Cloning Definitions and Explanations

Adult stem cells
Stem cells found in different tissues of the developed, adult organism that remain in an undifferentiated, or unspecialized, state. These stem cells can give rise to specialized cell types of the tissue from which they came, i.e., a heart stem cell can give rise to a functional heart muscle cell, but it is still unclear whether they can give rise to all different cell types of the body.

Allogeneic transplantation
Cell, tissue or organ transplants from one member of a species to a genetically different member of the same species.

Autologous transplantation
Cell, tissue or organ transplants from one individual back to the same individual. Such transplants do not induce an immune response and are not rejected.

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Blastocyst
A very early embryo consisting of approximately 150 cells. The blastocyst is a spherical cell mass produced by cleavage of the zygote (fertilized egg). It contains a fluid-filled cavity, a cluster of cells called the inner cell mass (from which embryonic stem cells are derived) and an outer layer of cells called the trophoblast (that forms the placenta).

Bone marrow stromal cell
Also known as mesenchymal stem cells, bone marrow stromal cells are a mixed population of cells derived from the non-blood forming fraction of bone marrow. Bone marrow stromal cells are capable of growth and differentiation into a number of different cell types including bone, cartilage and fat.

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Cell line
Cells that can be maintained and grown in culture and display an immortal or indefinite life span.

Cell type
A specific subset of cells within the body, defined by their appearance, location and function.

i) adipocyte: the functional cell type of fat, or adipose tissue, that is found throughout the body, particularly under the skin. Adipocytes store and synthesize fat for energy, thermal regulation and cushioning against mechanical shock
ii) cardiomyocytes: the functional muscle cell type of the heart that allows it to beat continuously and rhythmically
iii) chondrocyte: the functional cell type that makes cartilage for joints, ear canals, trachea, epiglottis, larynx, the discs between vertebrae and the ends of ribs
iv) fibroblast: a connective or support cell found within most tissues of the body. Fibroblasts provide an instructive support scaffold to help the functional cell types of a specific organ perform correctly.
v) hepatocyte: the functional cell type of the liver that makes enzymes for detoxifying metabolic waste, destroying red blood cells and reclaiming their constituents, and the synthesis of proteins for the blood plasma
vi) hematopoietic cell: the functional cell type that makes blood. Hematopoietic cells are found within the bone marrow of adults. In the fetus, hematopoietic cells are found within the liver, spleen, bone marrow and support tissues surrounding the fe tus in the womb.
vii) myocyte: the functional cell type of muscles
viii) neuron: the functional cell type of the brain that is specialized in conducting impulses
ix) osteoblast: the functional cell type responsible for making bone
x) islet cell: the functional cell of the pancreas that is responsible for secreting insulin, glucogon, gastrin and somatostatin. Together, these molecules regulate a number of processes including carbohydrate and fat metabolism, blood glucose levels and acid secretions into the stomach.

Cloning
The process in which an organism produces one or more genetically alike copies of itself by asexual means. Cloning may occur by propagation of cuttings, as in the case of plants; continual budding, as in the case of hydra; fission, as in the case of bacteria and protozoa; parthenogenic asexual reproduction as in the case of aphids; or somatic cell nuclear transfer, as in the case of higher order animals such as mammals. The term cloning can also be applied to a group of cells undergoing replication by repetitive mitoses (cell divisions).

Also see entries for Reproductive cloning and Therapeutic cloning below.

Cytoplasm
The part of the cell not including the nucleus.

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Differentiation
The process of development with an increase in the level of organization or complexity of a cell or tissue, accompanied with a more specialized function.

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Ectoderm
The outer of three germ layers of the early embryo that gives rise in later development to the skin, cells of the amnion and chorion, nervous system, enamel of the teeth, lens of the eye and neural crest.

Embryo
The product of a fertilized egg, from the zygote until the fetal stage.

Embryoid bodies
Spheroid colonies seen in culture produced by the growth of embryonic stem cells in suspension. Embryoid bodies are of mixed cell types, and the distribution and timing of the appearance of specific cell types corresponds to that observed within the embryo.

Embryonic germline cells
Embryonic germline cells, also called EG cells, are pluripotent stem cells derived from the primitive germline cells (those cells that give rise to eggs and sperm). Their properties are similar to those of embryonic stem cells.

Embryonic stem cell
Also called ES cells, embryonic stem cells are cells derived from the inner cell mass of developing blastocysts. An ES cell is self-renewing (can replicate itself), pluripotent (can form all cell types found in the body) and theoretically is immortal.

Endoderm
The inner of three germ layers of the early embryo that gives rise in later development to tissues such as the lungs, the intestine, the liver and the pancreas.

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Fetus
The stage in development from the end of the embryonic stage, 7-8 weeks after fertilization, to developed organism that ends at birth.

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Germ layers
The three germ layers are the endoderm, mesoderm and ectoderm and are the three precursory tissue layers of the early, primitive embryo (which form at approximately two weeks in the human) that give rise to all tissues of the body.

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Hematopoietic stem cells
The precursors of mature blood cells that are defined by their ability to replace the bone marrow system following its obliteration (for example, by g-irradiation) and can continue to produce mature blood cells.

