Stem Cells: How To Build a Body

Until recently I didn’t know much about stem cells except that they produce other kinds of cells and that the medical research on them was controversial. But in the context of the history of life, it turns out, their importance is as fundamental as you can get.

It took more than a billion years for the first cell with a nucleus to come together. Since then, the only reliable source for a new cell has been another cell. Every cell is an offspring—true for plants as well as animals.

An embryonic stem cell (Wikipedia)

An embryonic stem cell
(Wikipedia)

But while cells are specialized for one task or another, they are not always very good at dividing and reproducing. Muscle cells, blood cells, and nerve cells don’t reproduce at all. Other cells in the body divide only under some circumstances or only a limited number of times.

But reproduction is the stem cell’s specialty. When it divides, it produces another stem cell, ready for the next round, along with a muscle cell or blood cell or nerve cell or a cell of another organ. It looks the part for such flexibility—blob-like, unstructured, not committed until needed.

Stem cells are stationed throughout the body, small groups of them in each organ, like local hospitals on call to repair the sick and damaged. They are a profound piece of bodily engineering, a design for the long-term, like a futuristic dream-car that carries little 3-D printers throughout the engine and chassis to create new parts and replace the old parts automatically and on-board.

In human embryos, in contrast to adults, stem cells literally build the body. When an embryo is only a few days old, its stem cells begin to form all—all—of the specialized cells needed in a body, some two hundred of them.

In this root tip, the number 1 marks the relatively unstructured stem cells in the meristem. (Wikipedia)

In this root tip, 1 marks the relatively unstructured stem cells in the meristem.
(Wikipedia)

Plants have stem cells too. Located near the tips of the roots and stems in a layer called the meristem, plant stem cells divide into both specialized cells for the plant and additional stem cells. In short, stem cells are the place where a plant grows.

One of the wonders of any living thing is the sheer variety of its parts, the inventory of its tubes, organs, fluids, surfaces, protrusions, electric circuits and rigid pieces. As we pause to appreciate this profusion, sing the praises of the smudgy cell that creates and repairs them all.

Our Actual “Eve”

She lived about 150,000 years ago in southern Africa. These days she is known as Mitochondrial Eve. The “Eve” part is a little misleading since unlike the Biblical Eve, Mitochondrial Eve wasn’t the first or only woman alive at the time, and there were also plenty of men around. Still, Mitochondrial Eve was an actual person to whom every human today, male as well as female, can be traced back on his or her mother’s side—from mother to mother’s mother and so on.

But interesting as such a linkage may be to scientists, how significant is Mitochondrial Eve for us? See what you think.

Mitochondria in a cell (Flickr)

Mitochondria in a typical cell. The long thread of genetic DNA in the nucleus is shown, but the unrelated DNA inside the mitochondria is not. (Flickr)

Mitochondria (my-de-KAHN-dree-ah) are particles inside of cells that produce energy for the cell. Originally independent cells themselves, mitochondria were engulfed by larger cells long ago, proved useful, and made themselves at home.

When they did so, mitochondria brought with them their own bits of DNA. These strands are not related to, and are much smaller than, the complex DNA in a cell’s nucleus that make up our genes. But like all DNA molecules, as mitochondrial DNA makes copies of itself, it sometimes mutates; copying errors occur and the DNA changes slightly. As a result, mitochondrial DNA, handed down through generations of female humans, forms a record of our ancestry separate from our genes.

If this is difficult to visualize, a rough analogy is the battery in a car. These 12-volt energy-units that power the starter motor come in different brands with serial numbers and other codes on them. Over the years, independently of changes in cars themselves, battery manufacturers make changes to car batteries. Now imagine that you had no other way of telling the age of a car that had, say, been crushed beyond recognition. One option w ould be to dig out what was left of the car battery to find its codes or numbers. The car battery would date the car.

But if a particular version of mitochondrial DNA is passed down through women, how is it that males also carry it?  Because this DNA comes in the cell that each human grows from, and that cell is our mother’s. Fathers, through their sperm, contribute some of the genetic DNA that creates the new person, but the cell that begins to divide and multiply is mom’s, complete with her formulation of mitochondria DNA.

At the time that Mitochondrial Eve lived, of course, other mothers were passing along their own mitochondrial DNA to their own children, to their daughters’ children, etc. What happened to all those versions? Why is it that today’s humans everywhere carry the same version, the same mutation, of mitochondrial DNA? Apparently all those other lines of mitochondrial DNA fizzled out. Some mothers bore only sons, with no daughters to carry on their cell line. Other women had no children at all. The single remaining “brand” of mitochondrial DNA has been traced back to an approximate place and time five thousand generations ago. It is as if over the years all brands of car batteries went out of production except one, and that one is now installed in all cars.

What are we to make of all this? Compared to the Biblical Eve and her list of firsts—first woman, first human to be curious, first mother—Mitochondrial Eve wasn’t a forerunner of any of our significant traits. It’s that other  DNA, the genetic DNA from our mother and father, that plays a role in the color of our eyes and our musical aptitude.

Still, as biomedical historian Siddhartha Mukherjee puts it The Gene: An Intimate History:  “I find the idea of such a founding mother endlessly mesmerizing.”

It is mesmerizing to know that a small identifier in each of us can be traced back to a single human mother long ago. In theory, any diverse group of living things has a common ancestor after whom its descendants branched off. But that common ancestor may be difficult to “relate” to. The ancestors of all primates (not only humans but also monkeys, baboons, and chimps) lived about 60 million years ago and looked something like a squirrel with large eyes. I don’t feel the warmth.

On the other hand, I like the idea of being descended from a mother a long time ago from whom all other humans today are matrilineally descended as well, and whom we could, if we saw her, recognize as one of us.