Adipochondrocytes in rabbit auricular cartilage

Chondrocytes are described as one cell population in different cartilage types. The auricular cartilage in mouse and rat contains unique chondrocytes similar in morphology to white adipocytes and known as lipochondrocytes. Lipochondrocytes were not mentioned in other species. The current study aimed to explore the existence of this cell type in rabbits. The auricles of adult male white rabbits were harvested and processed for histological examination with light and electron microscopy. With the light microscopy, the auricular cartilage of adult rabbits contained central large rounded adipocyte-like chondrocytes, termed in the current study adipochondrocytes The adipochondrocytes were embedded in relatively wide lacunae and had large lipid droplets with a rim of cytoplasm. The scanning electron microscopy confirmed this result. With the transmission electron microscopy, the adipochondrocytes showed dark nucleus and electron-dense cytoplasm with few organelles and cytoplasmic processes. The adipochondrocytes of the auricular cartilage in adult rabbits were unique cell type and different from chondrocytes in other cartilage subtypes. This result should be considered during cartilage transplant. Further studies are suggested to investigate the development and physiological roles of adipochondrocytes in the auricular cartilage in rabbits.

💡 Research Summary

This paper reports the discovery and detailed characterization of a novel cell type in the auricular cartilage of adult New Zealand White rabbits, which the authors term “adipochondrocytes.” Historically, only mouse and rat auricular cartilage were known to contain “lipochondrocytes,” large chondrocytes packed with lipid droplets that resemble white adipocytes. The authors hypothesized that similar cells might exist in other small mammals and set out to investigate rabbit ear cartilage using a combination of light microscopy, scanning electron microscopy (SEM), and transmission electron microscopy (TEM).

Specimens were obtained from fifteen healthy six‑month‑old male rabbits. Tissue samples were fixed either in neutral buffered formalin for routine histology or in glutaraldehyde followed by osmium tetroxide for electron microscopy. Light‑microscopic staining (H&E, PAS, alcian blue, safranin‑O, Masson’s trichrome, orcein) revealed that the central zone of the cartilage is populated by large, rounded cells occupying wide lacunae. These cells contain a single, often massive lipid droplet that, when processed without osmium fixation, dissolves and leaves an empty space rimmed by a thin layer of cytoplasm, giving a classic “signet‑ring” appearance. Osmium‑fixed sections preserved the droplets, confirming their lipid nature. The nuclei are flattened and peripheral; occasional binucleated cells were observed. The surrounding perichondrium is thin and collagenous, and smaller ovoid cells are interspersed between the large adipochondrocytes.

SEM confirmed the overall architecture seen in light microscopy. Lacunae appear either fully open, partially open, or closed. Individual adipochondrocytes display a spectrum of lipid content: some have one large droplet, others contain numerous small droplets, and a few appear lipid‑free. The surface topography shows occasional cytoplasmic processes extending from the cells into the matrix.

TEM provided ultrastructural insight. Adipochondrocytes possess a dark, electron‑dense cytoplasm with few organelles, mainly rough endoplasmic reticulum, Golgi apparatus, and secretory granules. The nucleus is dark and peripheral. Large lipid droplets are surrounded by an amorphous, electron‑dense cytoplasmic matrix. Numerous plasma‑membrane extensions project into a proteoglycan‑rich territorial matrix. The territorial matrix stains strongly with alcian blue, PAS, and safranin‑O, indicating a high content of sulfated glycosaminoglycans and proteoglycans. The interterritorial matrix contains collagen and elastin fibers, as shown by Masson’s trichrome and orcein staining, and appears as a network of dense granules interspersed with elastic material in TEM.

Morphometric analysis measured cartilage thickness (≈118 µm), lacunar diameter (mean ≈ 33 µm, range 17–54.5 µm), lipid droplet diameter (mean ≈ 20.5 µm, range 12.5–28.6 µm), and the adipochondrocyte‑to‑ECM area ratio (≈ 49 %). Cellular density was about 1 × 10⁴ cells per mm². Thus, adipochondrocytes occupy roughly half of the cartilage volume, a proportion far greater than that of typical chondrocytes in other cartilage types.

In the discussion, the authors argue that “adipochondrocyte” is a more descriptive term than “lipochondrocyte” because the cells closely resemble white adipocytes in morphology. They note that while lipid accumulation could suggest a role in energy storage, previous work in rats showed that lipid content does not diminish after severe caloric restriction, implying a structural rather than metabolic function. The authors propose that the lipid droplets may contribute to the mechanical resilience of the external ear. Moreover, the cells share several features with dark hypertrophic chondrocytes observed in growth plate cartilage—electron‑dense cytoplasm, developed RER and Golgi, and active secretion of extracellular matrix components—yet they are distinguished by their abundant lipid stores.

The paper concludes that adipochondrocytes represent a distinct, active cell population in rabbit auricular cartilage, capable of robust proteoglycan synthesis. Their presence may have implications for cartilage transplantation and tissue‑engineered cartilage models, where cell source heterogeneity can affect construct properties. The authors call for further molecular and functional studies to elucidate the developmental origin, metabolic pathways, and potential regenerative roles of adipochondrocytes.