ORTHOPEDIC INTERFACE TISSUE ENGINEERING: BUILDING THE BRIDGE TO INTEGRATED MUSCULOSKELETAL TISSUE SYSTEMS - Advanced Biomaterials: Fundamentals, Processing, and Applications - page 584

Subsequent conditioned media studies have revealed that both autocrine and
paracrine factors were responsible for the changes in phenotype observed during
osteoblast - fi broblast co-culture (Shan, Wang, and Lu 2007). In addition to fi bro-
blasts and osteoblasts, chondrocytes are also present at the interface.
Recently, Jiang et al. evaluated osteoblast and chondrocyte interactions using
a monolayer-micromass model (Jiang, Nicoll, and Lu 2005). This co-culture model
permits direct physical contact between these two cell types, while maintaining
the required 3D chondrocyte culture using the micromass culture. Similar to the
fi ndings of Wang et al. (Wang et al. 2007), osteoblast mineralization potential was
signifi cantly reduced due to heterotypic cellular interactions. These results col-
lectively suggest that osteoblast-fi broblast interactions modulate cell phenotypes
and may lead to trans-differentiation. It is likely that osteoblast-fi broblast and
osteoblast-chondrocyte interactions are key modulators of cell phenotypes at the
graft-to-bone junction. It is not known, however, which or if any of these cells are
directly responsible for interface regeneration. Fibroblast trans-differentiation
was anticipated as the ACL-bone insertion site fi brocartilage may be derived
from ligamentous tissue during development (Nawata et al. 2002). The hetero-
typic cellular interactions most likely also have a down-stream effect, either in
terms of cell trans-differentiation into fi brochondrocytes or in the recruitment
and differentiation of pluripotent cells for fi brocartilage formation.
While osteoblast - fi broblast interactions resulted in phenotypic changes and
the expression of interface-relevant markers, a fi brocartilage - like interface was
not formed in vitro in the above osteoblast-fi broblast co - culture study. Thus,
chondrogenic cells such as fi brochondrocyte precursors, chondrocytes, or stimu-
lated stem cells may be involved in interface regeneration. When Lim et al. (Lim
et al. 2004) coated tendon grafts with mesenchymal stem cells embedded in a
fi brin gel, the formation of a zone of cartilaginous tissue between graft and bone
was observed, suggesting a potential role for stem cells in fi brocartilage forma-
tion. Expanding upon their co-culture model, Wang et al. (Wang and Lu 2005)
later evaluated the effects of cellular interactions on stem cell differentiation by
tri - culturing fi broblasts, osteoblasts and bone marrow-derived mesenchymal
stem cells (MSCs). In the tri-culture model, fi broblasts and osteoblasts were
seeded on cover-slips on the left and right sides of a culture well, and the MSCs
were loaded into the hydrogel insert and maintained in 3D culture.
Under the infl uence of osteoblast-fi broblast interactions, MSC proliferation
remained unchanged, and these cells exhibited a similar level of alkaline phos-
phatase activity and proteoglycan synthesis as that of the insertion fi brochondro-
cytes. Moreover, under stimulation by osteoblast-fi broblast interactions, MSCs
produced a type II collagen-containing matrix. These observations suggest that
osteoblast - fi broblast interactions may indeed promote the differentiation of
MSCs into insertion fi brochondrocytes and facilitate the eventual repair of the
ligament - to - bone junction.
Findings from these in vitro examinations of heterotypic cellular interactions
utilizing novel cellular interaction models provide preliminary validation of
the hypothesis that osteoblast-fi broblast interactions promote the induction of