Lady Gaga Revisited: How one cell becomes an entire person

3 Aug

DISCLAIMER: This latest entry has not yet been rubber stamped by the White Coats who check this blog for scientific accuracy. Corrections to lies below will come soon.  Reader comments, questions and corrections are welcome.

Months ago we began this blog by introducing a puzzle:

As if by wizardry, Lady Gaga arose from a single cell that was smaller than the period at the end of this sentence.  This ought to strike us as incredible, not only because Lada Gaga is composed of trillions of cells, but because her cells have various features and functions, so that her eye balls are distinct from her tuchus.  And yet the cells in her body, whether they make up her liver, legs or lashes, all contain the exact same DNA.

If somehow Lady Gaga had only type of cell—that is, she had grown to full size with cells that never “differentiated,” – we postulate that she would look something like the Fruit of the Loom grape man.

That is to say, she would be a blown up version of the blob she was when she was about 32-cells big, making her way toward her mother’s uterus by way of the fallopian tube.  The sac of cells at this stage is called the morula, latin for mulberry, because it looks like a cluster of seeds.

So why isn’t Lady Gaga a grape man?

The answer lies in the fact that within her cells, different genes can be switched on or off, allowing or blocking the creation of proteins that determine what a cell looks like or how it behaves.  As an embryo and a growing person, huge numbers of Lady Gaga’s genes have turned on and off at just the right moment in development like musicians cued in by a conductor.

So if cellular identity depends on which genes in its nucleus are turned on and off, what’s controlling the switches?

The answer: It depends. Sometimes a cell’s own internal machinery brings about a chemical change or hardwires possibilities for gene expression like the barricades on a highway that allow certain exits to be open or closed.   Along certain chromosomes we and other animals have 180-nucleotide-long stretches called Hox genes, which are master regulators that specify where our body structures develop.  Wonky fruit flies that have legs growing out of their heads have mutated or missing Hox genes.

Sometimes environmental factors like temperature or oxygen levels will flip a genetic switch.  Or sometimes, the cue comes from other cells.

Our cells communicate with one another by sending and receiving proteins or signaling molecules. The interactions of cells with their nearest neighbors determine which genes become activated.  And these little local “decisions” determine whether cells will die or duplicate, move, or differentiate to create specific kinds of tissue.

What we see in this complex dance is basically call and response.  One or many cells might send out a signaling molecule, or a call that says, “There needs to be a shift in gene expression.”  A cell that receives that call might in response modify its gene expression to be like the cell that is calling it, or to become a new kind of cell.

The sonic hedgehog gene, for example, creates proteins (SHH) that play an important role in the patterning of our bodies and especially the development of our limbs, fingers and toes.  Different concentrations of SHH cause different types of cells to be formed in the developing embryo.

Genes are a bit like an artist’s color palette.  It’s not until a cell picks up a set of colors that you actually get the beauty or the functionality of what a cell can do.

In the next few blog entries, we will take a close look at this artistry by tracing the early development of the embryo.  Through a timeline of the first month of pregnancy, we will see how the process of differentiation transformed Lady Gaga from speck to spawn.

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