Apoptosis causes lumen formation during angiogenesis in vitro

The establishment of new capillaries by sprouting from preexisting small blood vessels, a process called angiogenesis, is an absolute requirement for the development and remodeling of normal and diseased tissues. Endothelial cells (EC are the major cell type involved in this process. During angiogenesis complex interactions between EC and different cell types (e.g., monocytes, fibroblasts, smooth muscle cells) and growth factors (e.g., vascular endothehal cell growth factor) take place (reviewed in: Folkman and D'Amore, 1996). Cultivated under appropriate conditions, such as apical contact with fibrillar collagen, EC rapidly establish patent blood vessel-like structures resembling capillaries. This in vitro behavior is considered to be angiogenic and has been used to test the effects of biological compounds, like extracellular matrix (ECM) molecules and growth factors, on angiogenesis (Montesano et al., 1983; Madri and Pratt, 1986). Recently it has become apparent that apoptosis (programmed cell death) and the removal of deceased cells are essential for tissue remodeling in transient structures (Mori et al., 1995). Cell death by apoptosis is supposed to be generally noninflammatory (Steller, 1995), but apoptotic EC, unlike other cells undergoing apoptosis, seem to be potentially proinflammatory (Hebert et al., 1998; Bombeli et al., 1999). We have studied the occurrence of apoptotic cells and the distribution of inflammation-related adhesion molecules on their surfaces during angiogenesis in vitro induced by apical contact with a gel of type I collagen. Our observations indicate that apoptosis is instrumental in lumen formation in this in vitro system. We suggest that the removal of cellular material arising by apoptosis may constitute a general mechanism for modeling of the vascular lumen during angiogenesis in vivo.