Like the complex, multi-cellular tissues found within a person, Organovo’s human tissues are highly dynamic. New cells are created through cell division; they mature and integrate into the tissue, forming connections with surrounding cells and contributing functionality throughout their lifespan. As individual cells within the tissue age, they eventually undergo cellular senescence and death—much as they would in a living tissue inside the body.
The dynamic nature of bioprinted tissues is highlighted by a series of images of a multi-layered 3D tissue that mimics the layered architecture of the blood vessel wall. Within this thick tissue, cell division is balanced with cell death, while the layered structure and overall tissue dimensions remain stable over time.
Human blood vessel wall analogues were bioprinted into multi-well transwell plates (Corning). Tri-layered architecture consisted of a thin layer of human fibroblasts (representing the adventitia), a 250-micron layer of human vascular smooth muscle cells (representing the media) and a thin coating of human vascular endothelial cells (representing the intima). The laminar geometry was maintained in culture over time. These 3D human tissues are dynamic, exhibiting both cell proliferation (KI67, lower-right panel) and cell death (Tunel, upper-right panel) over time while retaining tissue-like density and compartmentalized architecture.
Native tissues inside the body contain multiple cell types that are often arranged into a specific pattern or structure driven by the function(s) required of that tissue. For example, a native blood vessel has three distinct cellular layers—a thin outer fibrous coating, composed of fibroblast cells and their extracellular matrix; a media, composed of smooth muscle cells that can expand and contract in response to physical or chemical stimuli, and a thin inner layer of endothelial cells that line the lumen of the vessel and act as a barrier between the blood and underlying tissue.
As can be appreciated in the images, bioprinting enables construction of tissues layer by layer, ensuring that each layer contains the relevant cell type(s) and has dimensions that approximate those of native tissue. When multiple cell types are present and organized properly, tissue-specific functions are often enhanced. For example, the production of albumin by bioprinted 3D liver tissue is significantly greater compared to simple 2D cultures established with the same cells.
Bioprinted vascular conduit after 28 days of bioreactor conditioning, highlighting the layered architecture and deposition of extracellular matrix (blue color). Image is Masson’s Trichrome (20x).