# Research Progress in the Construction and Application of In Vitro Vascular Models
###### Abstract
Endothelial progenitor cells and hematopoietic stem cells first form blood islands, and then the edge of the island differentiates into multiple small endothelial cell clusters that invade the surrounding tissue. These expanded cell clusters eventually connect to form a basic vascular network [19; 22]. Stable vascular endothelial cells are tightly interconnected by transmembrane adhesive proteins such as VE-cadherin, N-cadherin, claudins, and tight junction proteins, which enhance the permeability barrier function of endothelial cells to better control substance exchange and cell migration [23]. In contrast to vasculogenesis, angiogenesis refers to the formation of new blood vessels from existing vascular networks, which typically occurs during key points in morphogenesis, injury, and pathogenesis [24]. Therefore, angiogenesis is more common than vasculogenesis in adults. Sprouting angiogenesis and intussusceptive angiogenesis (non-sprouting angiogenesis) are the two primary ways to form new blood vessels during angiogenesis, and both occur in physiological or pathological tissues and organs [25; 26]. The human body has an oxygen-sensing mechanism that allows for timely physiological adaptations to fluctuations in oxygen concentration. Therefore, sprouting angiogenesis is typically caused by hypoxia-inducible factors (HIFs) or inflammation. HIFs are heterodimeric transcription factors consisting of \(\alpha\) and \(\beta\) subunits. HIF-1\(\beta\) is not regulated by oxygen concentration, while HIF-1\(\alpha\) is produced in large quantities under hypoxic conditions and forms heterodimers with HIF-1\(\beta\). The gradual accumulation of HIF activates the expression of many angiogenic genes and initiates the angiogenic process [27]. Angiogenic factors mainly include vascular endothelial growth factor, fibroblast growth factor, angiopoitein, and chemokines [28]. Among these, vascular endothelial growth factor is the most important factor in angiogenesis, occurring in the early stage of the angiogenic cascade response, and is associated with the initial activation of endothelial cells [29]. Additionally, potential genetic factors and hemodynamic stimuli can also promote angiogenesis to some extent [30]. In addition, intussusceptive angiogenesis refers to the process by which existing blood vessels invaginate through the vessel wall to form tissue columns in the lumen, eventually splitting the original vessel into two parallel vessels. This process is therefore also known as split angiogenesis [31; 32]. The specific process is as follows: first, the endothelial cells on both sides of the blood vessels touch each other to form a contact zone, and then, the endothelial cell membranes gradually become thinner and fuse to form tiny holes. Supporting cells such as mesenchymal stem cells, fibroblasts, and pericytes then invade and form columnar or rod-like structures to fill the gap between the two new blood vessels. Eventually, new vascular channels are formed in the original blood vessels as the hole structures gradually enlarge and merge [33; 34]. Although shear stress and blood flow velocity have been shown to play a critical role in this process, we currently know little about the cellular and molecular mechanisms of intussusception angiogenesis and need to establish experimental models for research [26; 35]. Compared to sprouting angiogenesis, intussusceptive angiogenesis occurs at a faster rate. During intussusceptive angiogenesis, endothelial cells appear to become larger and thinner, and the process does not involve endothelial cell proliferation, requiring less energy. As a result, most tumors form vessels rapidly through this process [31; 36]. Traditional animal models and two-dimensional in vitro models are insufficient in replicating the human pathological and physiological environment accurately. Additionally, the introduction of vascular networks allows for continuous supply of nutrients and oxygen to cells in tissue engineering constructs.