Stem cell
Stem cells or stem cells are cells found in all multicellular organisms that have the ability to divide (through mitosis) and differentiate into various types of specialized cells, in addition to self-renewing to produce more stem cells. In mammals, there are several types of stem cells that can be classified based on their cellular potency, that is, the number of different cell types in which they exist. that can differentiate. In adult organisms, stem cells and progenitor cells are involved in regenerating or repairing the body's tissues.
General information
Stem cells —in English stem cells (where stem means trunk, often being translated as “stem cells”)— have the ability to divide asymmetrically, giving rise to two daughter cells, one of which has the same properties as the original mother cell (self-renewal) and the other acquires the ability to differentiate if environmental conditions are right.
Most tissues of an adult organism have a resident population of adult stem cells that allow their periodic renewal or regeneration when tissue damage occurs. Some adult stem cells are capable of to differentiate into more than one cell type, such as mesenchymal stem cells and hematopoietic stem cells, while others are direct precursors of the cells of the tissue in which they are found, such as stem cells of the skin, intestinal muscle, or the gonadal stem cells (germ stem cells).
Embryonic stem cells are those that form part of the internal cell mass of a 4-5 day old embryo. These are pluripotent, which means that they can give rise to the three germ layers: ectoderm, mesoderm and endoderm. A fundamental characteristic of embryonic stem cells is that they can be maintained (in the embryo or under certain culture conditions) indefinitely, forming a cell identical to themselves when dividing, and maintaining a stable population of stem cells. There are experimental techniques where embryonic stem cells can be obtained without this implying the destruction of the embryo.
Types of stem cells
Taking into account their potency, stem cells can be classified into six types:
Totipotent stem cells
They can grow and form a complete organism, both the embryonic components (such as the three embryonic layers, the germinal lineage and the tissues that will give rise to the yolk sac), and the extraembryonic (such as the placenta). That is, they can form all cell types. The totipotent stem cell par excellence is the zygote, formed when an egg is fertilized by a sperm.
Pluripotent stem cells
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They cannot form a complete organism, but they can form any other type of cell corresponding to the three embryonic lineages (endoderm, ectoderm and mesoderm). They can, therefore, form cell lineages. They are in different stages of embryonic development. The most studied pluripotent stem cells are embryonic stem cells (in English embryonic stem cells or ES cells) which can be isolated from the cell mass internal layer of the blastocyst. The blastocyst is made up of an outer layer called trophoblast, made up of about 70 cells, and an inner cell mass made up of about 30 cells, which are embryonic stem cells that have the ability to differentiate into all cell types that They appear in the adult organism, giving rise to tissues and organs. Currently, they are used as a model to study embryonic development and to understand the mechanisms and signals that allow a pluripotent cell to form any fully differentiated cell in the organism.
Embryonic multipotent cells
They are pluripotent embryonic stem cells that are derived from the gonadal buds of the embryo. These gonadal buds are found in a specific area of the embryo called the gonadal crest, which will give rise to the gonads, ovary or testicle, and the ovules and spermatozoa respectively. They have a differentiation capacity similar to that of embryonic stem cells, but their isolation is more difficult.
Induced Pluripotent Stem (iPS) Cells
Today, human adult cells can be manipulated and cells with induced pluripotency (iPS) can be generated. It has been seen that they have the same potential for growth and differentiation as embryonic stem cells, and they will gradually replace or broaden the biotechnological possibilities dreamed of for the embryonic The commitment of Shinya Yamanaka, designer of this technology and winner of the Nobel Prize for its discovery, in relation to its use for other purposes, is an example of the ethics and responsibility of the researcher and assumes that science triumphs at the service of man.. The technical advantages of iPS are many, the most important being: they do not induce immunological rejection, which opens the possibility of creating specific drugs for a given patient; it does not require the use of human eggs, the technique is very easy to perform and its cost is low.
Multipotent stem cells
They are those that can only generate cells of the same layer or lineage of embryonic origin (for example: a bone marrow mesenchymal stem cell, having a mesodermal nature, will give rise to cells of that layer such as myocytes, adipocytes or osteocytes, among other). Another example is hematopoietic stem cells—blood stem cells that can differentiate into multiple blood cell types.
Unipotent Stem Cells
Also called progenitor cells, they are stem cells that have the ability to differentiate into only one type of cell. For example, muscle stem cells, also called satellite cells, can only differentiate into muscle cells.
