Phagocytosis of dying chondrocytes by osteoclasts in the mouse growth plate as demonstrated by annexin-V labelling

Abstract

Endochondral ossification in the epiphyseal growth plate of long bones is associated with programmed cell death (PCD) of a major portion of the chondrocytes. Here we tested the hypothesis that at the ossification front of the epiphyseal growth plate osteoclasts preferentially phagocytose chondrocytes that are undergoing PCD. We injected biotin-labelled annexin-V (anx-V-biotin, an early marker of PCD) intravenously in young adult mice. After 30 min of labelling, long bones were recovered and the tissue distribution examined of anx-V-biotin-labelled cells in the growth plate using ABC-peroxidase histochemistry. Positive staining for anx-V-biotin was detected in hypertrophic chondrocytes still present in closed lacunae at some distance from the ossification front. At the ossification front, chondrocyte lacunae were opened and close contacts were seen between tartrate-resistant acid phosphatase-positive osteoclasts and hypertrophic cartilage cells. Osteoclasts were significantly more frequently in contact with anx-V-biotin-labelled chondrocytes than with unlabelled chondrocytes. Osteoclasts also contained labelled and unlabelled phagocytic fragments within their cytoplasm. We conclude that in the growth plate osteoclasts preferentially phagocytose hypertrophic chondrocytes that are dying, suggesting these dying cells may signal osteoclasts for their removal.

Cancer and the Microenvironment

“…Fig. 6C demonstrated that OCs indeed removed apoptotic and dying myeloma cells by phagocytosis, confirming previous reports (37,38). The phagocytic activity of OCs did not affect their viability or their ability to support myeloma cell growth and survival.…”

Osteocyte and bone structure

“…Evidence that osteoclastic attack is directed toward apoptotic osteocytes has been reported in the growing skeleton in relation to bone renewal [44], and under pathologic conditions [45,46]. Similar to macrophages, osteoclasts are attracted to apoptotic cells that expose phosphatidylserine on their outer cell surface at an early stage of apoptosis [47]. In the case of osteoclasts, the apoptotic cells that attract them are osteocytes and hypertrophic chondrocytes of the growth plate [47].…”

“…Similar to macrophages, osteoclasts are attracted to apoptotic cells that expose phosphatidylserine on their outer cell surface at an early stage of apoptosis [47]. In the case of osteoclasts, the apoptotic cells that attract them are osteocytes and hypertrophic chondrocytes of the growth plate [47]. Exposure of phosphatidylserine on osteocytic cell "fingers" in canaliculi abutting on the wall of the cutting cone might therefore be the signal that urges the osteoclasts to continue absorption in that direction.…”

Chondrocyte apoptosis enhanced at the growth plate: a physeal response to a diaphyseal fracture

“…Chondrocytes of the human growth plate experience a life cycle that ranges from resting to proliferating before becoming hypertrophic cells and finally undergoing programmed cell death (Ballock and O'Keefe 2003;Forriol and Shapiro 2005;Shum and Nuckolls 2002), with their apoptotic bodies supposedly being phagocytosed by osteoclasts (Bronckers et al 2000). Until now, opinions regarding the ultimate fate of terminally differentiated hypertrophic chondrocytes have been conflicting.…”