Omnis cellula e cellula, with a twist

Identical? It would seem. But look closely and I think you will agree that it is not so.

Virchow, Schwann, Remak, and Redi were right … Omnis cellula e cellula, id est, All cells come from cells. And yet not all leaves on the same plant are identical. I wonder if you wonder why?

If the component cells of an organism arise from a single progenitor, how is it that individuals can be made up of so many different kinds of cells, cells that look and behave so differently? And how, while we’re at it, can it be that identical twins are not exactly the same, at least as far as things other than their genes are concerned?

The answer to the first question is gene expression. Let us suppose that there is a single gene for each trait. Why does a liver cell act like a liver cell or a cardiac cell act like a cardiac cell? What makes cells behave differently, given that each and every one contains the same genetic information? Consider that the genes that make liver cells behave like liver cells are turned on in liver cells, and the genes that make cardiac cells look and behave like cardiac cells are turned off in liver cells. The obverse is also true. These genes are regulated because the chemical milieu of a liver is such that cells which occur there act like liver cells. There’s something about the chemical milieu of the heart that cells which occur there act like cardiac cells. It is the environment within which a cell finds itself that determines just how that particular cell will function. So that is why, if they all source to a single progenitor, the many trillions of cells which make up the human body develop into a myriad of types.

But what about the other question? How it is that the fingerprints of identicle twins can differ, given that they possess the same genetic material? The answer is in the fine details of embryonic development. Consider that developing embryos cannot occupy the same position at the same time. Because this is so, each experiences a slightly different developmental environment. The ridges and troughs which form the fingerprints develop in the dermal layer of the skin. The dermis is sandwiched between two other layers (the epidermis and the subcutaneous layer) and buckles and folds as the skin moves during development. The pattern of the developing fingerprint is unique because each time it enfolds, even in genetically identical individuals, it does so chaotically. Development is a highly plastic phenomenon.

So, what does this all have to do with the image of the leaves of a Striped Maple seedling? Contrary to what our eyes may tell us, no two of the leaves on this delicate individual can be the same. Their component cells surely have the same genes but these are being expressed in a slightly different way each and every time a leaf develops to unfurl.

leaf

9 thoughts on “Omnis cellula e cellula, with a twist

    • As to your question … I wish! One problem … no one is knocking at my door … and I can’t believe that anyone ever will. They say that some of those who are most happy, professionally, are those who have the luxury of being paid to do what they truly enjoy. I truly enjoy the writing … but will never be lucky to find a situation in which I can make any sort of living at it. Maybe in my next life Elke. I’ll take the question as a complement … thank you. D

  1. When I read your title, I thought first of Omnia Gallia and its three parts: but it wasn’t human history you were concerned with here. The detail about the fingerprints fascinated me. While I’ve known that identical twins aren’t strictly identical, I’d never considered fingerprints. Your explanation was clear, and interesting.

    If I simply had glanced at your lovely leaf photo, I might have assumed that the upper edge of the one on the right had been nibbled by a snail or other creature. A closer look made your point: it’s natural variation, not damage. To borrow from the computer world, it truly is a feature, and not a bug!

  2. Although my knowledge and writing ability don’t match up with what you have written here, I have wondered and remarked similarly regarding my favorite Painted Trilliums. The petals always have such variety and when closely regarded show slight differences in the colored venation and the way the petal edges ruffle or not. The same goes for the sepals and leaves, although the petals are where one’s eyes land most easily. As far as how the genetic codes instruct the cells in their formation and function, I have always been amazed by that although without the understanding you display here. A very admirable post, David…as well as a very nice image.

  3. Your posts always enlighten me. You are able to describe complex subjects with ease. Will never look at a leaf in quite the same way again! Your recent images are pleasingly simple yet bold. I am a fan.

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