In addition to the potency criterion, stem cells can also be classified as to whether they are found in the embryo or in adult tissues.
Adult stem cells are found in adult tissues and organs and have the ability to differentiate into adult cells of the tissue in which they are found. In humans, around 20 different types of adult stem cells are known to date, which are responsible for regenerating tissues that are constantly worn out, such as skin, blood, intestines, myocardium, or tissues that have suffered damage (such as for example the liver).
This classification includes unipotent stem cells, such as hematopoietic stem cells from the bone marrow (responsible for blood formation). In the bone marrow itself, but also in umbilical cord blood, peripheral blood and body fat, another type of adult stem cell has been found, called mesenchymal, which can differentiate into numerous cell types of the three embryonic derivatives (muscular, vascular, nervous, hematopoietic, bone, etc.).
Oligopotent stem cells
These can only differentiate into a few cell types, such as lymphoid or myeloid stem cells.
Methods for obtaining stem cells
There are different techniques for obtaining stem cells. Embryonic stem cells and some adult stem cells can be isolated from their original location in embryos or tissues and maintained in special culture conditions more or less indefinitely. Sources that are routinely used or have just begun to be postulated are:
- Cryoconserved embryos: Cryopreservation or cryoconservation is a method that uses liquid nitrogen (-196 °C) to stop all cell functions and thus keep them for years. These embryos are from assisted human reproduction treatments, which when more of the necessary fertilizers can be donated by patients who undergo this treatment. These cryopreserved embryos in blastocyst phase can be preserved for five years, as regulated by R.D. 413/1996.
- Individual blastomers: with this technique, first tested in mice and then in humans, it is possible not to destroy the embryo. Mouse fertilized eggs were used to grow until they were 8 to 10 cells. One of these cells is extracted and cultivated. This technique has achieved two stable cell lines that showed a normal cariotype and presented characteristic markers of pluripotentiality. The embryo from which this cell is obtained is completely viable so it can be implanted in a uterus and follow normal development.
- Partogenesis: this reproductive process is not given in mammals. However, partenogenesis may be induced in mammals through chemical or physical in vitro methods. As a result of this activation, a cell mass called partnote can be obtained from those that can isolate pluripotent stem cells. This technique is only applicable in women.
- Obtaining on the basis of individual donors: Recent research has described that the [muscular stem cells] survive and maintain their properties after a post mortem freezing process.
Somatic cell reprogramming
In addition to the expansion of stem cells obtained from the body, techniques have been developed to reprogram somatic cells into pluripotent stem cells.
- Somatic cell reprogramming by nuclear transfer or transplant. It consists of extracting a nucleus from an unfertilized egg and replacing it with the nucleus of an adult somatic cell. When found in an enabling environment, the cytoplasm of the egg, this core is capable of reprogramming. One advantage of this technique (in its biomedical applications) is to obtain stem cells that contain the same genetic strength as the patient and thus prevent rejection problems. This technique has been successfully carried out in multiple animal and human species. This method has been successfully used for what is known as therapeutic cloning.
- Fusion of somatic cells and embryonic stem cells. Hybrids between various somatic cells and embryonic stem cells share many characteristics with stem cells, indicating that the multipotent phenotype is dominant in the products resulting from the merger. This type of hybrid cells, also called heterocariots, are valuable for the study of the genetic and biochemical mechanisms involved in multipotence.
- Reprogramming for defined transcription factors or Induced pluripotent stem cells. In 2006 the group of Dr. Shin'ya Yamanaka of Kyoto University showed that it is possible to reschedule adult somatic cells to stem cells through the ectopian expression of transcription factors, generating so-called induced pluripotent stem cells or iPS cells (of induced pluripotent stem cells in English). In the original protocol, embryonic fibroblasts (MEF) and adult fibroblasts after infection with retrovirus were successfully reprogrammed for transcription factors Oct4, Sox2, c-myc and Klf4.
Cord Stem Cells
From the umbilical cord, a population of multipotent stem cells can be isolated that possess embryonic (express the transcription factors OCT-4 and Nanog) and hematopoietic (express the CD45 leukocyte marker) characteristics. These adult stem cells can differentiate into cells of the blood and immune system.
