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Lecture 25

BIOL201 Lecture Notes - Lecture 25: Cell Culture, Glycosaminoglycan, Keratinocyte

Biology (Biological Sciences)
Course Code
Warren Gallin

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Stem-Cell Engineering
When cells are removed from the body and maintained in culture, they generally maintain their
original character. Keratinocytes continue to behave as keratinocytes, chondrocytes as
chondrocytes, liver cells as liver cells, and so on. Each type of specialized cell has a memory of
its developmental history and seems fixed in its specialized fate, although some limited
transformations can occur, just as in the intact tissues we discussed earlier. Stem cells in culture,
as in tissues, may continue to divide, or they may differentiate into one or more cell types, but
the cell types they can generate are restricted. Each type of stem cell serves for the renewal of
one particular type of tissue. For some tissues, such as the brain, it was long thought that
regeneration is impossible in adult life because no stem cells remain. There seemed to be little
hope, therefore, of replacing lost nerve cells in the mammalian brain through the genesis of new
ones, or of regenerating any other cell type whose normal progenitors are no longer present.
Recent discoveries have overturned this gloomy judgement and have led to a more optimistic
perception of what stem cells can do and how we may be able to use them. The change has come
from several findings that demonstrate exceptional forms of stem-cell versatility that could
scarcely have been suspected from knowledge of the normal life histories of cells in tissues. In
this last section of the chapter, we examine these phenomena and consider the new opportunities
they create to improve on nature's own mechanisms of damage repair.
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ES Cells Can Be Used to Make Any Part of the Body
As described in Chapter 21, it is possible through cell culture to derive from early mouse
embryos an extraordinary class of stem cells called embryonic stem cells, or ES cells. ES cells
can be kept proliferating indefinitely in culture and yet retain an unrestricted developmental
potential. If they are put back into an early embryonic environment, they can give rise to all the
tissues and cell types in the body, including germ cells. They integrate perfectly into whatever
site they may come to occupy, adopting the character and behavior that normal cells would show
at that site. One can think of development in terms of a series of choices presented to cells as
they follow a road that leads from the fertilized egg to terminal differentiation. After their long
sojourn in culture, the ES cell and its progeny can evidently still read the signs at each branch in
the highway and respond as normal embryonic cells would.
Cells with properties similar to those of mouse ES cells can now be derived from early human
embryos and from human fetal ovaries and testes, creating a potentially inexhaustible supply of
cells that might be used for the replacement and repair of mature human tissues that are
damaged. Whether or not one has ethical objections to such use of human embryos, it is worth
considering the possibilities that are opened up. Setting aside the dream of growing entire organs
from ES cells by a recapitulation of embryonic development, experiments in mice suggest that it
will be possible in the near future to use ES cells to replace the skeletal muscle fibers that
degenerate in victims of muscular dystrophy, the nerve cells that die in patients with Parkinson's
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disease, the insulin-secreting cells that are lacking in type I diabetics, the heart muscle cells that
die in a heart attack, and so on.
Mouse ES cells can be induced to differentiate into a variety of cell types in culture (Figure 22-
57). When treated with a carefully chosen combination of signal proteins, for example, the ES
cells differentiate into astrocytes and oligodendrocytes, the two main types of glial cells in the
central nervous system. If the treated ES cells are injected into a mouse brain, they can serve as
progenitors of these cell types. If the host mouse is deficient in myelin-forming
oligodendrocytes, for example, the grafted cells can correct the deficiency and form myelin
sheaths around axons that lack them.
Figure 22-57
Production of differentiated cells from mouse ES cells in culture. ES cells derived from an early
mouse embryo can be cultured indefinitely as a monolayer, or allowed to form aggregates called
embryoid bodies, in which the cells begin to specialize. Cells (more...)
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Epidermal Stem Cell Populations Can Be Expanded in
Culture for Tissue Repair
It is a long way still from this sort of success in mice to routine treatments for human diseases.
One of the main difficulties lies in immune rejection. If ES-derived cells of one genotype are
grafted into an individual of another, the grafted cells are likely to be rejected by the immune
system as foreign. Methods of dealing with this problem have been developed for the
transplantation of organs such as kidneys and hearts. Immunological problems—and some
ethical problems—can, however, be avoided altogether if the right kinds of stem cells can be
obtained from the patient's own body.
A simple example is the use of epidermal stem cells for repair of the skin after extensive burns.
By culturing cells from undamaged regions of the skin of the burned patient, it is possible to
obtain epidermal stem cells quite rapidly in large numbers. These can then be used to repopulate
the damaged body surface. For good results after a third-degree burn, however, it is essential to
provide first of all an urgent replacement for the lost dermis. For this, dermis taken from a human
cadaver can be used, or an artificial dermis substitute. This is still an area of active
experimentation. In one technique, an artificial matrix of collagen mixed with a
glycosaminoglycan is formed into a sheet, with a thin membrane of silicone rubber covering its
external surface as a barrier to water loss, and this skin substitute (called Integra) is laid on the
burned body surface after the damaged tissue has been cleaned away. Fibroblasts and blood
capillaries from the patient's surviving deep tissues migrate into the artificial matrix and
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gradually replace it with new connective tissue. Meanwhile, the epidermal cells are cultivated
until there are enough to form a thin sheet of adequate extent. Two or more weeks after the
original operation, the silicone rubber membrane is carefully removed and replaced with this
cultured epidermis, so as to reconstruct a complete skin.
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Neural Stem Cells Can Repopulate the Central Nervous
While the epidermis is one of the simplest and most easily regenerated tissues, the central
nervous system (the CNS) is the most complex and seems the most difficult to reconstruct in
adult life. The adult mammalian brain and spinal cord have very little capacity for self-repair.
Stem cells capable of generating new neurons are hard to find in adult mammals—so hard to
find, indeed, that until recently they were thought to be absent.
It is now known, however, that CNS neural stem cells capable of giving rise to both neurons and
glial cells do persist in the adult mammalian brain. Moreover, in certain parts of the brain they
continually produce new neurons to replace those that die (Figure 22-58). Neuronal turnover
occurs on a more dramatic scale in certain songbirds, where large numbers of neurons die each
year and are replaced by newborn neurons as part of the process by which a new song is learned
in each breeding season.
Figure 22-58
The continuing production of neurons in an adult mouse brain. The brain is viewed from above,
in a cut-away section, to show the region lining the ventricles of the forebrain where neural stem
cells are found. These cells continually produce progeny that (more...)
In experiments with rodents, adult neural stem cells have been harvested from the brain, grown
in culture, and then implanted back into the brain of a host animal, where they produce
differentiated progeny. Remarkably, it seems that the grafted cells adjust their behavior to match
their new location. Stem cells from the hippocampus, for example, implanted in the olfactory-
bulb-precursor pathway (see Figure 22-58) give rise to neurons that become correctly
incorporated into the olfactory bulb, and vice versa. These findings hold out the hope that, in
spite of the extraordinary complexity of nerve cell types and neuronal connections, it may be
possible to use neural stem cells to repair at least some types of damage and disease in the central
nervous system.
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