Stem cells offer both promise and challenge
Each of us starts life as a single cell: a fertilized egg. That cell that divides in two; the two divide to make four, four to make eight, 16, 32 – 60 trillion.
The earliest cells of the embryo are “totipotent,” with the potential to become anything: eyes or lips, heart or toes, brains or buttocks. Over time, however, the a cell makes decisions – it “differentiates,” turning on some genes, turning off others. Following instructions from those genes, the cell matures into a muscle cell, a brain neuron, a skin cell. Once a cell differentiates, its fate is sealed – once a brain cell, always a brain cell – and it is no longer totipotent – its potential is more limited.
Mow the lawn and it grows right back. Put a houseplant cutting in water, and it grows roots.
Some lizards, caught by the tail, escape by breaking off the appendage. Months later, the tail has re-grown. Salamanders can even re-grow missing feet.
Like early embryos, plants and “lower” animals contain cells that, like Peter Pan, have never grown up – they’ve avoided differentiating. “Stem cells” in the animal, meristem cells in plants, remain young forever.
From them, later generations of cells branch, building up their populations to replace missing tissues.
Humans have cells that can do the same – to a point.
Red blood cells (RBCs) have a tough life, repeatedly squeezed through capillaries so fine it would take 10 or so, side-by side, to equal the width of a hair.
Bent and scraped against the capillary walls as it’s pushed through, an RBC wears out in three or four months. To replace worn-out RBCs, forever-young stem cells in the bone marrow divide over and over, manufacturing two or three million new RBCs every second.
Hematopoietic stem cells (“hemato-” meaning “blood;” “-poietic” meaning “creating” – from the same root as “poetry”) are only “pluripotent” – they can form different types of blood cells, but not, say, brain or kidney cells.
With luck and care our brains and livers and hearts have enough of all the types of cells they need. But all too often, disease, accident, or simple wear-and-tear kill off some cells, and we can’t see or hear or think or move as well as we might.
Suppose we could use forever-young stem cells to treat victims of Parkinson’s disease, who have lost the brain cells that would prevent their movement problems; to diabetics, who have lost the pancreatic islet cells that secrete insulin; to those who have lost hair follicle cells, and are growing bald.
We’ve actually been using stem cell therapy for half a century. Patients receiving bone marrow transplants to jump-start a failing blood-production system, and, more recently, burn victims receiving skin grafts, are actually healed by stem cells residing in the marrow or skin.
Looking at what’s happening in the labs, the therapeutic possibilities of both partially differentiated stem cells and embryonic stem cells appear phenomenal.
Embryonic stem cells are harvested from “extra” embryos resulting from in-vitro fertilization (techniques allowing infertile couples to conceive). Embryos could also be created using some of the techniques used in cloning (researchers emphasize they will use the techniques only to obtain stem cells, not for “reproductive cloning” – cloning of human beings).
Either way, in the process of obtaining stem cells, the embryo dies. There has been much discussion of this within scientific, medical and religious circles. Pending public support and regulation of stem cell research, the issue deserves a calm, well-thought-out public discussion.
Alan Stahler trained as a biologist and is an amateur astronomer. He teaches enrichment classes for children and adults at Sierra Friends Center. His science programs can be heard at noon on alternate Tuesdays on KVMR-FM (89.5).
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