Stem cells from the umbilical cord are relatively easy to obtain and have low immunogenicity, due to the low expression of the major histocompatibility complex (MHC), which is why they have begun to be used in therapies to cure various diseases:
- Autoimmune diseases like lupus.
- Hematological diseases such as sickle cell anemia.
- Diabetes.
In addition, they have numerous advantages: they can be stored for approximately 15 years, they can be converted into any type of cell, they are more accepted by relatives than marrow cells, they do not have viruses, they are easily obtained without cause ethical dilemmas and the number of cells obtained is greater than that of those extracted from the marrow.
Stem cells from amniotic fluid
Thanks to the latest scientific advances, it was shown that amniotic fluid contains differentiated and undifferentiated embryonic and extraembryonic tissue cells derived from ectoderm, mesoderm, and endoderm. The typology and characteristics of the amniotic fluid cells vary according to the moment of gestation and depending on the existence of possible fetal pathologies. Recently, there has been evidence of experiments demonstrating the presence of mesenchymal fetal stem cells with differentiating potential towards cellular elements derived from three embryonic leaves, for example.
Amniotic fluid stem cells expand easily in culture, maintain genetic stability and can be induced to differentiate (studies by Paolo De Coppi, Antony Atala, Giuseppe Simoni, etc.) also into hematopoietic cells. For this reason, they represent a new source of cells that could have multiple applications in tissue engineering and cell therapy, especially for the treatment of congenital anomalies in the perinatal period.
Stem cells from amniotic fluid are not ethically controversial and can be kept for one's own use.
Dental Stem Cells
Described by Shi in 2003, dental stem cells are of mesenchymal origin and are found in the dental pulp of primary or permanent teeth, making them an easily accessible source. In addition, unlike stem cells of hematopoietic origin, mesenchymal cells have great plasticity to become nerve or heart cells, which has attracted a large number of researchers to establish a possible gene therapy. Various public and private dental stem cell banks have been created in various countries for the purpose of cryopreserving these cells.
Stem cell preservation
In May 2004, the world's first stem cell bank opened in England. Many countries followed, and before the end of 2005, there were more than 100 banks in the world.
There are different types of stem cells, depending on when the cells are alive. There are two preservation options that need to be considered. These alternatives will give you and your children access to the various therapies that are available today and in the future.
- Hematopoietic stem cell (relative to the origin of blood and its components): the blood of the umbilical cord collected during delivery is a rich source of hematopoietic stem cells. These cells have multiple therapeutic applications for the treatment of blood diseases and autoimmune diseases.
- Mesenchymal stem cell (relative to cellular tissue): these cells are multipotent, because they can be differentiated in a variety of types of cells with applications for the treatment of heart disease, bones, cartilage and muscles.
Stem cell treatments
Japanese scientist Shinya Yamanaka, laureate of the 2012 Nobel Prize in Medicine, warned reporters against the "enormous" risks of certain "stem cell therapies" that have not been tested and are being offered in clinics and hospitals in a growing number of countries.
Stem cells could have a multitude of clinical uses and could be used in regenerative medicine, immunotherapy and gene therapy. In fact, great successes have been obtained in animals with the use of stem cells to treat hematological diseases, type 1 diabetes, Parkinson's, neuronal destruction and heart attacks. But even in 2012 there were no conclusive studies in humans and the Spanish Medicines Agency, dependent on the Ministry of Health, warned in October 2012 about the risk of its indiscriminate use.
Some medical discoveries suggest that stem cell treatments can cure disease and relieve pain. There are some stem cell treatments, but most are still in an experimental stage. Medical research anticipates that one day with the use of technology, derived from research for adult and embryonic stem cells, it will be possible to treat cancer, diabetes, spinal cord injuries and muscle damage, among other diseases. Many promising treatments for serious diseases have been applied using adult stem cells. The advantage of adult stem cells over embryonic ones is that there is no problem in rejecting them, because normally the stem cells are taken from the patient. There is still a big problem both scientific and ethical about this.
In recent years, research has been carried out on the in vitro proliferation of umbilical cord stem cells to increase the number of stem cells and cover the need for transplantation. These studies are very promising and may allow the use of umbilical cord stem cells in gene therapy in the future: we can thus treat diseases caused by the deficiency or defect of a certain gene. Introducing a certain gene into proliferating stem cells in vitro and transplanting such cells into the recipient patient. The use of other types of cells as carriers of good genes in patients with diseases caused by genetic deficiencies or deficits, is being clinically tested.