Hematopoietic cell transplantation
The transplantation of hematopoietic stem cells with blood-forming potential. Hematopoietic stem cells provide rapid and sustained reconstitution of blood formation and are found in adult bone marrow, umbilical cord blood, peripheral blood and in fetal liver.

Heterologous
Not homologous or uniform. In the context of cells, heterologous is a mixed or divergent cell population or of a divergent origin.

Histocompatible
A tissue or organ from a donor (the person giving the organ or tissue) that will not be rejected by the recipient (the patient in whom the tissue or organ is transplanted). Rejection is caused because the immune system of the recipient sees the transplanted organ or tissue as foreign and tries to destroy it. Tissues from most people are not histocompatible with other people. In siblings, the probability of histocompatibility is higher, while identical twins are almost always histocompatible.

Homologous
Similar or uniform, often used in the context of genes and DNA sequences. In the context of stem cells, the term homologous recombination is a technique used to disable a gene in embryonic stem cells.

Homologous recombination
A technique used to inactivate a gene and determine its function in a living animal. The process of homologous recombination is more efficient in embryonic stem cells than in other cell types. It is achieved by introducing a stretch of DNA that is similar or identical (homologous) to part of a gene and to some of the DNA surrounding the gene, but different (not homologous) to a specific section of the gene. The DNA is then introduced into the stem cells and the stretch of homologous DNA will recognize the similar sequences of the gene within the cell, and replace it. But the cell is then left with a piece of DNA in the gene that has the wrong sequence and this interrupts the function of the gene. The gene is then said to be knocked out. From these embryonic stem cells, an entire mouse can be made by injecting the altered stem cells into a blastocyst, and implanting the blastocyst into a female mouse. This is one way to make genetically manipulated mice and other animals with altered gene function. These experiments are crucial to understand how specific genes work and interact in living animals.

Human embryonic stem cell
A stem cell that is derived from the inner cell mass of a blastocyst and can differentiate into several tissue types in a dish. They are similar to embryonic stem cells from the mouse; however, in the mouse, it is possible to inject those cells into a blastocyst, to make a new mouse, while this is not, and should not, be possible in humans for ethical reasons. Human embryonic stem cells are harder to grow than mouse embryonic stem cells.

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Inner cell mass
A small group of cells attached to the wall of the blastocyst (the embryo at a very early stage of development that looks like a hollow ball). Embryonic stem cells are made by isolating and culturing the cells that make up the inner cell mass. In development. it is the inner cell mass that will eventually give rise to all the organs and tissues of the future embryo and fetus, but do not give rise to the extra-embryonic tissues, such as the placenta.

In vitro fertilization
A procedure where an egg cell (the oocyte) and sperm cells are brought together in a dish (i.e. in vitro), so that a sperm cell can fertilize the egg. The resulting fertilized egg, called a zygote, will start dividing and after a several divisions, forms the embryo that can be implanted into the womb of a woman and give rise to pregnancy.

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Mesemchymal stem cell
Also known as bone marrow stromal cells, mesenchymal stem cells are rare cells, mainly found in the bone marrow, that can give rise to a large number of tissue types such as bone, cartilage (the lining of joints), fat tissue, and connective tissue (tissue that is in between organs and structures in the body).

Mesoderm
The middle of three germ layers that gives rise later in development to such tissues as muscle, bone, and blood.

Morphology
Study of the shape and visual appearance of cells, tissues and organs.

Multipotent stem cells
Stem cells whose progeny are of multiple differentiated cell types, but all within a particular tissue, organ, or physiological system. For example, blood-forming (hematopoietic) stem cells are single multipotent cells that can produce all cell types that are normal components of the blood.

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Neural stem cell
A type of stem cell that resides in the brain, which can make new nerve cells (called neurons) and other cells that support nerve cells (called glia). In the adult, neural stem cells can be found in very specific and very small areas of the brain where replacement of nerve cells is seen.

Nucleus
A part of the cell, situated more or less in the middle of the cell, that is surrounded by a specialized membrane and contains the DNA of the cell. This DNA is packaged into structures called chromosomes, which is the genetic, inherited material of cells.

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Oligopotent progenitor cells
Progenitor cells that can produce more than one type of mature cell. An example is the myeloid progenitor cell which can give rise to mature blood cells, including blood granulocytes, monocytes, red blood cells, platelets, basophiles, eosinophiles and dendritic cells, but not T lymphocytes, B lymphocytes, or natural killer cells.

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Parthenogenesis
A form of reproduction where an egg develops without the fusion of sperm with the egg cell. Parthenogenesis occurs commonly among insects and other arthropods. Artificially inducing parthenogenesis with human eggs may be a means to isolate stem cells from an embryo, without fertilization.

Plasticity
A phenomenon used to describe a cell that is capable of becoming a specialized cell type of different tissue. For example, when the same stem cell can make both new blood cells and new muscle cells.

Phenotype
The description of the characteristics of a cell, a tissue or an animal; as black and white fur of a mouse are two phenotypes that can be found. The phenotype is determined by the genes (or the genotype) and by the environment. For example, short stature is a phenotype that can be genetically determined (and therefore inherited from the parents), but can also be caused by malnourishment during childhood (and therefore be caused by the environment).

