Cell differentiation and regeneration
Red and white blood cells in a large vessel
The number of cells
from any organism
ranges from one to
trillions.
However, even the
most complex
organisms have a
relatively small (~200)
catalog of
differentiated cell
types with specialized
function (bone,
muscle, nerve).
Cell differentiation: the process by which an undifferentiated cell reaches
its specialized function. It occurs during histogenesis. Cell differentiation
is stable. Most differentiated cells cannot transform into other cell types (it
can happen during regeneration).
Cell division and differentiation
Cell differentiation occurs continuously in adult organisms. Most organisms
live much longer than the individual cells from which they are composed. As
cells die, new cells differentiate for replacement.
The rate of cell turnover differs dramatically in different tissues. The lining of
the small intestine is completely replaced every few days. However, neurons
are long lived and don’t recycle.
Differentiated cells are produced by 2 methods:
1. Some differentiated cells
divide. Hepatocytes are liver
cells that make bile and detoxify
chemicals. They are long lived
and divide slowly.
However, after damage by
toxins or injury, hepatocytes
grow rapidly. If you remove 2/3
of the liver, it regenerates in 1-2
weeks.
Stem cells
2. Other differentiated cells arise from a pool of undifferentiated stem cells.
Stem cells have 3 properties:
1.
2.
3.
They are undifferentiated.
They have a capacity for self
renewal and divide slowly.
They form committed progenitor
cells that divide a few times but are
committed to form a specific tissue.
Renewal by stem cell differentiation is
common (blood cells, epithelia, and
spermatogonia).
Stem cells are usually hidden in a safe,
sequestered site away from injury. Stem
cells of the intestine lie at the base of
the Crypts. They continuously release
committed progenitor cells that form
the intestinal villi.
Differentiation of blood cells
Hematopoiesis: (hemat = blood, poien = to make), the blood of vertebrates
contains many different types of cells with distinct functions. All mature
blood cells are short lived and must be replaced continuously from stem
cells. In humans, the hematopoietic stem cells produce billions of blood
cells each hour to replace the aging cells.
Hemangioblast: an embryonic stem cell that gives rise to blood vessels
and universal blood stem cells.
Universal blood stem cells: form myeloid and lymphoid precursors.
Myeloid precursors form several types of differentiated cells including red
blood cells which transport O2 and CO2. They also make platelets for
coagulation of blood, and monocytes / granulocytes that serve a protective
role.
Lymphoid precursors make lymphocytes that are involved in B and T cell
immunity.
The overall scheme for
hematopoiesis.
The embryonic stem cell,
the hemangioblast, gives
rise to angioblasts that
make both vessels and
universal blood stem cells.
The universal stem cells
renew and also form the
myeloid and lymphoid
precursors.
How is hematopoiesis regulated?
Blood cells and vessels are derived from mesoderm. BMP-4 is a protein that
promotes ventral development. It combines with other cytokines including
fibroblast growth factor and activin to induce hematopoesis.
The SLC gene was discovered as over
expressed in human leukemia, and it
appears to be required early in the process
of stem cell development. Knock out the
gene in mice = they fail to form blood cells.
Pluripotent stem cells and progenitor cells
express transcription factors/switch genes
that direct pathways of differentiation.
GATA proteins regulate the decision to form
progenitors or remain as stem cells.
GATA-1 induces RBCs. GATA-2 blocks RBC
differentiation and induces stem cells.
Colony stimulating factors (CSF-1) are
cytokines that direct expression of specific
transcription factors for myeloid cells.
Erythrocytes mature in bone marrow from precursors
called erythroblasts.
Step 1: erythrocyte burst-forming cell forms
from the myeloid stem cells and can make up
to 5000 erythrocytes (red blood cells) if the
CSF IL-3 is present.
Step 2: the burst-forming cells respond to
another CSF known as erythropoietin, which
controls the total number of divisions.
More erythropoietin is made when a person
requires more O2. For example, when one is
high above sea level or sick with anemia.
The erythroblast is filled with hemoglobin and
loses organelles including the nucleus to form
the mature red blood cell.
Billions of old red blood cells are removed
from the blood each day by apoptosis
(programmed cell death) and must be replaced.
Genetic control of muscle cell differentiation
Myo D is a master regulator of muscle cell differentiation. If you inject
Myo D DNA into a fibroblast it turns into a muscle cell. It is a member of a
myogenic family (Myo D, myogenin, myf-5, and MRF-4).
These are transcription factors (basic helix-loop-helix) and activate genes
that are needed for muscle cell differentiation.
The basic region binds to DNA, the HLH region causes dimer formation with
other HLH proteins such as E proteins = induces muscle differentiation.
Another member of the HLH family is id. This is an inhibitor of
differentiation. It has the HLH domain but there is no basic region to bind
DNA. It binds to other HLH proteins and blocks their function = prevents
muscle cell differentiation.
Knock out mice have confirmed the importance of these genes in muscle
cell differentiation. Myo D- / myf-5- mice die after birth due to a lack of
skeletal muscle. Myogenin– mice also die at birth due to disorganized
muscle fibers (fibers are not aligned and don’t work properly).
