Are paralysed chondrocytes really dying?

ABSTRACT

Paralysed chondrocytes are cells with uncommon morphology that recently have been seen among the non-hypertrophic chondrocytes of rabbit growth cartilage. Paralysed chondrocytes characterised by condensed nucleus and well developed rough endoplasmic reticulum and Golgi apparatous and numerous secretory vesicles. Because, in some examples, the cytoplasm of these cells appeared empty except from few organelles and the nucleus was condensed, paralysed chondrocytes were considered as an example of physiologically dying cells. Studying of paralysed chondrocytes is difficult due to the absence of an in vitro model in which the paralysed ehondrocytes could be induced. The aims of the current study were to establish a system of culture for induction of paralysed chondrocytes and to investigate if these cells are really dying. Chondrocytes were isolated from the growth cartilage of fetal equines, centrifuged and cultured as pellets in either 10% fetal calf serum or 10% horse serum for 28 days and processed for light and electron microscopy. Different cell types were counted and expressed as a percentage to the total cell number. Growth kinetics including the pellet weight and thickness and the cellular density were evaluated. After 7 days in culture, paralysed chondrocytes with similar morphology to those described in the rabbit growth cartilage could be identified in pellets in each serum type, however, the proportion of the cells was different. In pellet cultured with 10% fetal calf serum, more than 50% of the cells were paralysed chondrocytes but in 10% horse serum, less than 10% of cells were of paralysed type. At day 14, about 50% of the cells in pellets cultured in either serum type differentiated into hypertrophic dark chondrocytes and the proportion of paralysed cells was markedly decreased. After 21 days in each culture, more than 70% of the cells were hypertrophic dark chondrocytes and no paralysed chondrocytes could be observed. Interestingly, the pellets in both serum types showed similar growth kinetics. The paralysed chondrocytes may be not dying and they likely to be an immature form of hypertrophic dark chondrocytes. It is better to use the term immature dark chondrocytes instead of paralysed cells. This culture system will be useful for further molecular studies on paralysed chondrocytes and to explore the functions of these cells