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Giant crystals have been found within the mitochondria of cartilage cells in horses, with nothing similar seen in the cartilage of any other species investigated so far.
Mitochondria are membrane-bound structures that are effectively power generators within a cell, converting oxygen and nutrients into adenosine triphosphate, which powers the cell’s metabolic activities.
Researchers from Austria and Germany have reported their discovery in the journal Histochemistry and Cell Biology.
The crystals in the mitochondria of chondrocytes − the latter being the only cells found in healthy cartilage − show dark contrast in transmission electron microscopy imaging – a technique in which a beam of electrons is transmitted through a specimen to form an image.
The crystals, which can be so large they result in enlarged mitochondria that can stretch the length of a cell, show a granular substructure of regularly aligned 1 to 2-nanometre small units.
Further analysis revealed a high content of nitrogen, indicating a protein. Their chemical composition has yet to be fully clarified.
Sylvia Nürnberger and her colleagues said the outer shape of the crystals was geometrical, with some profiles revealing hexagonal cross sections.
They were elongated, spanning a length of several micrometres through the whole cell.
“In some chondrocytes, several crystals were found, sometimes combined in a single mitochondrion,” they reported.
The crystals were preferentially aligned along the long axis of the cells, thus appearing in the same orientation as the chondrocytes in the tissue.
“Although no similar structures have been found in the cartilage of any other species investigated,” they reported, “they have been found in cartilage repair tissue formed within a mechanically stimulated equine chondrocyte construct.”
The crystals were mainly located in the surface regions of cartilage, especially in joint regions of well-developed superficial layers, more often in yearlings than in adult horses.
They believe the crystals are related to the high mechanical stress in the horse joint and potentially also to the increased metabolic activity of immature individuals.
Discussing their findings, the study team said crystals in cartilage cells have never been described in other species, with cats and dogs having already been ruled out in previous research. The study team went as far as screening for crystals in cartilage from other species, but found none in chickens, rats, pigs, sheep, calves, or humans.
They noted that crystals within mitochondria have also been seen in other cell types but, in contrast to the crystals seen in horse cartilage, were clearly smaller than the mitochondria, frequently compartmented, and they occupied only a part of the mitochondrial organelle.
Crystals also have often been described within the mitochondria of lower organisms such as protozoa and invertebrates, and have even been seen in other vertebrates, including mammals. However, these crystals have either only an inner crystalline structure or also a crystalline outer shape, and seem more common in the presence of liver and muscle conditions such as ischaemia, protein deficiency or starvation.
“They did not seem to be the reason for the size increase of the mitochondria. On the contrary, in the equine chondrocytes the increasing size of the crystals may be the obvious reason for the enlargement of the mitochondria, since the crystals completely fill the intramitochondrial space and stretch the mitochondrion to an enormous size, sometimes to the full length of the cell.
“Further, in horse chondrocytes, giant mitochondria only appear in relation with giant crystals.”
The crystals presented special characteristics in terms of their size and appearance in chondrocytes, which are cells with low metabolic activity under no particular hormonal influence.
“The distribution of the crystals suggested a relation to areas of high mechanical stress, which could locally be the case in defects and in transplanted areas.
“Crystals were found especially in yearlings, suggesting also a developmental component, probably due to the anatomical changes and activities during growth.”
They said further studies on the composition of the crystals were necessary to understand the origin and reason for their development and physiological correlations.
Members of the study team are affiliated with a range of institutions, including the Medical University of Vienna.
Giant crystals inside mitochondria of equine chondrocytes
S. Nürnberger, C. Rentenberger, K. Thiel, B. Schäd, I. Grunwald, I. Ponomarev, St. Marlovits, Ch. Meyer, and D. Barnewitz
Histochem Cell Biol. 2017; 147(5): 635–649. doi: 10.1007/s00418-016-1516-6 PMCID: PMC5400799