J R Soc Med 2002;95:5-8
doi:10.1258/jrsm.95.1.5
© 2002 Royal Society of Medicine
Stem cells for all seasons? Experimental and clinical issues
Sandro Eridani MD
Institute of Advanced Biomedical Technology—National Research
Council, Milan, Italy
Correspondence to: Professor Sandro Eridani, ITBA-CNR, Via Fratelli Cervi 93,
20090 Segrate, Italy E-mail:
eridani{at}itba.mi.cnr.it
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INTRODUCTION
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Stem cells, by definition, are capable of total or partial differentiation
as
well as self-renewal. The most primitive stem cells, in many
people's
opinion, are those derived from the blastocyst inner
cell
mass
1. Such
embryonic germ cells can be maintained in culture
for a long time if an
appropriate cytokine, leukaemia inhibitory
factor, is present in the culture
medium. They remain pluripotent;
but, if the cytokine is removed, they can be
modulated to produce
various cell lines, from blood islands to neurons and
epithelia
2.
These
cells therefore meet the criteria of being able to divide
symmetrically to
expand their number and to divide asymmetrically
in order to self-renew and
give rise to a differentiated
progeny
3.
However,
there is no evidence that blastocyst cells self-renew
in vivo and
obviously they are not working over the entire lifespan
of the organism, as
stem cells should. Therefore they cannot
be regarded as true stem cells, while
among so-called germ-line
stem cells (oocytes and sperm-producing cells) only
male spermatogonial
cells are present in mammals over the lifetime: oocytes
are
produced in a finite number near the time of birth. I confine
this review
to cells that can self-renew for the entire lifespan
of the organism.
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SOMATIC STEM CELLS
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During embryonic development pluripotential stem cells become
progressively
more restricted, giving rise to stem cells with
narrower commitments. Somatic
stem cells appear relatively late
in development: haemopoietic stem cells are
seen first in the
yolk sac, then in the paraortic region, later in fetal
liver,
finally in spleen and bone marrow. In irradiated embryos, repopulating
stem
cells tend to increase successively through the same sites (yolk
sac,
liver, marrow)
5.
Adult stem cells are often situated in
‘niches’ where they are
available to start a differentiation
pathway; the environment of such niches
seems able to exert
a critical influence on their biochemical and
developmental
potential. For instance, when adult haemopoietic stem cells
from
mouse bone marrow are injected into the inner mass of the
mouse blastocyst,
they express fetal rather than adult
haemoglobin
6.
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NEURAL STEM CELLS
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Evidence for the presence of adult stem cells in the central
nervous system
dates back to the 1960s, when postnatal neurogenesis
was found in rat and
guinea-pig
hippocampus
7.
Further research
revealed neural stem cells in other regions such as the
forebrain
ventricular wall: in the subventricular zone, facing the lumen,
a
layer of proliferating subependymal cells may represent either
a stem cell
compartment or a stream of progenitor cells in
transit
8,9.
It
is therefore possible that such cells serve as an endogenous
source for new
neurons and glia in the adult mammalian forebrain.
A potential for neuron
generation also seems to exist in the
striatum, thalamus and
hypothalamus
10.
What is the function of these persistent stem cells in the adult brain? A
distinct possibility, so far overlooked, is slow and continuous replacement of
neural cells: sphere-forming cells, a population of neural precursors produced
at various stages of embryo development, may persist as a ‘leftover
supply’ in the brain and other tissues until the necessity arises to
proliferate and
differentiate11.
Very recently, Rietze et
al.12 reported
that stem cells purified from mouse adult brain appeared capable of generating
either neural or non-neural cells. It has long been known that neural stem
cells immortalized in culture can be induced to yield both neurons and glial
cells13. Clearly,
therefore, central nervous system stem cells, preserved in culture, offer a
source of replenishment for depleted
tissues14. We
should also note the use of fetal cells which have undergone initial neural
differentiation for treatment of Parkinson's disease. This method, however,
requires so much fetal tissue that it is impracticable on a large scale;
therefore, for this purpose too, the focus is on stem cells in
culture15.
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HAEMOPOIETIC STEM CELLS
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Many classes of haemopoietic stem cells have been identified,
capable of
giving rise to particular sets of blood cells: among
the earliest to be
proposed were the so-called CFU-S (colony-forming
units spleen), capable of
repopulating the entire haemopoietic
system in lethally irradiated mice. An
analogous class of cells
in human beings, including a subset of long-term
self-renewing
cells, has been shown able to reconstitute haemopoiesis in
patients
who have received lethal doses of radiation or
chemotherapy
16,17.
At a less primitive level there are oligopotent stem cells that can produce
either a common lymphoid progenitor or a common myeloid precursor, which in
turn may give rise to an erythroid or megakaryocytic or granulo-monocytic
precursor. Furthermore, so-called colony-forming cells can produce colonies of
a restricted type, such as BFU-Es and CFU-Es for cells of the erythroid
lineage, CFU-GMs for granulocytes and monocytes, and CFU-MKs for
megakaryocytes11. A
stem cell acting as a lymphoid progenitor has also been
identified19.
The potential of such haemopoietic stem cells has been exploited
clinically, not only in blood neoplastic disorders but also in genetic
diseases of the immune
system20 and now
treatment of solid
tumours21 (see
later).
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STEM CELL PLASTICITY
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Adult stem cells can be highly versatile, capable of modulation
from one
type to another. Neural stem cells, for instance, can
transform under
selective stimulation into blood cells of various
sorts
22,
and
haemopoietic stem cells injected into irradiated hosts have
yielded glial
cells
25 and hepatic
oval cells
24 among
others.
