In Vivo, In Vitro, In Silico: Computational Tools for Product and Process Design in Tissue Engineering - Computational Modeling in Tissue Engineering - page 3

vitro process to be impervious to a wide scale of external perturbations, which are
often unavoidable in an artificial environment. Additionally developmental pro-
cesses exhibit a multistage character, meaning cells will differentiate in charac-
teristic stages, with a distinct morphology and marker genes. Accordingly, the in
vitro process could be divided into a series of sequential subprocesses, each cor-
responding to a specific stage in developmental biology. This would make the
process highly observable, by for example determining expression of certain
marker genes, and highly controllable, since growth factors could be added when
the cells are at a stage where they are competent to respond to them. Another
concept advocating the use of in vitro developmental processes is path-depen-
dence, or the dependence of one developmental stage on the previous ones. This
means that the optimal conditions of the successive stage are provided by its
predecessors. These conditions consequently do not have to be incorporated in the
process design and will make the process more autonomous. An example of path-
dependence can be found in endochondral ossification, where first a cartilage
anlage is formed which creates optimal conditions for the invading ossification
front [ 9 ]. Furthermore, some intermediate tissue forms have a great robustness
thanks to intrinsic factors, allowing them to be treated as individual modules. The
regulative, self-controlled behaviour exhibited by these modules will likely lead to
a high product consistency. Several modular forms appear during development,
including cellular modules like cartilage condensations and multicellular modules
with a spatially extended and heterogeneous cell population like the growth plate.
The growth plate is a developmental centre that integrates many signalling
pathways in order to regulate the patterning and growth of the skeleton. As a cell
progresses throughout the growth plate, going from the long bone's epiphysis
towards the diaphysis, its shape and function changes drastically [ 10 ]. At the
epiphysis, a pool of small round chondrocytes makes up the resting zone. These
cells differentiate into more rapidly proliferating flat chondrocytes, forming pro-
liferative columns. The resting and proliferating chondrocytes secrete structural
proteins, such as collagen type II, that form a hyaline cartilage matrix. Towards the
diaphysis, chondrocytes differentiate further into prehypertrophic, secreting Indian
Hedgehog (Ihh), and thereafter hypertrophic chondrocytes [ 11 ]. Hypertrophic
chondrocytes remodel the cartilage matrix into a calcifying matrix comprising
primarily collagen type X (Col-X). At terminal differentiation, the cells will induce
invasion and resorption of the hypertrophic cartilage as well as the start of vas-
cularisation by excreting proteins like Matrix Metallopeptidase 13 (MMP13) and
Vascular Endothelial Growth Factor (VEGF) [ 12 ]. The evolution the chondrocytes
undergo is reminiscent of the developmental process of endochondral ossification,
indicating these events can be recapitulated using adult stem cells [ 13 - 17 ]. Indeed,
implantation of articular chondrocytes (mixed with osteoblasts) in mice has been
shown to result in formation of a structure similar to that of the growth plate [ 18 ].
Lenas et al. [ 4 - 8 ] extensively discuss how the growth plate developmental
process can be used as a template for robust and reliable bone TE processes.
Proof-of-concept for the basic idea that aggregates or constructs stem cells of
(embryonal and postnatal bone marrow derived) after in vitro differentiation along