Tissue engineering - Orthopaedic Biomaterials in Research and Practice - page 229

cartilage repair. Thus, there many different types of cells that can be
used. Cells can be classified based on their source:

• Allogenic cells: cells from the donor of the same species; for exam-

ple, dermal fibroblasts from human foreskin have been shown to
be viable for engineering of skin.

• Autologous cells: cells from the same individual in which the cells

will be reimplanted. Since the cells are from the same individual,
they pose the least risk of rejection and pathogen transmission.
However, the challenge is that autologous cells are not always
available. For example, individuals that are very ill, or who have a
genetic disease, may not be able to provide useful cells. The need
to surgically remove these cells may also expose these individuals
to surgical site infections or postoperative pain.

• Xenogenic cells: cells from the donor of another species; for exam-

ple, animal cells such as bovine, equine, and porcine tissues have
been used extensively for cardiovascular implants.

Other categories of cells include stem cells, syngenic cells, pri-
mary cells, and secondary cells. Stem cells are undifferentiated cells
that can be divided in culture into different specialized cells. These
may be used for repairing diseased or damaged tissue or used to grow
new organs. Syngenic cells, which are also known as isogenic cells,
are those derived from genetically identical donors, such as twins or
clones. Primary and secondary cells refer to the source, where primary
ones are from an organism compared with secondary ones, which are
from a cell bank.

Skin grafts

One of the milestones of tissue engineering is the development and wide-
spread use of skin grafts. Part of the drive and desire to develop skin
grafts for plastic and reconstructive surgery was to help treat injuries
from military wars. Extensive investigation of tissue healing through
cellular effects, as well as through the cultivation of cells outside the
body, has helped fuel growth in this area. As such, cell biology and espe-
cially in vitro cell culture became the mainstay of what can be consid-
ered tissue engineering.
In the 1950s, animal studies showed that the products of a culture
of epidermal cells could be applied to a graft bed to reconstitute an
epidermis. However, a challenge at that time was the inefficient means
of cell cultivation, which provided insufficient cells to sustain trans-
plantation. Then, in the 1960s and 1970s, growth factors were found to
promote greater proliferation of epidermal cells when the factors were
added to the culture medium. In the late 1970s, cultured cells were
shown that they could be grown in sheets in a petri dish and trans-
ferred intact, rather than as disaggregated cells, to a graft wound bed.
During that same period, the use of fibroblasts to condense a hydrated