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Filaments and granules in mitochondrial vacuoles in chondrocytes
Abstract:
As chondrocytes degenerate mitochondria frequently become vacuolated. Vacuoles seem to form by an invagination of mitochondria into their own matrix followed by cytoplasmic invasion. The bounding membrane of vacuoles and the electron dense granules which are intermittently dispersed around its periphery are positive when stained with phosphotungstic acid and with silver nitrate methenamine after periodic acid oxidation, indicating the presence of glycoprotein; they are also positive after staining with colloidal iron and bismuth nitrate, indicating the presence of acid proteoglycans. Within vacuoles and apparently arising from their glycoprotein proteoglycan boundary are small filaments composed of granular and filamentous protions. Vacuoles and their contents are exocytosed thus accounting for the presence of granules and filaments in pericellular lacunae. In pericellular lacunae and in the extracellular matrix, filaments interconnect with granules or with collagen fibrils, or both. Filaments decrease in number as the distance from the cell increases, while granules decrease in size but remain numerous. Filaments and granules decrease in the extracellular matrix as collagen fibrils aggregate and as the matric calcifies. Thus, they may play a role in aggregation of collagen fibrils. Filaments and granules also decrease with increasing age. Filaments and granules do not seem to be derived from Golgi vacuoles. They are present in costal, tracheal, and epiphyseal cartilage in three different species of animals.
Insights
Mitochondria in degenerating chondrocytes form vacuoles containing glycoprotein and proteoglycan filaments. These extracellular components may aid collagen aggregation and are found across various cartilage types and species.
Area of Science:
- Biochemistry
- Cell Biology
- Histology
Background:
- Chondrocyte degeneration is linked to mitochondrial abnormalities.
- Vacuolated mitochondria are a common observation in aging or damaged cartilage.
Purpose of the Study:
- To investigate the composition and origin of vacuoles within degenerating chondrocytes.
- To determine the role of these vacuoles and their contents in the extracellular matrix of cartilage.
Main Methods:
- Transmission electron microscopy with specialized staining techniques (phosphotungstic acid, silver nitrate methenamine, colloidal iron, bismuth nitrate).
- Analysis of mitochondrial vacuole formation and exocytosis.
- Observation of extracellular matrix components in relation to cell proximity and age.
Main Results:
- Mitochondria form vacuoles through invagination and cytoplasmic invasion.
- Vacuole membranes and associated granules contain glycoprotein and acid proteoglycans.
- Filaments, granules, and proteoglycans are exocytosed into the pericellular space.
- Extracellular filaments and granules interact with collagen fibrils and decrease with age and matrix calcification.
Conclusions:
- Mitochondrial-derived vacuoles release components that may influence collagen fibril aggregation.
- These findings suggest a novel role for chondrocyte mitochondria in cartilage matrix remodeling.
- The observed structures are present in diverse cartilage types and animal species.