Cancer Treatments
Recently, stem cells found in umbilical cord blood have been used to treat cancer patients. During chemotherapy, most of the growing cells are killed by cytotoxic agents. The side effect of chemotherapy is what stem cell transplants try to reverse; the substance that is healthy within the patient's bone, the marrow, is replaced by those lost in the treatment. In most current treatments that use stem cells, it is preferable to obtain them from a donor with the same blood type than to use the patient's own. Only if it is necessary to use one's own stem cells (always as a last resort and if a donor with the same blood type was not found) and if the patient does not have their own collection of stem cells (umbilical cord blood) stored, then the containing substance in the bones will be removed before chemotherapy, and reinjected afterwards.
Immunohematology
Hematopoietic stem cell transplantation has been used successfully for 50 years to treat multiple diseases: thalassemia, sickle cell anemia, Fanconi anemia, inborn errors of metabolism, severe aplastic anemia, severe combined immunodeficiencies (SCID)... They have also been used for the treatment of tumors: acute myeloid and lymphoid leukemias, chronic myeloid leukemias, myelodysplasias, lymphomas, myelomas, solid tumors of the kidneys, breast, ovary and neuroblastoma, etc. Research on stem cells arose from the findings of Ernest A. McCulloch and James E. Till at the University of Toronto in the 1960s.
This is achieved by bone marrow transplantation. The bone marrow contains the precursor stem cells for blood and lymphatic cells. It used to be taken from the hip bone, but is now being taken from the peripheral blood after treatment with growth-stimulating factors. The success of a bone marrow transplant, like any other transplant, depends on HLA matching. But in addition to being able to produce rejection of the individual to the transplanted tissue, bone marrow transplantation presents the particularity that it can also occur in the opposite direction, rejection of the tissue transplanted to the individual (GVHD: graft versus host disease ).
However, GVHD rejection may have an advantage and be of interest as an immunotherapy, since it can recognize the malignant cells with which it competes as foreign and allow a more rapid remission of the leukemia.
After destroying the marrow by radiation or chemotherapy, the transplant is performed. New blood cells appear within two weeks and after several months (autologous) or more than a year (allogous transplants) immune function is restored.
It is also possible to use cordon stem cells for the same purpose.
- Immune Treatment for Diabetes: For treatment, you first take blood from a person with diabetes and then separate the cells from the immune system (the lymphocytes). Exposing these cells to stem cells of the umbilical cord of a baby that is not related, then return the lymphocytes to the patient's body is known to this as "mother cell education therapy," because by exposing to stem cells, errant lymphocytes seem to learn again how they should behave. Type 1 diabetes is an autoimmune disease that occurs when the immune system of the body attacks the beta cells of the pancreas, which produces insulin. This leaves people with type 1 diabetes with little or no insulin. They need insulin injections to survive. Stem cell therapy offers a solution to complications related to the immune system and Langerhans islets of the pancreas or in turn improve the acceptance of Langerhans islets. While the re-generative potential of stem cells can be used to make available a supply of self-recovery of insulin-producing cells that respond to glucose, their immunomodulatory properties can potentially be used to prevent, stop or reverse self-immunity, as well as improve the rejection of innate graft and prevent recurrence of the disease. It has been thought that any cure for type 1 diabetes would have to stop the auto-immune attack, while regenerating or transplanting beta cells taking into account this has been done different studies, trying to corroborate the effect of stem cells in patients with type 1 diabetes. For example, a current study was performed, data collected for four years on 9 patients with type 1 diabetes in China, to see how well treatment works, researchers measured peptide C, a fragment of protein that is a secondary product of insulin production. Two people with type 1 diabetes who received a stem cell education treatment shortly after diagnosis (five and eight months later) continued to have a normal C peptide production and did not need insulin four years after a single treatment. Another type 1 diabetes patient had had the disease for four years when he received the treatment. In any case, it had an improvement in peptide levels C, but it was not considered in remission. The other six people with type 1 diabetes experienced reductions in peptide levels C over time. The authors of the study said this suggests that more than one treatment may be needed. "Mother cell education therapy is a safe method" with long-term effectiveness, concludes by saying Dr. Yong Zhao.
