Stem Cells, Therapeutic Cloning, and Bioethics

Stem cells are unspecialized cells that can divide to produce more stem cells and can differentiate into specialized cell types; cloning produces genetically identical cells or organisms; and bioethics examines the moral questions raised by biotechnology. In brief: embryonic stem cells are pluripotent and can form almost any cell type, adult (tissue) stem cells are multipotent and form a limited range of cell types, and induced pluripotent stem cells (iPSCs) are adult cells reprogrammed to a pluripotent state. Therapeutic cloning aims to make stem cells genetically matched to a patient, while reproductive cloning aims to produce a whole new individual, as with Dolly the sheep.

Stem cell research promises to treat diseases that now have no cure, but it raises difficult ethical questions. This guide explains potency and the types of stem cells, how they are obtained and used, the difference between therapeutic and reproductive cloning, summarizes medical and agricultural applications, sets out the main ethical arguments in a balanced way, compares the cell types and works through a numerical example.

What are stem cells?

Stem cells have two defining properties: self-renewal (they can divide many times, producing more stem cells) and potency (they can differentiate into specialized cells, such as nerve, muscle or blood cells). Differentiation happens when particular genes are switched on and others off, as explained in the article on gene regulation. The cell types are described by their potency, i.e. the range of cell types they can form:

• Totipotent: can form all cell types of the organism and also the tissues of the placenta. The zygote and the cells of the very early embryo (up to about the 4-to-8-cell stage) are totipotent.
• Pluripotent: can form all cell types of the body, but not a whole organism. Embryonic stem cells are pluripotent.
• Multipotent: can form several related cell types. Blood-forming (hematopoietic) stem cells in bone marrow make all blood cells.
• Unipotent: can form only one type of cell, such as stem cells of the skin that produce only skin cells.

What types of stem cell exist?

Embryonic stem cells (ESCs)

They are taken from the inner cell mass of a blastocyst (an embryo of about 5 days), usually from embryos created by in vitro fertilization that are no longer needed. They can divide almost without limit in culture and can become any cell type, which makes them valuable. Their use is ethically controversial because the embryo is destroyed in obtaining them, and there is a risk that they form tumors (teratomas) if transplanted undifferentiated, as well as of immune rejection.

Adult (tissue-specific) stem cells

They are found in many tissues, such as bone marrow, skin, the lining of the gut, the brain and fat. They maintain and repair the tissue and are usually multipotent. Bone marrow transplants, used for leukemia and some other blood diseases for more than 50 years, are the most established stem cell therapy. Cord blood from the umbilical cord, which is rich in hematopoietic stem cells, can also be used. They are less likely to form tumors, and can be taken from the patient (autologous), which avoids rejection, but they are hard to isolate and to grow in the laboratory.

Induced pluripotent stem cells (iPSCs)

In 2006, Shinya Yamanaka showed that adult mouse skin cells can be reprogrammed into pluripotent cells by introducing just four genes (Oct4, Sox2, Klf4 and c-Myc, known as the Yamanaka factors). Human iPSCs followed in 2007. These cells can be made from a patient’s own cells and avoid the destruction of embryos, and so they are used to model diseases, test drugs, and develop cell therapies. Yamanaka shared the 2012 Nobel Prize in Physiology or Medicine with John Gurdon, who had shown in the 1960s that the nucleus of a specialized frog cell can direct the development of a whole tadpole when it is transferred to an egg.

Stem cell types compared

What is cloning?

A clone is a group of genetically identical cells or organisms. Natural clones include identical twins and organisms produced by asexual reproduction. In biotechnology, the main technique is somatic cell nuclear transfer (SCNT): the nucleus is removed from an egg cell, and replaced with a nucleus from a body (somatic) cell of the individual to be cloned. The egg is stimulated, usually by a small electric shock, to divide.

• Reproductive cloning: the resulting embryo is implanted into a surrogate mother to produce a live animal that is genetically almost identical to the donor of the nucleus. Dolly the sheep, born in 1996 at the Roslin Institute in Scotland, was the first mammal cloned from an adult cell. She was one of 277 attempts (29 embryos implanted, and one lamb survived). Since then, cattle, pigs, horses, dogs, cats and monkeys have been cloned, with low success rates and often health problems. Human reproductive cloning is prohibited in most countries and is not considered safe or ethically acceptable by most scientists.
• Therapeutic cloning: the embryo made by SCNT is not implanted, but is used after about 5 days to obtain embryonic stem cells that are genetically matched to the donor of the nucleus. In principle, such cells could be turned into tissues for the donor without immune rejection. It has proved technically difficult in humans (the first successful derivation of human ESCs by SCNT was reported in 2013), and iPSCs have largely replaced it as the preferred route.

