Nuclear Transplantation

Question: Are differentiated animal cells totipotent?

An egg is removed from a Scottish blackface ewe.

Scottish blackface sheep (#2)

Question: Are differentiated animal cells totipotent?

An egg is removed from a Scottish blackface ewe.

8 The embryo develops and Dolly is born.

Nuclear Transplantation Sheep

Scottish blackface sheep (#2)

Scottish blackface sheep (#3)

Dorset sheep, genetically identical to #1

Conclusion: Differentiated animal cells are totipotent in nuclear transplant experiments.

Scottish blackface sheep (#3)

8 The embryo develops and Dolly is born.

Dorset sheep, genetically identical to #1

Conclusion: Differentiated animal cells are totipotent in nuclear transplant experiments.

I An enucleated egg from a black mouse

„.is fused with the nucleus of an agouti mouse.

I An enucleated egg from a black mouse

„.is fused with the nucleus of an agouti mouse.

.and transplanted into an albino "surrogate mother".

Mouse Genetics

.and transplanted into an albino "surrogate mother".

19.5 Cloned Mice Because so much is known about mouse genetics and molecular biology, cloned mice may be useful in studies of basic biology.

touched off a flurry of controversy, but cloning is not a new scientific concept. The idea of totipotency was accepted long before Dolly was born, but achieving it is an impressive technical achievement.

An example of nuclear totipotency gone awry occurs in a human tumor called a teratocarcinoma. Here, a differentiated cell dedifferentiates to form an unspecialized cell. Then it divides, forming a tumor, as occurs in most cancers. But some cells in the tumor redifferentiate to form specialized tissue arrangements. So the tumor can form a large mass of cells inside the abdomen, with some of the cells forming kidney

Tumor With Hair And Teeth

tubules, others hair, and still others teeth! How this redifferentiation occurs is not clear.

Stem cells can be induced to differentiate by environmental signals

Genomic equivalence implies that a differentiated cell stays specialized because of its environment, not because of its genes, and that appropriate environmental changes could result in a new pattern of differentiation. In normal development, a complex series of signals results in the patterns of differentiation we see in a newborn organism. If these signals could be described in enough detail, we should be able to understand how any cell type becomes any other.

In plants, the growing regions at the tips of the roots and stems contain meristems, which are clusters of undifferentiated, rapidly dividing cells. These cells can give rise to the specialized cell types that make up roots and stems, respectively. Plants have many fewer (15-20) cell types than animals (as many as 200). Most plant cell types differ in the structure of their cell walls, whereas most animal cell types have specific cytoplasmic characteristics and many cell-specific proteins.

In mammals, stem cells are found in adult tissues that need frequent cell replacement, such as the skin, the inner lining of the intestine, and the blood system. As they divide, stem cells produce cells that differentiate to replace dead cells and maintain tissues. In the body, stem cells have limited abilities to differentiate. The stem cells in bone marrow, for example, produce only the various types of red and white blood cells, while the stem cells in the nervous system produce only the various types of nerve cells.

Can one kind of stem cell be manipulated by its environment to produce cells that differentiate into cells of another tissue type? The answer appears to be yes. For example, when stem cells from the brain were transplanted into the bone marrow of mice whose bone marrow stem cells had been depleted, they proceeded to act like bone marrow stem cells, producing blood cells. In the reverse experiment, bone marrow stem cells were implanted into the brains of mice, where they formed nerve cells. These experiments indicate that some component of the environment—presumably acting through intercellular signals—determines what a stem cell will do.

The stem cell populations that are closest to totipotency are not the ones found in adults, but those of the early embryo. In mice, these embryonic stem cells can be removed from an early embryo (called a blastocyst) and then induced to differentiate in some particular way. Normally, these cells are formed a few days after fertilization, and their fate in the developing embryo is soon determined. Before that time, however, they are virtually totipotent. Such cells can be grown in definitely in the laboratory and, when injected back into a mouse blastocyst, will mix with the resident cells and differentiate to form all the cell types of the mouse. This kind of experiment shows that blastocyst cells do not lose any of their developmental potential while growing in the laboratory.