Acute Myocardial Infarction
Acute myocardial infarction belongs to the acute coronary syndromes, these are characterized by presenting a clinical picture composed of an ischemic condition (lack of irrigation) to some area of the myocardium, which leads to necrosis of the same, this is due to an initial obstruction that can be acute and total of any of the coronary arteries that supply it. Acute myocardial infarction is considered the leading cause of death in men and women worldwide. Many of the contributing factors are caused by a poor diet and by leading a very sedentary life, it is stated that many eating problems can be avoided by eating a healthy diet and constant physical exercise.
- Follow-up protocol for treatment: The protocol includes the following tests: (a) myocardial damage markers (creatincinase, isoenzyme MB of creatincinase and troponin T) for 24 h after the procedure; (b) basal and dobutamine echocardiography at pre-implant low doses and at 6 months; (c) magnetic resonance pre-implant and 6 months; (d) ECG Holter at 3 weeks and 6 months, and (e) clinical visit with analytics, ECG and chest X-ray at 3 weeks and 6 months. In the first patient, catheterism and ventriculography were also performed at 6 months.
- Cellular implant: At 10-15 days of the infarction, bone marrow extraction is performed by repeated punctures of the posterior iliac crest, prior disinfection of the skin with ioda povidone, with a puncture trocar connected to a 20 ml syringe. In each puncture, about 5 ml of bone marrow are aspirated. The mononuclear fraction is obtained through the centrifuge of Ficoll before smoothing the erythrocytes with water. Cell suspension is reused in RPMI-1640 with 2% autologous plasma. The number of cells is adjusted to 1 x 106/ml. Mononuclear cells are transferred to a teflon bag and incubated overnight at 37 °C with CO2 at 5%. The next day, they centrifuge and heparinize, and the viability with tritan blue is valued. The implantation of cells in the infarct region is carried out within 10-15 days. The left coronary artery is channeled with the catheter-guide and a coaxial catheter-balon is introduced that inflates to 2-4 atmospheres in the previously repaired segment with stent. Later the guide wire is removed and this light is used for the infusion of the cells. The suspension of stem cells is inserted into a 50 ml syringe that is connected to the infusion catheter. Then, periods of 2 min occlusion of the left coronary artery with slow infusion of the suspension (1 ml/min) are alternated with periods of 1 min reperfusion. The injection of mesenchymal estromal cells contributes to the healing of wounds through various mechanisms, including the stimulation of angiogenesis.
Veterinary use
In 2011 in Brazil, a maned wolf, hit by a truck, underwent stem cell treatment at the Brasilia Zoological Garden, this being the first recorded case of the use of stem cells to heal wounds in a wild animal.
Cloning
Cloning is the act of transferring the nucleus of a somatic cell of a patient to the cell without a nucleus of an egg donor. This transfer will act like a fertilized egg and begin the cell division process.
Stem cell controversy
The fact that these cells currently involve the use of human embryos and fetal cadaveric tissue calls for careful consideration of ethical issues related to the progress of biomedical research. In contrast, medical research believes that it is necessary to proceed with embryonic stem cell research because the resulting technologies could have great medical potential, and that excess embryos created by in vitro fertilization can be donated for research. This, in turn, produced conflicts with the Pro-Life movement (Pro-Life), who adjudicate the protection of human embryos. The constant debate has made authorities around the world seek regularity in their work and highlight the fact that embryonic stem cell research represents an ethical and social challenge.
According to many religious and ethical systems, human life begins at fertilization. According to their arguments, any intentional measure to stop development after conception is considered as the destruction of a human life. Other critics don't have a moral problem with human stem cell research, but fear a precedent for human experimentation. Some critics support the idea of research, but want strict legal rules to prevent genetic experimentation on humans, such as cloning, and to ensure that human embryos are only obtained from appropriate sources. Preventing human stem cell research from becoming a slippery slope toward human genetic experiments is considered by most in society to be an important point in the human stem cell controversy.
Within the medical community, there are different positions, among them that «blastocysts or embryos are living organisms that within 9 months will be human beings with rights, therefore, it is unethical to destroy the blastocyst or embryo to obtain the stem cells", while others consider that in the early age of an embryo what you have is a sprout of cells with their internal mass.
