Stem cells and meristems

Why are plant stem cells used every day in nurseries, while human embryonic stem cells are still rarely used in hospitals? Nurseries use them to turn one good plant into thousands of identical copies. Embryonic stem cells could, in principle, become almost any cell in the body. Most people guess the science just is not ready yet. That is only part of the answer. By the end, the nursery and the hospital can be weighed side by side.

A stem cell is an undifferentiated cell, one that has not yet become specialised. It can divide to make more cells like itself, and it can differentiate into other types of cell. Embryonic stem cells come from an early embryo. They can differentiate into almost any kind of cell the body has, and the word for that is pluripotent. Adult stem cells, such as those in bone marrow, the soft tissue inside some bones, can form many types of cell, including blood cells, but not every type. Which could become a nerve cell: embryonic stem cells or bone marrow stem cells? Embryonic stem cells. They can become almost any cell in the body, nerve cells included. If that felt straightforward, one report on a Foundation paper backs you up: "The majority of responses correctly identified that, of the cell types listed, embryonic stem cells are pluripotent". Plants have stem cells too, and they sit in meristems. A meristem is found at the growing tips of shoots and roots. A meristem cell can differentiate into every type of plant cell, and it keeps that ability for the plant's whole life.

In medicine, stem cells may be able to help conditions such as diabetes and paralysis, where cells have been damaged or lost. A student writes that stem cells get into damaged nerve tissue, adapt, and repair the damaged cells. What is wrong with saying the stem cells adapt and repair the damaged cells? Nothing adapts and nothing gets repaired. The stem cells divide, and the new cells differentiate into nerve cells, which replace the damaged ones. One examiner's report on a Foundation paper says a stem cell question "was not answered well due to weak use of language". It adds: "The idea of stem cells 'adapting' was a common occurrence". The fix: two verbs every time. The stem cells divide, and the new cells differentiate into the cell that is needed. Bone marrow transplants already work this way. Stem cells in donated marrow give a patient new blood cells. But donated cells carry different genes, so the patient's body can attack them. That is called rejection. Therapeutic cloning gets round that. An embryo is made with the same genes as the patient, and stem cells from that embryo are grown into the cells the patient needs. Why would the patient's body not reject cells made by therapeutic cloning? They carry the patient's own genes, so the body does not attack them. And the cloning makes cells for treatment, never a copy of the person.

Before any stem cell treatment is used, it has to be weighed. Viruses can be passed on with the cells, and donated cells can be rejected. The treatment is costly, and it may not work. Some people also have ethical or religious objections to using embryos, because an embryo is destroyed when its stem cells are taken. Each objection only counts where it applies, so every case has to be checked on its own. First case: bone marrow from a donor is given to a patient whose own marrow cannot make healthy blood cells. Is rejection a risk here, and does the embryo objection apply? Rejection is a risk, because the donor's genes are different. No embryo is used, so that objection does not apply. Viruses from the donor are a risk too. Second case: stem cells from a patient's own bone marrow are used to give that same patient new blood cells. Does rejection or the embryo objection apply this time? Neither. No one else's genes go in, and no embryo is involved. Third case: cells from an embryo made by therapeutic cloning replace damaged nerve cells in a paralysed patient. Which risk or objection applies to the embryo case that did not apply before? The embryo objection. An embryo is made and then destroyed, and some people object to that. Rejection is not a risk, because the embryo has the patient's genes. A report on a Higher paper records this trap: "Misconceptions were common, particularly references to ethical concerns about embryonic stem cells, which were not relevant in this context." The fix: for every objection, ask whether it applies to this case before you write it down.

A strong answer weighs a treatment as a chain: the claim, the evidence, what it shows, what it does not show, and then a judgement. Maya has type one diabetes. She cannot make enough insulin, the hormone that controls her blood sugar, so she injects it several times a day. Doctors suggest making insulin-producing cells for her by therapeutic cloning. Can you build the chain for her, from the claim to a judgement, before I show you? The claim: the treatment could free her from daily injections. The evidence: embryonic stem cells can differentiate into almost any cell, including insulin-producing ones, and an embryo made by therapeutic cloning has her genes. What it shows: the cells would not be rejected, and the treatment could last. What it does not show: viruses could be passed on with the cells, the treatment may not work, and it is costly. And here the embryo objection does apply, because an embryo is made and destroyed. Now two judgements to compare. One says stem cells will cure Maya. The other says the treatment is worth researching, but not yet a replacement for her injections. Which judgement is stronger, the first or the second, and why? The second. It weighs the risks against injections, which are proven and safe. The first ignores every risk the chain turned up.

Meristem cells can become any type of plant cell, so a whole new plant can be grown from a few of them. Stem cells from a meristem can be used to grow many identical new plants quickly and cheaply. The new plants are clones: genetically identical to the parent plant. That has two big uses. A rare plant species can be cloned to protect it from extinction. And a crop plant with a useful feature, such as resistance to a disease, can be cloned in large numbers for farmers. A botanic garden has the last twenty plants of a rare tree. A banana grower has found one plant that resists a fungal disease. What one risk do the garden and the banana grower share? Every clone has the same genes, so anything able to attack one of them can attack them all. And no gene has been changed. Cloning copies a plant's genes exactly; genetic engineering changes them, and that is a different technology. About half of students get this right. One report on a Higher paper records that "forty-nine per cent of students correctly identified producing a large number of identical plants as one economic use of plant stem cells." It goes on: "There was some confusion of genetic engineering and stem cell technology, as most other students selected to create genetically modified crops as their answer." The fix: cloning copies genes, and genetic engineering changes them, so keep the two apart. Now the opening question: why is it routine in a nursery but rare in a hospital? In a nursery there is no embryo and no rejection, so the only risk is the plants' shared genes. In a person, viruses, rejection, cost and the embryo objection can all apply.

If the context rule has landed, a stem cell treatment this video never described should be no trouble. Stem cells from Jo's own fat grow new knee cartilage for Jo. Which objections apply? Not rejection and not the embryo objection: the cells carry Jo's own genes, and no embryo is used. Cost, and whether it works, still count. Now try a plant: where in a plant are its stem cells found? In the meristems, at the growing tips of the shoots and the roots. A different one to end: a rose is cloned a thousand times. Has any gene changed? No gene has changed. Cloning copies genes; changing them would be genetic engineering. And the nursery and the hospital: a plant clone has only its shared genes to weigh, while a stem cell treatment for a person has every link of the chain to weigh.

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That completes our chapter on Cell division, specialisation and stem cells. Next chapter: Enzymes, digestion and metabolism. Hopefully that's next in your feed, but if not, search for ScholaFly B I oh three, oh oh.

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