Cell differentiation and regeneration

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).

Red and white blood cells in a large vessel

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

Stem cells have 3 properties:

1. They are undifferentiated.2. They have a capacity for self

renewal and divide slowly.3. 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. Stemcells of the intestine lie at the base of the Crypts. They continuously release committed progenitor cells that form the intestinal villi.

2. Other differentiated cells arise from a pool of undifferentiated stem cells.

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 -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 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.