Harnessing Cell–Biomaterial Interactions for Osteochondral Tissue Regeneration - Tissue Engineering: Cell-Surface Interactions for Tissue Culture - page 75

used because they are able to differentiate along multiple cell lineages, can pro-
liferate readily, and can be easily harvested.
5.1.1 Bone-Marrow-Derived Stem Cells
The bone marrow contains both hematopoietic stem cells and MSCs. The cloning
abilities of MSCs along with their multilineage capacity suggest their potential for
use in tissue engineering applications [ 42 , 43 ]. The method used to culture MSCs
and the associated culture conditions can affect the phenotype of the cells.
Therefore, to identify a homogenous population of MSCs, the Mesenchymal and
Tissue Stem Cell Committee of the International Society of Cellular Therapy
proposed minimal criteria for human designation of MSCs as follows: (1) MSCs
must adhere to tissue culture plastic; (2) MSCs must be positive for CD105, CD73,
and CD90 and negative for CD45, CD34, CD14, or CD11b, CD79a or CD19, and
HLA-DR; (3) MSCs must be capable of differentiating into osteoblasts, adipo-
cytes, and chondroblasts under standard in vitro conditions [ 44 ]. The ability of
MSCs to differentiate into chondroprogenitor cells as well as to form cartilage in
vivo has led to continued efforts to utilize MSCs for cartilage engineering [ 45 - 47 ].
As previously discussed, the process of condensation drives cell-cell interac-
tions in vivo and initiates chondrogenesis. Therefore, MSCs have been cultured as
pellets, aggregates, or spheroids to promote chondrogenic cell-cell interactions.
MSCs are able to express type II collagen and aggrecan with low expression levels
of type I collagen when they are in pellet culture [ 48 , 49 ]. Cell culture conditions,
such as the culture method and the presence of chondrogenic factors, play a key
role in directing MSCs towards a chondrogenic lineage. For example, MSCs
exposed to dexamethasone, an anti-inflammatory agent for MSC chondrogenesis,
and transforming growth factor (TGF)-b 1 /TGF-b 3 together in pellet culture dem-
onstrated an increased level of aggrecan and type II collagen expression compared
with the presence of the individual chondrogenic factors [ 50 , 51 ]. MSCs in pellet
culture in the presence of dexamethasone and TGF-b 1 /TGF-b 3 have also shown
increased levels of type X collagen expression and alkaline phosphatase activity,
which indicates the pellet culture may induce hypertrophic chondrocyte differen-
tiation under certain conditions [ 49 , 52 ]. However, when MSCs we co-cultured
with mature chondrocytes in a pellet culture with dexamethasone and TGF-b 3 ,
type II collagen expression was found to be significantly higher than with culture
of chondrocytes alone [ 53 ]. Human MSC pellets cultured with dexamethasone and
TGF-b 1 along with conditioned medium from human chondrocyte pellets induced
type II collagen expression and lowered type X collagen expression, when com-
pared with MSC pellets without conditioned medium from chondrocytes [ 51 ].
These co-culture systems provide a mechanism to expose MSCs to cartilage
proteins and ECM molecules produced by chondrocytes, which may facilitate the
initiation of chondrogenesis. Additionally, exposure of MSC pellets to 10 ng/mL
parathyroid-hormone-related peptide has been shown to reduce type X collagen
expression, which is responsible for inhibiting chondrocytes from transiting from a