As humans, it is natural to categorize our environment. Usually, these groups include perceptible differences.
-The red shirt vs. the blue shirt (not the near infrared vs. far infrared shirt)
-The quiet (to our ears) sound vs. the loud sound
Since we don’t readily see or feel microscopic differences in our body, for most of us our default is to think of bone as bone, fat as fat, and blood vessels as blood vessels.
However, as with all of the above, blood vessels aren’t all the same. They differ in size, branching, permeability and the fluid forces they experience, and all of this affects cellular behavior. Furthermore, vessels in different organs may develop from different sources in utero, supporting the notion that there are attributes of blood vessels that are organ-specific.
I have previously blogged about the importance and challenges of vascularization in tissue engineering. Here, I want to focus on engineering organ-specific vasculature.
This is one of the foci of Dr. Ying Zheng’s lab at the University of Washington. Her lab is interested in studying kidney vasculature, particularly the peritubular capillaries, which are highly susceptible to damage from drugs and toxins that lead to either acute or chronic kidney disease.
To make a representative in vitro model, Dr. Zheng’s group decided they needed to: use the right cells, in the right geometry, in the most representative biomaterial.
This may all sound intuitive, but for many labs, the default endothelial cell line comes from large human umbilical veins (HUVECs), as these are generally quite easy to isolate and grow. However, as Dr. Zheng’s lab suspected, an endothelial line from the small kidney vessels is more likely to represent kidney microvasculature (peritubular capillaries) in vitro. They thus developed a technique to isolate human kidney peritubular microvascular endothelial cells (HKMECs).
In their first publication on the subject this year, they compare their HKMECs to HUVECs. They show that HKMECs have different expression of over 1500 genes compared to HUVECs, and importantly express a gene called plasmalemma (PV1), which is fundamental to the architecture of the kidney peritubular vasculature.
Since they were trying to mimic capillaries, which are small, highly branched blood vessels, they created a microfluidic system in which a branched grid is used to create a negative imprint in a stiff collagen gel, and the cells are then smoothly seeded throughout. This enabled them to 1) mimic capillary architecture and 2) permit perfusion, both of which gave a more realistic scenario. They noted that in these capillary systems, the HKMECs formed “fenestrae,” which are tiny holes in the vasculature that increase permeability. Not only were these on the same size scale as those found in vivo, when the same experiment was repeated with HUVECs, there were no fenestrae present. Clearly, the kidney specific endothelium was uniquely able to form a kidney-specific microvessel architecture.
Dr. Zheng’s group improved upon their kidney microvascular networks in their most recent publication by adding in one more organ-specific feature: extracellular matrix (ECM).
ECM is made up of a mixture of structural proteins like collagen, laminin and fibronectin. In addition to providing a framework for cells to bind, it also leads to specific cell signals. Each tissue has its own ECM composition, and this explains, in part, why they also have different tissue densities (e.g. liver is much stiffer than pancreas).
Rather than using a hydrogel made solely out of collagen I for their microfluidic networks, Dr. Zheng’s lab mixed in a gel formed from digested kidney tissue. They found that HKMECs were more quiescent (in a state of inactivity or dormancy) on the kidney hydrogels vs. collagen alone, suggesting that the cells were happier in their natural ECM environment.
Altogether, Dr. Zheng’s work is an impressive reminder of some fundamental pillars of tissue engineering, namely bringing together cells and scaffolds in a physiologically relevant culture environment. It is also a strong reminder that while we often use a ‘one-size-fits-all’ approach in the lab, this does not always lead to the best tissue models.
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