Effect of Pulse Rate on Collagen Deposition in the Tissue-Engineered Blood Vessel
AMY SOLAN, M.S.,1 SHANNON MITCHELL,1 MARSHA MOSES, Ph.D.,2 and
LAURA NIKLASON, M.D., Ph.D.1,3
http://www.liebertonline.com/doi/pdf/10.1089/107632703768247287?cookieSet=1
One of the challenges in tissue engineering blood vessels is to culture blood vessels that
take on the characteristics of real blood vessels. In particular, a good blood vessel must be able to withstand
the mechanical forces from the fluid that flows through it. It is known that the
mechanical strength of blood vessels come primarily from the ECM (which includes
collagen and elastin). The more collagen there is, the greater the rupture strength
of the vessel. Smooth muscle cells are also responsible for making components of
the ECM. Collagen content is also affected by the proteinase MMP-1 and TIMP-1
(an MMP-1 inhibitor). Knowing this, the group studied the effect of pulsatile stresses on the
development of blood vessels cultured in vitro.
To do this, they tissue engineered blood vessels polyglycolic acid (PGA) mesh and
subjected them to various pulsatile stresses (including static flow – 0 bpm, adult
heart rates – 90 bpmf fetal heart rates – 165 bpm). They then calculated the radial
distension with measurements of the changing diameter of the blood vessel at
different time points. They also measured collagen content by its dry weight and
found that vessels grown under pulsatile conditions (especially 90 bpm and 165 bpm)
had significantly higher collagen content than vessels grown in static conditions.
However, the rate of the pulses did not seem to matter in blood vessel development.
They also measured the amount of MMP-1 and TIMP-1, both of which were significantly
higher than the static flow condition than the pulsatile conditions.
This experiment was useful because it shows that collagen content is increased in
vessels subjected to pulsatile conditions, making the vessels thicker and probably
more suitable for grafts. In class, we've seen that the conditions in which we grow our cultures
can affect how the culture and our results turn out (like the collagen in the serum). Similarly in this
experiment, the environment affected the mechanical and physical properties of the blood vessel.
It would be interesting to see how these vessels (made this way where cells are forced to make
collagen) would compare to vessels made on gels with a just a high collagen content. Also,
this experiment quantified the collagen content by dry weight, but did not check for protein
expression. Since TIMP-1 and MMP-1 are responsible for collagen breakdown, it may be
interesting to see whether the cells constantly make collagen (that TIMP-1 and MMP-1 break
down) or if collagen content is relatively constant after some time.
5 comments:
What was the structure of the tissue engineered blood vessels besides the mesh? Did they line them with cells to help make it more like a native blood vessel?
Collagen content helps with the mechanical strength of a lot of tissues (bone, blood vessels, tendons, etc). But the strength often depends on the way that the fibrils are laid out and their structure. Did they mention, or do you know, what kind of structure the collagen needs to be in to increase the mechanical strength of blood vessels?
To the first question: To see which results were statistically significant (such as the effect of different pulsatile rates), they looked at the p values from the Tukey LSD test. They also compared the experimental collagen content values to collagen content of normal, native arteries.
To the second question: The engineered blood vessel was a silicone tube with PGA mesh surrounding it. That entire thing is then attached to a glass bioreactor and a non-degradable sleeve. The silicone tubing was able to provide the pulsatile strain. They did seed each vessel with SMCs.
They only mentioned that applying the stresses increased the collagen content. However, other experiments on collagen gels showed/mentioned that aligned collagen reduces contraction of the gel and improves the strength of the gel. Other factors on collagen include its length (shorter collagen fibers are better) and the amount of cross-linking within the gel. These are only studies on gels, but I think they would apply to blood vessels as well.
Did this paper mention results of increased collagen production in other types of tissue under pulsatile stress? It would be interesting to see if this stress could affect gene expression for cardiac muscle or other systems subjected to repetitive stresses in the body.
It would be interesting to see which aspect had more influence on mechanical strength: shear collagen content or (like An-chi and you discussed) the structure and cross-linking of the collagen. Also, does the pulsatile stress influence only collagen production or do you think it also influences the structure and cross linking of the collagen present?
They didn't look at other tissue-type cells. In this study, they made cultures with three different cell types: human aortic smooth muscle cells, human dermal fibroblasts, and rat aortic smooth muscle cells. The reason they chose these cells and did the study was because there haven't been many experiments studying cardiovascular cells in 3D in general, so maybe in the near future. They did find however, that even between the rat and human SMCs, the mechanical properties differed (not the collagen content) after a few days - the human SMCs seemed to "detiorate" quicker than the rat SMCs after day 4 of the pulsatile stress. It would be interesting to see what other factors affect similar cell types of different species, and why that may be. It might be interesting to compare that data to data of other cell types, too.
I think it would have an affect on the structure since collagen gels usually contract a bunch after a few days. As for the cross-linking, I think that would depend on other factors. In this study, they looked at MMP-1 and TIMP-1, so there may be factors that cells secrete to regulate the collagen matrix. The collagen structure may depend on the cell movement/migration too. I wonder if cells move along collagen or through it.
Post a Comment