Introduction
The meniscus is critical to functioning of the knee(Makris et al., 2011; Clark and Ogden, 1983). Variations in blood supply divide the meniscus into red, white, and red-white zones, each with distinct repair capacities. (Chahla et al., 2021; Kunze et al., 2021). Lesions in the red zone heal much more easily than those in the white zone. While the meniscus is fully vascularized during prenatal development, only 10–25% of the mature meniscus retains its vascularization, with even fewer vessels present in degenerated states (Clark and Ogden, 1983). Despite the known decline in vascular capacity during meniscus development and degeneration, the vascularization of the embryonic meniscus has not been histologically described, and the mechanisms underlying this diminished process remain unclear.
NOTCH signaling pathway is one of the most essential signals for embryonic development and vascular formation in lots of tissues (Siebel and Lendahl, 2017). Distribution of NOTCH signaling always leads to vascular remodeling with tissue specificity, which has also presented with unique function in vascular development of bones (Ramasamy et al., 2014). DLL1, as a ligand for NOTCH signaling, is capable of regulating endothelial identity, especially arteries (Limbourg et al., 2007), which is essential for blood flow in ischemic regions and maintenance of multipotent stem cells (Chakrabarti et al., 2018). However, the significance of these signaling pathways in the meniscus has not been explored to date.
Considering the potential therapeutic methods in meniscus vascularization, single cell data of embryo meniscus in E24w and E35w was analyzed, focusing on characteristics of endothelial cells. The distribution of endothelial cells at different stages of embryonic meniscus development revealed the diminishing process. Classification of artery/veins was identified through development. DLL1/NOTCH1 was introduced by single cell analysis and combined data with osteoarthritis meniscus. Experiments in vitro were conducted to prove critical role of DLL1/NOTCH1 in meniscus angiogenesis both in development and degeneration.