Biomedical Engineering Reference
In-Depth Information
modifi cation or formulation in vitro prior to their medical use. Examples include Carticel (Box 14.6)
and Apligraf, a skin substitute used in the treatment of certain ulcers, which is composed of
keratinocytes and fi broblasts derived from human neonatal foreskin tissue and bovine collagen.
14.10.1 Stem cells
The therapeutic application of stem cells has long been a dream of medical sciences, but recent
discoveries and technical advances have brought this dream somewhat closer to being a reality.
Stem cells are usually defi ned as undifferentiated cells capable of self-renewal that can differenti-
ate into more than one specialized cell type.
Such cells are often classifi ed on the basis of their original source as either embryonic or adult
stem cells. As the name suggests, embryonic stem cells are derived from the early embryo, whereas
adult stem cells are present in various tissues of the adult species. Much of the earlier work on
embryonic stem cells was conducted using mouse embryos. Human embryonic stem cells were
fi rst isolated and cultured in the laboratory in 1998. Research on adult stem cells spans some four
decades, with the discovery during the 1960s of haematopoietic stem cells in the bone marrow
(Chapter 10). However, the exact distribution profi le, role and ability to manipulate adult stem
cells (particularly those outside of the bone marrow) are subjects of intense current research, and
for which more questions remain than are answered.
Embryonic stem cells are derived from pre-implant-stage human embryos, usually at the blastocyst
stage (the blastocyst is a thin-walled hollow structure containing a cluster of cells, known as the inner
cell mass, from which the embryo arises). These embryos are invariably ones initially generated as part
of in vitro fertilization procedures but which are destined to be discarded, either due to poor quality or
because they are in excess to requirement. There are an estimated 400 000 in vitro fertilization-produced
embryos in frozen storage in the USA alone, of which some 2.8 per cent are likely to be discarded.
Culture of human embryonic stem cells starts with the recovery of the blastocyst's inner cell
mass (Figure 14.17). One common recovery procedure is termed 'immunosurgery'. The process
.. .
. .
Inner cell mass
IVF
(day 0)
Blastocyst
(day 5)
.. .
.
.
. .
Recovery of
inner cell mass
Culture on feeder cell layer
Figure 14.17 Overview of the generation and culture of human embryonic stem cells. IVF: in vitro fertilization.
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