In addition to the ethical problems associated with the destruction of the blastocyst, the fact that a large number of eggs are needed to create embryos, which will then be destroyed, and how these eggs are obtained, is also unethical. The egg donor is first treated with some drugs and hormones so that she creates many eggs that will be donated. These drugs can bring health problems which is unethical to harm a patient knowingly.
The natural, primary and main purpose of medicine and technical-scientific progress is the defense and protection of human life. Science makes sense to the extent that it conforms to natural ethics safeguarding life. A science without the guide of ethical criteria ends up turning against the human being, for whose service it was born.
Points of view
The debates have sparked the Pro-Life movement, which cares about the rights and status of an embryo as an early human. This movement believes that stem cell research instrumentalizes and violates what they call the sanctity of life and should be considered murder. The fundamental ideas of those who oppose these investigations are the defense of what they call the inviolability of human life and that human life would begin when a sperm fertilizes an egg to form a single cell.
Some research uses embryos that were created but not used in in vitro fertilization to derive a new line of stem cells. Most of these embryos tend to be destroyed, or stored for long periods, spending their time of life. In the United States alone, around 400,000 embryos have been estimated in this state.
Medical research indicates that stem cells have the potential to dramatically alter approaches to understanding and treating disease, and to alleviate suffering. In the future, most medical research anticipates the use of technologies derived from stem cell research to treat various. Spinal cord injuries and Parkinson's are two examples that have been recognized by famous people (for now, Christopher Reeve and Michael J. Fox).
In August 2000, the US National Institutes of Health said:
[...] Research on multipotent stem cells [...] promise new treatments and possible cures for many diseases and injuries, such as parakinson, diabetes, heart problems, multiple sclerosis, burns and spinal injuries. NIH believes that the medical potential of pluripotent stem cells will benefit medical technologies and will be consistent with ethics.
Recently, research at Advanced Cell Technology in Woecester was able to obtain stem cells from a mouse without killing the embryos. If this technique is improved it will be possible to eliminate some of the ethical problems associated with embryonic stem cell research.
In 2007, another technique was discovered by research teams from the United States and Japan. Human skin cells have been reprogrammed to function more like embryonic cells when introduced to a virus. Extracting and cloning stem cells is expensive and complex, but the new method of reprogramming is much cheaper. However, the technique can alter the DNA of new stem cells, causing skin cancer.
In 2007 work began with induced pluripotent stem cells ("IMCP") by manipulating only four genes; later, it has been possible to reduce the number to only two of those four genes; and even, just by introducing into the cell the four proteins encoded by the four genes. The process consists of extracting a cell from the patient to be treated, manipulating said 4 or 2 genes or introducing the four proteins encoded by those four genes, cultivating them and introducing them into the patient or causing their differentiation towards the cell type that is needed (one or more, since the stem cells thus created behave like embryonic cells). There is no human experience yet and the small but certain risk of tumors remains to be resolved.
Policies on stem cells and cloning by country
Therapeutic/embryonic cloning goes hand in hand with this topic. However, there are countries that are opposed to both cloning or only one or neither. They are also opposed to stem cell experimentation. For example:
- European Union: yes embryonic cell lines, no therapeutic cloning.
- United States: The creation of mobile lines is legal but without public funds. The legality of therapeutic cloning depends on the state in which it is found. "As of August 1, 2001, government funds will not be used for embryonic stem cell research, and from that time only pre-existing cell lines will be used by August 1, 2001," Bush said at a press conference. Although Bush adopted this position, he did not object to private institutions experiencing embryonic stem cells. This is why proposition 71 emerged in November 2004 in California, which authorizes the establishment of the Institute for Regenerative Medicine in California for a period of ten years.
- UK: yes embryonic cell lines. Yes to therapeutic cloning.
- Sweden: yes embryonic cell lines. Therapeutic cloning is legal.
- Israel: legal embryonic cell lines and therapeutic cloning.
- China: legal embryonic cell lines and therapeutic cloning.
- Brazil: legal embryonic cell lines created by in-vitro fertilization with 3 years of age/ Therapeutic cloning is not legal.
- South Korea: yes embryonic cell lines. Allowed by the country's health minister.
- Singapore: legal embryonic cell lines if the blastocyst is destroyed 14 days after fertilization. Therapeutic cloning is legal.
- Australia: Yes embryonic cell lines, therapeutic cloning is not legal.