Vascularized Tissue Organoids

Abstract

Tissue organoids hold enormous potential as tools for a variety of applications, including disease modeling and drug screening. To effectively mimic the native tissue environment, it is critical to integrate a microvasculature with the parenchyma and stroma. In addition to providing a means to physiologically perfuse the organoids, the microvasculature also contributes to the cellular dynamics of the tissue model via the cells of the perivascular niche, thereby further modulating tissue function. In this review, we discuss current and developing strategies for vascularizing organoids, consider tissue-specific vascularization approaches, discuss the importance of perfusion, and provide perspectives on the state of the field.

Keywords: endothelial; extracellular matrix; microvessel; organoid; perfusion; spheroid; stem cell; vascular network; vascularization; vessel.

Figure 4

Microvessel Isolates. Individual microvessel fragments (A) are isolated from discarded lipoaspirates. Implanted vessels rapidly undergo angiogenesis to form a perfused network (B). When embedded in 3D matrices in vitro and cultured, sprouts (black arrow) form from parent vessel fragments (white arrow) (C), and grow to form a neovascular network. Figure adapted from LeBlanc et al. [85].

Figure 5

Vascularized adipose organoid. Microvessel fragments were combined with MSC-derived pre-adipocytes in an organoid format. 4,4-difluoro-4-bora-3a,4a-diaza-s-indacene (BODIPY) stain (A) shows lipid accumulation in differentiated adipocytes (A), while lectin stain shows the vasculature (B). Adapted from Strobel et al. [50].

Figure 1

Common methods of organoid fabrication. The “hanging drop” method involves placing drops of cell suspension on a plate and inverting it such that cells settle and aggregate within the drop (A). In recent years, the use of non-adherent microwell plates has become perhaps most frequent (B). Plates are commercially available, but can be custom made as well. In some cases, the use of a rotating bioreactor can also promote cell aggregation (C).

Figure 2

Vascularization Strategies: EC-only approaches frequently result in primitive EC tubes, rather than mature vasculatures (A). Adding in perivascular cells, such as MSCs, can add some stability to the network (B). Some groups have more recently derived tissues from stem cell aggregates (C). Stem cells can, given the correct stimuli, differentiate into a spectrum of cell types, which can mimic the cellular complexity found in vivo. This cellular complexity, when it is achieved, can lead to a more mature neovascular phenotype. Alternatively, incorporating whole microvessel fragments can also bring in the required cellular and structural complexity (D) needed to mimic the native vasculature (E).

Figure 3

Native microvasculature. Microvascular fragments are shown in (A) with phase contrast and fluorescent microscopy. Multiple cell types, including ECs, macrophages, and pericytes can be seen on vessel fragments. Native microvasculatures contain a complex, hierarchal structure, shown in (B). Panel B adapted from Strobel et al. [29].

Figure 6

Microfluidic organoid culture. Microfluidic devices in Homan et al. applied fluid flow to organoids, which increased vessel growth. While flow was largely external, some degree of intraluminal perfusion was evident. Adapted from Homan et al. [13].