Biomedical Engineering Reference
In-Depth Information
for large proteins, and possibly even cells, to pass through. Fibrovascular tissue will pass
through any material with pores greater than 10 nm. Pore size and distribution are rarely
regular, and abnormalities will both change general material properties as well as fluid flow
rate across the device.
A major focus in tissue engineering is controlling cell organization and regeneration. The
more control the researcher has over cell development, the greater the capabilities and the
wider the range of applications of the bioartificial tissue. Stem cells provide an opportunity
for researchers to develop tissues essentially from scratch. Stem cells both build and main-
tain cells in vivo and possess the ability to be used for tissue generation ex vivo. Some back-
ground information on this new technology is provided following, and specific applications
to tissue engineering are available in later chapters.
1.5.3 Stem Cell Research
In recent years, stem cells have become the topic of both intense controversy and incred-
ible excitement within the research community. The potential for stem cell technology is
apparently limitless, with some known possibilities shown in Figure 1.7. Cells may be used
to test drugs on different types of tissues, to understand how to prevent birth defects, and
to potentially replace and regenerate damaged tissue in the body. The possibilities truly
seem endless.
In actuality there are two different types of stem cells.
come from
embryos, which are mostly supplied by in vitro fertilization clinics four to five days following
fertilization. At this point, stem cells will either self-regenerate or commit and differentiate.
Self-renewal
Embryonic stem cells
means that the stem cell will reproduce with no developmental
commitment. Essentially, the stem cell remains a stem cell.
or
regeneration
Differentiation
is the expression
of tissue or cell-specific genes.
For the majority of tissues in the human body, cells will differentiate terminally. In some
cases, however, dynamic operation is required, and, as such, a population of
adult stem cells
is maintained for regeneration purposes. The two most common types of adult stem cells
are those of the hematopoietic system (blood renewal) and the intestinal epithelia. These
cell types are similar in that they both occur in very large numbers and have short life
spans. Stem cells are required to maintain this dynamic population.
Researchers control stem cell development and differentiation within cultures by a num-
ber of means. For embryonic stem cells, the difference between self-regeneration and differ-
entitation, surprisingly enough, is the concentration of a single essential protein, or growth
factor. Leukemia inhibitory factor (LIF), in high enough concentrations, will cause embry-
onic stem cells to regenerate indefinitely in cultures. This is an interesting fact, because it
proves that stem cell development is not an intrinsic predetermined state but rather is
induced by extrinsic factors.
With an executive order in 2009, President Barack Obama lifted an eight-and-a-half-year
ban on government-funded stem cell research, earning praise from the science community
for opening the door to potential cures for some of mankind's most debilitating diseases. Both
tissue engineering and stem cell research represent just a sampling of the breakthrough
biologically focused ventures currently being explored by today's biomedical engineers.