Adult stem cells have unexpected potency
Recently, it was discovered that adult stem cells can produce a variety of
differentiated cell types. They are not limited to the cell types in the tissue
from which they are derived.
Ependymal cells line the fluid
filled ventricles of the brain
and appear to be stem cells.
When mouse neural stem cells
are injected into the
bloodstream, they form
myeloid cells and
lymphocytes. The injected
cells were labeled with a
reporter gene for b-galacto
sidase so they could be
distinguished from host cells.
Stem cells from bone marrow
can give rise to a variety of
tissues such as liver,
adipocytes, and chondrocytes.
Medical importance
The ability of stem cells to multiply and produce a wide range of differentiated
cell types is potentially of great medical importance.
When signals that direct stem cell differentiation become better understood, it
may be possible to use the cells to replace damaged or diseased tissue.
Examples include Alzheimer’s disease, Parkinson’s disease, loss of brain
tissue after stroke or injury, inducing b cells to treat diabetes, and restoring
cartilage that is damaged by arthritis.
Human embryonic stem cells are particularly interesting. They are found in the
inner cell mass of the early blastula. They divide infinitely and produce many
types of differentiated cells.
In the future, it may be possible to clone the cell of a patient who has suffered a
heart attack. This could be used to create a blastocyst by nuclear transfer to an
oocyte. Stem cells from the inner cell mass could be harvested and induced to
form cardiac muscle. These could be transplanted into the patient’s heart
muscle to repopulate the scar.
Currently, research with embryonic stem cells is not funded by the US
government, and political issues prevent rapid progress in this area by US
scientists.
Recent work has questioned the value of adult stem cells
Over the past two years, evidence has mounted that adult cells may be
almost as malleable as embryonic cells. For example, blood precursor cells
can form other tissues, such as brain cells, if they are first incubated with
embryonic stem cells.
Several recent studies (within the last several months) have raised doubts
about the validity of those results. Rather than switching their fate — a
phenomenon known as transdifferentiation — the adult cells might actually
be fusing with the embryonic cells to become an entirely new type of cell.
Fused cells might be too abnormal to be of medical use.
The fusion argument is likely to come up in the Senate in debates there
over a bill introduced by Sam Brownback (Republican, Kansas) that would
ban human cloning. The nuclear-transfer procedure used in cloning could
also be used to produce genetically compatible embryonic stem cells for
treating disease in individual patients. Therapeutic cloning versus
reproductive cloning of new individuals.
Brownback has argued that adult stem cells make this unnecessary, as
they can be taken directly from the patient.
Regeneration
Many animals have an extraordinary ability to regenerate body structures
(starfish or newts). There are 2 basic types of regeneration:
Epimorphosis: characteristic of regenerating limbs. It is characterized by
dedifferentiation of remaining tissue, increased cell division to make more
tissue, and differentiation into all of the cell types that are needed.
Morphallaxis: occurs exclusively through repatterning of tissues and requires
no new cell division. Often makes a smaller structure.
Epimorphic regeneration
How does regeneration work in salamanders? When a limb is amputated, the
remaining cells construct a new limb to exactly match the previous one.
After amputation a plasma clot forms. Adjacent cells migrate to cover it and
form an apical ectodermal cap. In contrast to mammals, no scar forms.
Cells beneath the cap dedifferentiate (bone, muscle, blood) and detach from
one another. The mass of unifferentiated cells is a regeneration blastema.
The undifferentiated cells proliferate
and resemble the progress zone of a
growing embryonic limb. There is a
similar pattern of Hox gene expression
and growth factor expression
including FGF and SHH.
Retinoic acid is produced by the
blastema and specifies the proximal
position on which to build. Too much
retinoic acid causes excess limb
growth.
Morhallactic regeneration
Hydra is a small fresh water organism
with a tube body, a hypostome (head
region), and a basal disc (foot). These
organisms can produce sexually, but
they usually multiply by budding.
When a hydra is cut in half, both ends
regenerate a new body. If a slice is cut
out of the middle, both ends
regenerate a hypostome and foot.
However, there is no cell growth, so
the organism will be much smaller.
The remaining cells simply reorganize
to form a new, smaller hydra.
Medical advances in regeneration
Humans can regenerate some tissues (liver,
peripheral nerve). Children even retain the ability to
regenerate finger tips. However, most tissues cannot
be regenerated. The ability to regenerate human
tissue would be a major medical breakthrough.
Bone regeneration: bone heals but it can’t regenerate
to fill in a gap. A new technique involves grafting a gel
containing parathyroid hormone. This stimulates
bone regeneration and is used successfully in dogs.
Nerve regeneration: CNS has no ability to regenerate
neurons but peripheral nerves do. When an axon of a
peripheral nerve is cut, the remaining cell
regenerates. This follows the Schwann cells (cells
that insulate axons) to find the proper synapse.
When the spinal cord is damaged, oligodendrocytes
release factors that block axon regeneration leading
to permanent paralysis. Two genes, Nogo-1 and MAG,
are responsible. Antibodies to these proteins support
partial regeneration.