Pluripotent stem cells
Stem cells that can become all the cell types that are found in an implanted embryo, fetus, or developed organism, but not embryonic components of the trophoblast and placenta (these are usually called extra-embryonic).

Post-implantation embryo
Implanted embryos in the early stages of development until the establishment of the body plan of a developed organism with identifiable tissues and organs.

Pre-implantation embryos
Fertilized eggs (zygotes) and all of the developmental stages up to, but not beyond, the blastocyst stage.

Progenitor cell
A progenitor cell, often confused with stem cell, is an early descendant of a stem cell that can only differentiate, but it cannot renew itself anymore. In contrast, a stem cell can renew itself (make more stem cells by cell division) or it can differentiate (divide and with each cell division evolve more and more into different types of cells). A progenitor cell is often more limited in the kinds of cells it can become than a stem cell. In scientific terms, it is said that progenitor cells are more differentiated than stem cells.

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Regenerative medicine
Medical interventions that aim to repair damaged organs, most often by using stem cells to replace cells and tissues damaged by aging and by disease.

Reproductive cloning
Somatic cell nuclear transfer used for the production of a fetus and delivery of a live offspring that is genetically identical the donor of the somatic cell DNA.

Also see entries for Cloning and Therapeutic cloning.

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Somatic cells
All the cells within the developing or developed organism with the exception of germline (egg and sperm) cells.

Somatic cell nuclear transfer
A technique in which the nucleus of a somatic cell (any cell of the body except sperm cells and egg cells) is injected, or transfered, into an egg, that has had its nucleus removed. If the new egg is then implanted into the womb of an animal, an individual will be born that is a clone. The clone has the identical genetic material as the somatic cell, which supplied the nucleus that carries the genetic material. This procedure is very inefficient and was first developed for agricultural purposes. However, in human medicine, this technique can be used to isolate embryonic stem cells from eggs that have undergone nuclear transfer. When the somatic cell is supplied from the cells of a person, the stem cells isolated from the developing eggs can be used to make a tissue that will not be rejected by that person, because they have the same genetic material. In this way, 'customized' embryonic stem cells could be made for everyone who needed them.

Stem cells
Cells that have both the capacity to self-renew (make more stem cells by cell division) as well as to differentiate into mature, specialized cells.

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Therapeutic cloning
Somatic cell nuclear transfer for the isolation of embryonic stem cells. The embryonic stem cells are derived from the blastocyst (before it becomes a fetus) and can be instructed to form particular cell types (e.g. heart muscle) to be implanted into damaged tissue (e.g. heart) to restore its function. If the stem cells are placed back into the individual who gave the DNA for the somatic cell nuclear transfer, the embryonic stem cells and their derivatives are genetically identical and thus immunocompatible (they will not be rejected).

Also see entries for Cloning and Reproductive cloning.

Thermocycling
An instrument or device that repeatedly alternates through multiple temperatures required for an iterative, temperature-dependant chemical process such as the polymerase chain reaction.

In the case of a Polymerase chain reaction (PCR), using a thermocycler, you can replicate DNA without using a living organism. The technique, which is in-vitro can be used to exponentially multiply the number of DNA cells. Commmonly these cells are used for genetic manipulation.

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Totipotent stem cells
Stem cells that can give rise to all cell types that are found in an embryo, fetus, or developed organism, including the embryonic components of the trophoblast and placenta required to support development and birth. The zygote and the cells at the very early stages following fertilization (i.e., the 2-cell stage) are considered totipotent.

Transdifferentiation
The ability of a particular cell of one tissue, organ or system, including stem or progenitor cells, to differentiate into a cell type characteristic of another tissue, organ, or system; e.g., blood stem cells changing to liver cells.

Transplantation biology
The science that studies the transplantation of organs and cells. Transplantation biologists investigate scientific questions to understand why foreign tissues and organs are rejected, the way transplanted organs function in the recipient, how this function can be maintained or improved, and how the organ to be transplanted should be handled to obtain optimal results.

Trophoblast
The tissue of the developing embryo responsible for implantation and formation of the placenta. In contrast to embryonic stem cells, the trophoblast does not come from the inner cell mass, but from cells surrounding it.

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Umbilical cord stem cells
Hematopoietic stem cells are present in the blood of the umbilical cord during and shortly after delivery. These stem cells are in the blood at the time of delivery, because they move from the liver, where blood-formation takes place during fetal life, to the bone marrow, where blood is made after birth. Umbilical cord stem cells are similar to stem cells that reside in bone marrow, and can be used for the treatment of leukemia, and other diseases of the blood. Efforts are now being undertaken to collect these cells and store them in freezers for later use. However, one problem is that there may not be enough umbilical cord stem cells in any one sample to transplant into an adult.

Unipotent stem cells
Stem cells that self-renew as well as give rise to a single mature cell type; e.g., spermatogenic stem cells.

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Zygote
The cell that results from the union of sperm and egg during fertilization. Cell division begins after the zygote forms.

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