The clone is not necessarily identical in every respect: mitochondrial DNA comes from the egg, and environment and chance influence development and behavior.

What are the applications of biotechnology?

• Medical: stem cell transplants for blood diseases; skin grafts grown from cultured cells for burn victims; trials of cell therapies for spinal cord injury, diabetes, heart disease and eye diseases; recombinant medicines; gene therapy; diagnostic tests; organoids (miniature organ-like structures grown from stem cells) for drug testing.
• Agricultural: transgenic and gene-edited crops, cloned elite breeding animals, tissue culture of plants, biopesticides.
• Industrial and environmental: enzymes, biofuels, bioplastics, bioremediation using microbes to break down pollutants.

What are the ethical issues?

Bioethics asks what we may do, and what we should do. Different views come from different values and are held by thoughtful people. Common questions include:

• Moral status of the embryo: some people believe that a human life with full moral status begins at fertilization, so destroying embryos for research is wrong. Others hold that an embryo of a few days, a ball of cells without a nervous system, does not have the same status as a person, and that the potential benefits of curing disease justify the research. Many countries allow ESC research under strict rules, some with limits (such as 14 days of development).
• Benefits and risks: are the potential medical benefits worth the risks of unproven treatments? Unregulated clinics selling unproven stem cell treatments have harmed patients.
• Consent and ownership: consent of the donors of embryos, eggs and tissues; the use of cell lines such as HeLa cells, taken from Henrietta Lacks in 1951 without her knowledge; patents on cells and genes.
• Justice and access: who will be able to afford new therapies?
• Animal welfare: cloning and transgenic animals suffer from health problems.
• Reproductive cloning and enhancement: concerns about human dignity, identity and the use of technology to select or enhance traits.

Scientists, ethicists, regulators and the public all take part in the discussion. Laws vary from country to country.

Worked example: expanding stem cells in culture

A laboratory starts with 5 × 10⁴ hematopoietic stem cells and needs 2 × 10⁹ cells for a transplant. The cells divide once every 24 hours in culture, assuming each cell divides.

1. Fold increase needed: 2 × 10⁹ ÷ 5 × 10⁴ = 4 × 10⁴ = 40 000.
2. Number of doublings: 2ⁿ = 40 000, so n = log₂(40 000) = ln(40 000) ÷ ln 2 = 10.6 ÷ 0.693 ≈ 15.3. About 16 doublings are needed (2¹⁵ = 32 768 is not enough; 2¹⁶ = 65 536 is more than enough).
3. Time: 16 doublings at 24 hours each is 16 days.
4. Check: 5 × 10⁴ × 65 536 = 3.3 × 10⁹ cells, which is greater than the 2 × 10⁹ needed.

In real cultures, not all cells divide, some die or differentiate, and the stem cell character can be lost with many divisions, so the actual time is longer. The calculation shows why growing enough cells for therapy takes weeks, and why cord blood, with small cell numbers, is often used for children or combined from two donors.

How did the field develop?

Ernest McCulloch and James Till discovered blood stem cells in mice in 1961. John Gurdon cloned frogs by nuclear transfer in 1962. E. Donnall Thomas performed the first successful bone marrow transplants for leukemia in the 1950s and 1960s. James Thomson derived human embryonic stem cells in 1998. Ian Wilmut, Keith Campbell and colleagues cloned Dolly in 1996, and she died in 2003 of a lung disease. Yamanaka’s iPSCs followed in 2006 and 2007. The first clinical trials with iPSC-derived cells began in Japan in 2014.

Which mistakes are common?

Mistake 1: Mixing up therapeutic and reproductive cloning

Therapeutic cloning makes stem cells and does not produce a baby. Reproductive cloning aims to make a new individual.

Mistake 2: Saying that all stem cells can become any cell type

Only pluripotent cells (embryonic and iPSC) can. Adult stem cells are limited.

Mistake 3: Thinking that a clone is an exact copy in every way

Clones have almost the same nuclear DNA, but environmental effects and mitochondrial DNA make them differ.

Mistake 4: Believing that ethical questions have a single scientific answer

Science gives facts. Decisions also depend on values, so there is legitimate disagreement.