Biomedical Engineering Reference
In-Depth Information
dentinogenesis. Cells that remain in the healthy portion of the pulp migrate to the injured
site, proliferate by the growth factors released from surrounding dentin matrix and attach
the necrotic layer to form osteodentin. Later, the cells attached to osteodentin differentiate
into odontoblasts to produce tubular dentin, thus forming reparative dentine. This early
mineralized tissue preserves the pulp integrity and serves as protective barrier upon the
injury (Nakashima, 2005).
When the tooth is further damaged, dentin regeneration becomes difficult as it requires a
healthy pulp. Thus, bigger traumas or advanced caries are clinically treated with root canal
therapy, in which the entire pulp is cleaned out and replaced with a gutta-percha filling.
However, living pulp is critical for the maintenance of tooth homeostasis and essential for
tooth longevity.
An ideal form of therapy might consist of regenerative approaches in which diseased or
necrotic pulp tissues are removed and replaced with regenerated pulp tissues to revitalize
the teeth. In particular, the regenerative pulp therapy would reconstitute the normal tissue
continuum at the pulp-dentine border by regulating the tissue-specific processes of
reparative dentinogenesis. Two types of dental pulp regeneration can be considered based
on the clinical situations: in situ regeneration of partial pulp or de novo synthesis of a total
pulp replacement (Sun et al., 2010).
Engineering and regeneration of dental pulp tissue still remain a difficult task. A regenerated
pulp tissue should be functionally competent: it should be vascularised, contain similar cell
density and architecture of ECM to those of natural pulp, be capable of giving rise to new
odontoblasts lining against the existing dentin surface, produce new dentin and be innervated.
The first step to engineer tissues is to isolate cells with the right phenotype and propagate
them in suitable culturing environments. DPSCs can be isolated by two methods: the
enzyme-digestion method and the explants outgrowth method. The first method involves
the collection of the pulp tissue under sterile conditions, the digestion with appropriate
enzymes (collagenase, dispase, trypsin), the seeding in culture dishes containing a special
medium supplemented with necessary additives, and then the incubation at 37°C. The
second method implies that the extruded pulp tissue is cut into 2 mm
3
, and directly
incubated in culture dishes containing the essential medium with supplements. A period of
two weeks is generally needed to allow a sufficient number of cells to migrate out of the
tissue. It has been demonstrated that cells isolated by enzyme-digestion have a higher
proliferation rate than those collected by outgrowth (Huang et al., 2006).
Once these cells are grown on a two-dimensional surface, it is possible to transfer them to a
three-dimensional scaffold construct. The scaffold provides a 3D environment for cells to
attach and grow, therefore mimicking the in vivo condition (Fig.2). An ideal scaffold should
be biocompatible, biodegradable, and have adequate physical and mechanical strength.
Then, it should be porous to allow placement of cells and effective transport of nutrients,
oxygen, waste as well as growth factors. Finally, it should be replaced by regenerative tissue
while retaining the shape and form of the final tissue structure (Saber, 2009).
Scaffolds can be fabricated from natural polymers or synthetic materials. The natural polymers
have advantages of good biocompatibility and bioactivity. On the contrary, synthetic matrices
enable precise control over the physiochemical properties such as degradation rate, porosity,
microstructure, and mechanical strength (Sharma & Elisseeff, 2004).
Examples of natural polymers are collagen, gelatin, dextran and fibronectin. Although
collagen is a commonly used matrix in which to grow cells in three-dimensions, several cell
types are known to cause the contraction of collagen. It has been demonstrated that pulp
