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.


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