCCs, or failure of cell-cell contact, results in mis-differentiation of mesodermal cells into brown adipocytes.” — Kawasaki et al., eLife, 2024
The researchers further demonstrated that mesodermal cells that successfully interacted with CNCCs activated their own Hh signaling, creating a self-reinforcing signaling relay. When this relay was disrupted at any point, the normal differentiation cascade also collapsed.
A Split Tongue, a Missing Frenulum
This ectopic brown fat did not simply “fill the wrong space.” It actively inhibited the formation of the tongue swelling, which should normally emerge around E11.5. When researchers mechanically replaced CNCCs and myoblasts with adipose tissue in the mandibles of normal mice, they successfully reproduced tongue clefts similar to those observed in the mutant mice.
These findings have direct clinical implications. Tongue abnormalities—including clefts, hamartomas, and ankyloglossia—can occur not only in syndromic conditions such as OFD1 syndrome, but also in non-familial conditions whose mechanisms have remained unclear. The study suggests that mechanical disruption of the same cellular processes—for example, physical pressure on the embryo—could produce phenotypes identical to those caused by genetic mutations.
The absence of the lingual frenulum, observed in 30 of the 58 mutant mice examined, was found to be controlled by a different signaling pathway: non-canonical Wnt signaling. Cells in the anterior mandibular region normally migrate along the lingual-buccal axis to form the frenulum. When CNCCs in this region were replaced by adipocytes, or when a non-canonical Wnt inhibitor was administered, the frenulum failed to form.
Even the variation in abnormalities among individual mutant mice could be explained by X-inactivation. Because the Ofd1 gene is located on the X chromosome, CNCCs in heterozygous female mice randomly express one of the two alleles. The result is a mosaic of normal and mutant CNCCs, which determines where muscle forms and where fat develops.
From Mice to Humans, From the Laboratory to the Clinic
The researchers then examined tongue tissue from a patient carrying the OFD1 G138S mutation. The findings were consistent: the patient's tongue protrusion contained ectopic brown adipose tissue, lacked normal tongue papillae, and showed no detectable expression of KRT1-5, a marker of papillary epithelium, above the affected region. What occurred in the mice also occurred in humans.
This study opens new possibilities for the management of ciliopathies. If tongue abnormalities are caused by a specific and localized failure of Hh signaling, then correcting ciliary function or pharmacologically activating the Hh pathway during the appropriate developmental period could, in theory, prevent malformations. It is a clue—still far from becoming a therapeutic protocol—that the future management of these conditions may depend not only on reconstructive surgery but also on much earlier molecular interventions.
The tongue is the first organ to encounter the world—long before we learn to speak, swallow, or taste. Understanding how it forms, cell by cell and signal by signal, is the first step toward ensuring that it can perform its functions perfectly.
Source DOI: http://: https://doi.org/10.7554/eLife.85042
Authors: Nanda Ayu, drg. Achmad Zam Zam Aghasy, M.Kes.
Photo: Pexels