Engineering Fibrous Tissues and Their Interfaces with Bone - Structural Interfaces and Attachments in Biology - page 326

GDF 5, 6, 7 : In vivo studies have shown that injection of GDF 5, 6, and 7 improve
healing capacity of injured tendons in rats and rabbits [ 117 - 120 ]. In a rat model,
GDF 5 and GDF 6 at concentrations of 0, 1, and 10 g were soaked on a collagen

sponge. After 14 days, it was seen that failure loads increased for rat Achilles
tendons implanted with the treated sponge when compared to untreated collagen
sponge controls. Histology revealed that the application of GDF 5 and GDF 6
resulted in cell-abundant regions distributed around the collagen sponge [ 117 ].
The introduction of these factors can also promote new tendon and ligament
tissue formation [ 121 ]. Another study used GDF 5, 6, and 7 implanted subcutane-
ously and in the quadriceps muscle of rat models. In the subcutaneous implant,
after 21 days, light microscopy and electron microscopy were utilized to observe
collagen type I with structure resembling newly formed tendon tissue. Northern blot
analysis and PCR revealed mRNA expression of elastin, decorin, and collagen type
I[ 121 ]. When implanted into the quadriceps muscle, light microscopy images
confirmed crimped neotendon-/neoligament-like tissue formation in GDF implants
after 10 days [ 121 ]. Overall, the results of this study support that GDF 5, 6, and 7
can influence differentiation of connective tissue precursor cells to tendon and
ligament cells.
Growth factor experiments have shown success in promoting cell proliferation,
differentiation, and matrix production in a variety of both in vitro and in vivo
models of tendon/ligament healing. However, further studies must be undertaken
to better understand the specific intracellular pathways involved and the effects of
combining different growth factors simultaneously or in a temporally controlled
manner before optimal growth factor regimens for improving fibrous tissue
production from cell-scaffold constructs can be identified.

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15.4.3.2 Bioreactors

To improve tissue formation within cell-scaffold constructs, bioreactor culture
systems have been explored for a variety of tissue engineering applications.
Bioreactors can be divided into two classes: constructs that can be directly
implanted into the body and the in vivo environment acts as the bioreactor, or
constructs that can be placed in an ex vivo cell culture system to control and
maintain the biochemical and physical environment [ 8 ]. An in vivo bioreactor
system provides a native environment for cell culture, but fine control of signals
is lost and understanding of regulatory pathways is not clear. An ex vivo bioreactor
allows for the control and introduction of biochemical and physical signals, which
can help promote cell proliferation, differentiation, and tissue development. In
particular, ex vivo tensile stimulation has been reported to induce cellular prolifera-
tion, fibroblastic differentiation, cellular alignment, and ECM synthesis and
remodeling [ 122 - 125 ]. In addition, this type of bioreactor may, in turn, provide
better understanding of tissue development associated mechanosensitive signaling
pathways. However, concern remains over scalability of ex vivo bioreactors for
clinical scale production of engineered constructs.