Nurturing Complexity: The role of vascularization in Organoid Advancements

Like all organoids, brain organoids appear to also have a significant obstacle where oxygen and nutrients cannot be delivered to the innermost parts of the organoid due to the brain organoids lacking vascularization. To address this, a number of practical methods have been conducted. An overview of just a handful of methodologies, is explained below:

The vascularization of organoids, a critical aspect of creating physiologically relevant miniature organs, often involves the co-culture with endothelial cells. Endothelial cells are derived from mesoderm and are required for the formation of new blood vessels.

This interaction mirrors the intricate interaction that occurs during embryonic development and early organogenesis, encouraging endothelial cells to self-assemble into capillary-like structures within the organoid.

In the context of human brain organoids, co-culturing with vascular cells, specifically a monolayer of endothelial cells, has been employed to achieve organoid vascularization. In 2018, Pham et al., successfully vascularized 34-day-old human brain organoids by embedding them in Matrigel, enriched with endothelial cells derived from human iPSCs, along with bone morphogenetic protein 4 (BMP4), vascular endothelial growth factor A (VEGF 165), and fibroblast growth factor 2 (FGF-2). After 34 days of co-culture, these brain organoids displayed a network of penetrating vessels expressing platelet endothelial cell adhesion molecule, a key endothelial marker.

Perhaps the most common method for organoid vascularization involves co-culturing human organoids with human umbilical vascular endothelial cells (HUVECs). In brain organoids vascularized through HUVEC co-culture for over 200 days, Shi et al., found that vascularization initially manifests in the ventricular zone (VZ)-like region, which houses neural stem and progenitor cells (NSPCs), mirroring the developmental patterns of the human brain vasculature. Vascularized brain organoids exhibit more robust growth compared to their non-vascularized counterparts, suggesting that vascularization promotes the proliferation of NSPCs and/or the survival of neurons, even in an in vitro environment.

Co-culture of organoids with stem cells differentiated toward an endothelial fate (right), resulting in cortical organoids with a vasculature (red). The control organoids (left) express few endothelial markers. Cakir et al., Nature Methods. Engineering of human brain organoids with a functional vascular-like system.

Bioprinting technologies allow researchers to precisely position cells and biomaterials to create intricate structures, including vascular networks. By using bioinks composed of endothelial cells and supportive matrix materials, researchers can fabricate organoids with predefined vascular patterns. Bioprinting allows for fine control over the architecture of the vascular network. It can create highly customized organoids with intricate vasculature.

Bioprinting technologies allow researchers to precisely position cells and biomaterials to create intricate structures, including vascular networks. By using bioinks composed of endothelial cells and supportive matrix materials, researchers can fabricate organoids with predefined vascular patterns. Bioprinting allows for fine control over the architecture of the vascular network. It can create highly customized organoids with intricate vasculature.

Bioprinting has emerged as a transformative technology for creating large-scale tissue constructs with intricate vascular networks. Two primary methods, Filament Deposition Fabrication (FFF) and Fused Deposition Modeling (FDM), are frequently employed to achieve this goal. These techniques involve depositing cell-laden hydrogel filaments in a controlled manner to form specific patterns, offering immense potential for tissue engineering.

Although bioprinting enables the creation of tissues with spatially organized cells and extracellular matrices, it faces challenges related to resolution. The printing resolution is limited, particularly in extrusion-based bioprinting, which has a resolution of about 100 µm, whereas the diameter of typical capillaries is approximately 10 µm.

When grown under fluidic shear stress, a network of blood vessels (red) develops to connect kidney cells (green) within and between organoids. Credit: Wyss Institute at Harvard University.

The vascularization of organoids represents a groundbreaking frontier in regenerative medicine and tissue engineering. As techniques continue to advance, organoids hold the potential to revolutionize drug testing, disease research, and transplantation. The integration of vascular networks within organoids brings us closer to realizing their full potential in reshaping the landscape of medical research and treatment.