Engineering Fibrous Tissues and Their Interfaces with Bone - Structural Interfaces and Attachments in Biology - page 328

15.5 Tissue Engineering of Fibrous Tissue
Interfaces with Bone
The enthesis, or site of attachment of the tendon or ligament to the bone, is a
common focus of tissue overuse injuries that can lead to rupture [ 133 ]. Engineering
fibrous tissue-bone interfaces is challenging as constructs must address the required
mechanical properties and physiology of two very disparate tissues. The native
interfacial tissue's fibrocartilaginous composition also differs significantly from the
tissues on either side (tendon/ligament and bone). However, enthesis tissue engi-
neering is similar to regenerating the midsubstance of the tendon or ligament in that
cells, scaffolds, and exogenous factors have been explored to recreate the insertion
site, although more complex combinations of these basic elements may be required
than for regular fibrous tissue engineering. This section will discuss concepts of co-
culture of different cell types, phasic and gradient scaffolds, and delivery of soluble
factors to regenerate the transition from fibrous tissue to bone.
15.5.1 Cells
Co-culture methods enable the interaction between two different cell populations in
a controlled system. Such systems have been proposed to aid interface tissue
engineering, as well as understanding the developmental processes resulting in the
formation of the fibrous tissue-bone insertion [ 46 ]. At the tendon-/ligament-bone
insertion site, fibroblasts, chondrocytes, hypertrophic chondrocytes, and osteoblasts
have been identified [ 12 , 13 , 18 , 134 ]. A study performed using a co-culture of
monolayer bovine osteoblasts with a condensed micromass of bovine chondrocytes
found that, over 21 days in vitro , cell number remained constant over time for both
cell types. In addition, there was little glycosaminoglycan deposition present in co-
culture experiments when compared to the chondrocyte culture control, and alkaline
phosphatase activity remained constant over time, as opposed to increasing over
time in the osteoblast-only control. It was shown that co-culture had no effect on
osteoblast collagen type I gene expression, but mineralization was delayed [ 135 ].
These results suggest that the interactions between the two cell types affect the
phenotype of both cells; however, further studies need to be performed to understand
the type of specific interactions that are responsible for the alterations observed.
Co-culture experiments have also been performed utilizing bovine ACL
fibroblasts and osteoblasts from trabecular bone fragments of neonatal calves
[ 136 ]. This co-culture model was created in a tissue culture well with a permeable
hydrogel divider in the middle. By day 7, fibroblasts and osteoblasts had migrated
to the interface region of the hydrogel. Alkaline phosphatase activity in the co-
culture peaked at day 14 and decreased through day 28, whereas the osteoblast
control culture demonstrated increasing intensity of alkaline phosphatase staining
over 28 days. Glycosaminoglycan deposition (measured by Alcian Blue staining)