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The mechanism of matrix vesicle (MV) formation by growth plate chondrocytes in primary cell culture was assessed both by using drugs which interfere with assembly or disassembly of microfilaments and microtubules, as well as by comparison of the composition of chondrocyte microvilli with MV. Cytochalasin D, which is known to inhibit assembly of actin microfilaments, was found to stimulate the release of alkaline phosphatase-rich MV. This stimulatory effect was confirmed by studies with 3Hpalmitate- and 32P-prelabeled cells which showed that cytochalasin D enhanced the release of labeled MV. In contrast, phalloidin, which blocks disassembly of microfilaments, suppressed release of cellular alkaline phosphatase into MV. The phospholipid composition of vesicles released by cells treated with cytochalasin D and phalloidin was virtually identical with that of the controls. In contrast, colchicine, which interferes with the assembly of microtubules, was found to cause fragmentation of the cells, producing large vesicles significantly different in lipid composition from MV. Microscopic studies revealed that cytochalasin D caused marked rounding and retraction of the cells, with evidence of actin withdrawal from the cell periphery. This led to cell surface blebbing and formation of small zeiotic bodies at the tips of cell processes. In contrast, phalloidin enhanced and stabilized the actin network within the cells. Chemical analysis of microvilli prepared from isolated chondrocytes revealed high levels of alkaline phosphatase and a phospholipid composition almost identical to MV. Electrophoretic profiles of microvillar proteins were again like that of MV, except for the presence of high levels of actin. This cytoskeletal protein was nondetectable in MV. Taken together with the effects of the drugs, the data indicate that cell surface microvilli are the precursors of MV and that retraction of the supporting microfilament network is essential for the release of these structures.
Hale et al. (Sun,) studied this question.