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.

It’s Diversity All the Way Down

“The most impressive aspect of the living world is its diversity. No two individuals in sexually reproducing populations are the same, nor are any two populations, species, or higher taxa [categories of organisms]. Wherever one looks in nature, one finds uniqueness.” So wrote Ernst Mayr in This is Biology, published in 1997.

Grains of sand under an electron microscope (wikipedia)

Grains of sand 
(wikipedia)

Part of his statement was a new idea to me. Clearly each species differs from the next. But I had not fully absorbed the notion that every organism, if it reproduces in pairs, is different from every other individual in its species. (Single-cell organisms like bacteria that divide into identical clones are the exception.) Every individual grass plant, every fish, every pure-bred dog, every ant is as different from another of its species as two human neighbors are. And, as Mayr adds, that makes uniqueness the order of the day.

But what about  diversity and uniqueness in the non-biological, inanimate world? “Nature” includes not only living things but also rocks, water, air, light and other forces and materials. They seem to be unique in their own ways. Snowflakes are famously singular. Clouds change constantly. So does the surface of the ocean. Air flows and spins. I’m pretty sure I’ve never seen two rocks that are identical. It’s a good bet that every asteroid, planet and star is different from others. Looking out over the desert, the ocean, or the skies, we always witness diversity in shape, motion, color and light if we look closely enough.

Diversity and fertility in grass (www.kvkcard.org)

(www.kvkcard.org)

Still, Mayr seems right that the diversity of living things  “impresses” us in a distinct way. Each organism succeeds at being alive, yet does so in a slightly different way from the others.

Moreover,  that booming variety, that hedge against species failure, comes on fast and strong. New life thrusts itself at us—in the new baby, in a puppy, among the trees springing up in corners of the yard, in the horde of ants and bees and birds of summer. In Origin of Species, Darwin wrote, “There is no exception to the rule that every organic being naturally increases at so high a rate, that if not destroyed, the earth would soon be covered by the progeny of a single pair.”

Diversity multiplied by fertility.