To repair cartilage, surgeons typically take a piece of cartilage from another part of the injured joint and patch the damaged area, this procedure depends on damaging otherwise healthy cartilage. Also, such autotransplantation procedures are little protection against age-dependent cartilage degeneration.
There must be a better way. Bioengineers want to discover more innovative ways to grow cartilage from patient’s own stem cells. A new study from the University of Pennsylvania might make such a wish come true.
This research, comes from the laboratories of Associate professors Jason Burdick and Robert Mauck.
“The broad picture is trying to develop new therapies to replace cartilage tissue, starting with focal defects – things like sports injuries – and then hopefully moving toward surface replacement for cartilage degradation that comes with aging. Here, we’re trying to figure the right environment for adult stem cells to produce the best cartilage,” said Burdick.
Why use stem cells to make cartilage? Mauck explained, “As we age, the health and vitality of cartilage cells declines so the efficacy of any repair with adult chondrocytes is actually quite low. Stem cells, which retain this vital capacity, are therefore ideal.”
Burdick and his colleagues have long studied mesenchymal stem cells (MSCs), a type of adult stem cell found in bone marrow and many other tissues as well that can differentiate into bone, cartilage and fat. Burdick’s laboratory has been investigating the microenvironmental signals that direct MSCs to differentiate into chondrocytes (cartilage-making cells).
A recent paper from Burdick’s group investigated the right conditions for inducing fat cell or bone cell differentiation of MSCs while encapsulated in hydrogels, which are polymer networks that simulate some of the environmental conditions as which stem cells naturally grow (see Guvendiren M, Burdick JA. Curr Opin Biotechnol. 2013 Mar 29. pii: S0958-1669(13)00066-9. doi: 10.1016/j.copbio.2013.03.009). The first step in growing new cartilage is initiating cartilage production or chondrogenesis. To do this, you must convince the MSCs to differentiate into chondrocytes, the cells that make cartilage. Chondrocytes secrete the spongy matrix of collagen and acidic sugars that cushion joints. One challenge in promoting MSC differentiation into chondrocytes is that chondrocyte density in adult tissue is rather low. However, cartilage production requires that the chondrocytes be in rather close proximity.
Burdick explained: “In typical hydrogels used in cartilage tissue engineering, we’re spacing cells apart so they’re losing that initial signal and interaction. That’s when we started thinking about cadherins, which are molecules that these cells used to interact with each other, particularly at the point they first become chondrocytes.”
In order to simulate this microenvironment, Burdick and his collaborators and colleagues used a peptide sequence that mimics these cadherin interactions and bound them to the hydrogels that were then used to encapsulate the MSCs.
According to Mauck, “While the direct link between cadherins and chondrogenesis is not completely understood, what’s known is that if you enhance these interactions early during tissue formation, you can make more cartilage, and, if you block them, you get very poor cartilage formation. What this gel does is trick the cells into think it’s got friends nearby.”
See L Bian, et al., PNAS 2013; DOI:10.1073/pnas.1214100110.