Embryonic stem cells growing in the laboratory can be induced to differentiate if the right signal is provided (Figure 19.6). For example, treatment of mouse embryonic stem cells with a derivative of vitamin A causes them to form nerve cells, while other growth factors induce them to form blood cells, again demonstrating their developmental potential and the roles of environmental signals. This finding raises the possibility of using stem cell cultures as sources of differentiated cells for clinical medicine. A key advance toward this use has been the ability to grow human embryonic stem cells in the laboratory.

A source for embryonic stem cells could be human embryos made for in vitro fertilization. This medical procedure is used by couples who want a child but cannot conceive naturally. Up to ten eggs are taken from the mother's ovaries and exposed to the father's sperm, with the hope that some early embryos will form. A few of these embryos are then implanted into the mother's uterus for development. Any remaining embryos not used for implantation could be a source of stem cells.

But a problem arises if these embryonic stem cells are induced to differentiate to form a tissue for transplantation— say, pancreatic tissue for a patient with diabetes. The cells and the recipient are genetically different, so the recipient's immune system may reject the transplanted cells . This problem has led to the proposal of therapeutic cloning, in which nuclear transplantation and stem cell technologies would be combined. This procedure would require several steps:

► Eggs are removed from a female donor.

► A cell is removed from the recipient.

► The entire cell, or its nucleus only, is fused with the enucleated egg.

► The egg is stimulated to divide.

► Embryonic stem cells form; these cells are genetically the recipient's.

► The stem cells are induced to differentiate into the desired tissue for transplantation.

Progress with this ambitious program has been slow but steady. The age of custom-made cells to replace those lost to disease or injury is rapidly approaching.

Genes are differentially expressed in cell differentiation

Nuclear transplantation, cell fusion, and plant cell cloning have demonstrated genomic equivalence in somatic cells of

Nuclear Transplantation

Colony of cartilage cells

19.6 The Potential Use of Embryonic Stem Cells in Medicine

Human embryonic stem cells can be cultured in the laboratory and induced to differentiate.Their use as transplants to replace damaged tissue is under intensive investigation.

Colony of cartilage cells

19.6 The Potential Use of Embryonic Stem Cells in Medicine

Human embryonic stem cells can be cultured in the laboratory and induced to differentiate.Their use as transplants to replace damaged tissue is under intensive investigation.

an organism. Molecular experiments have provided even more convincing evidence. For example, the gene for P-globin, one of the protein components of hemoglobin, is present and expressed in red blood cells as they form in the bone marrow of mammals. Is the same gene also present—but unexpressed—in nerve cells in the brain, which do not make hemoglobin?

Nucleic acid hybridization (see Figure 14.5) can provide an answer. A probe for the P-globin gene can be applied to DNA from both brain cells and immature red blood cells (recall that mature red blood cells lose their nuclei and DNA). In both cases, the probe finds its complement, showing that the P-globin gene is present in both types of cells. On the other hand, if the probe is applied to cellular mRNA, rather than cellular DNA, it finds P-globin mRNA only in the red blood cells, and not in the brain cells. This result shows that the gene is expressed in only one of the two tissues. Many similar experiments have shown convincingly that differentiated cells lose none of the genes that were present in the fertilized egg.

What leads to this differential gene expression? One well-studied example of differentiation is the conversion of un-differentiated muscle precursor cells, called myoblasts, into the large, multinucleated muscle fibers that make up mammalian skeletal muscles. The key event that starts this conversion is the expression of MyoDl (myoblast-determination gene 1). The protein product of this gene is a transcription factor (MyoD1) with a helix-loop-helix domain (see Figure 14.15), which not only binds to the promoters of muscle-determining genes to stimulate their transcription, but also acts on its own promoter to keep its levels high in the myoblasts and in their descendants.

Strong evidence for the controlling role of MyoDl in muscle fiber differentiation comes from experiments in which a sequence containing an active promoter adjacent to MyoDl is transfected into the precursors of other cell types. For example, if this sequence is added to fat cell precursors, the fat cells are reprogrammed to become muscle cells. Genes such as MyoDl that direct fundamental decisions in development, often by regulating genes on other chromosomes, usually encode transcription factors.

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Responses

  • jill
    Where the embryo develops in a sheep?
    8 years ago
  • AAPO
    How redifferentiation occurs?
    7 years ago
  • vanessa
    What are the steps of nuclear transplantation?
    3 years ago

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