Human mesenchymal stem cells have differentiated in culture
into
adipocytes, chondrocytes and
osteocytes
25. This
wide spectrum
of possible differentiation illustrates the impressive potential
of
early stem cells taken out of their microenvironment. If one
wishes to
achieve differentiation away from the expected lineage,
how crucial is culture
and how long must it continue to allow
dedifferentiation and a subsequent
shift of lineage? Lengthy
passage in culture does not seem essential: for
instance, marrow
stromal cells grown for only five days and injected into the
forebrain
of neonatal mice were able to differentiate into mature astrocytes,
and
possibly into neurons as
well
26.
Haemopoietic stem cells can now be purified to such an extent that single
bone-marrow-derived cells are available. Seemingly, one such cell can not only
achieve long-term haemopoietic repopulation of an irradiated host but also
differentiate into non-haemopoietic elements such as epithelial cells of
liver, lung and gastrointestinal
tract27.
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CLINICAL OBSTACLES TO STEM CELL TRANSPLANTATION
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Established methods
Transplantation of human haemopoietic stem cells, obtained from
various
sources, is now established practice for certain
disorders
28.
To
obtain sufficient quantities, these cells are usually exposed
in
vitro to a cocktail of
cytokines
29.
How frequent are such early clonogenic cells in blood or blood-forming
cells? The exact number is still uncertain, but a comparison of bone marrow,
peripheral blood and cord blood showed cord blood to be the richest source and
peripheral blood the
poorest30. Cord
blood clonogenic cells expand rapidly in vitro and are already used
in treatment of genetic disorders of haemopoiesis and
leukaemia31,32.
It seems important to avoid early depletion of the stem cell pool in
culture and attention has been focused on the action of negative regulators of
haemopoiesis, such as macrophage inflammatory protein 1-alpha (MIP-1-alpha)
and tumour necrosis factor
alpha33. MIP-1
alpha and leukaemia inhibitory factor protect the repopulating ability of
purified haemopoietic stem cells, and preliminary clinical trials to test
tolerance to MIP-1-alpha are in
progress34.
A new approach is to use a preparative regimen which though not totally
myeloablative allows engraftment of transplanted allogeneic cells. This can
result in a pronounced graft-versus-tumour effect, and early results are
encouraging. 10 of 19 patients with metastatic renal cell carcinoma showed a
clear
response35.
Experimental protocols
As regards the central nervous system, a widely used model in animals is
the lesioned striatum, in which cells at different stages of maturation have
been inserted36.
Virtually immortal human nervous stem cell lines are available, and when
implanted into the striatum of immuno-deficient mice were detectable in the
host parenchyma up to a year after
grafting14.
Implantation of dopamine-producing cells has been tried not only in mice but
also in patients with Parkinson's
disease37;
transplants of embryo-derived cells as well as adult neural stem cells have
also been successfully performed in laboratory models of demyelinating
diseases such as multiple
sclerosis38,39.
Lately, further evidence of the surprising potential of adult neural stem
cells, pointing to a ‘repertoire’ of differentiation very close to
that of embryonic
cells12,40,
has provided further stimulus to work in this
area41. Preliminary
clinical trials are contemplated on replacement of damaged cartilage or repair
of injured tendons. Some groups are trying to grow neural stem cells for
transplantation into the brain or spinal cord of patients with severe central
nervous system
damage14,42,43;
modulation by growth factors of cells derived from human microspheres (the
early aggregates of neural progenitors) could provide the almost unlimited
supply of enriched non-genetically-transformed neurons required for
transplantation
studies44.
Stem cell transplantation and modulation has potential applications in
various other clinical conditions. The turning point has been the recognition
and identification of stem cells in several different tissues, with a
plasticity that allows transdifferentiation: bone marrow cells, for instance,
may be capable of yielding epidermal cells, skeletal muscle cells and even
hepatic oval cells (precursors of mature liver
cells)24.
When stem cells are introduced in a new ‘niche’, they are
believed to undergo a process of reprogramming and differentiation in response
to signals promoted from the new
micro-envronment45.
The nature of such signals, however, remains obscure. It is noteworthy that
stem cells from different tissues, such as bone marrow and muscle exposed to a
given culture environment, yield cells with similar properties. We already
know that areas of muscle regeneration show an influx of progenitor cells from
other sources, and in immunodeficient mice transplanted marrow-derived cells
can migrate into such areas, differentiate and give rise to muscle
fibres47. This
suggests a possible means of treatment for degenerative disorders such as
muscular dystrophies. Another observation of great interest is that bone
marrow stem cells can lead to myocardial regeneration in mice with
experimentally induced
infarction48;
attempts to isolate the responsible cells are in progress.
Another ready source of adult stem cells with high proliferation potential
is the skin. Such cells may prove useful not only for repair of skin lesions
but also, after reprogramming, for transplantation into other tissues or
organs49.
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A QUESTION OF IDENTITY
|
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The finding of high plasticity in adult stem cells demands revision
of the
stem-cell
concept
50. When the
identity of cells capable
of repairing damaged tissues is scrutinized, we see
that not
all stem cells are equivalent; in the central nervous system,
for
example, most neural stem cells are regionally and temporally
restricted,
although it is possible to find and isolate rare
stem cells which are capable
of producing diverse cells
types
51.
In future, therefore, it will probably be necessary to use stem cells from
many different sources, for the repair of damaged tissues and organs. The main
limiting factor at present is the poverty of information on signals in host
tissue that promote optimal homing and activation of transplanted cells. A
promising development, in this context, is a very recent approach whereby a
synthetic matrix and controlled-release microparticles are assembled with the
progenitor cells before transplantation, thus creating an artificial
microenvironment: within such ‘neo-tissue’ each microparticle is
designed to supply agents that promote certain aspects of cell
function52. This
whole area of research is showing a welcome surge of
activity53.
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INTRODUCTION
SOMATIC STEM CELLS
