system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: When a zygote first begins to divide in the womb, how do the new cells "know" what to differentiate into? How does a "kidney cell" know that it's a "kidney cell" and not a "brain cell" or a "nose cell"?
gpt
Each cell in your body contains your entire DNA sequence, or genome, but not all genes are "turned on," so to speak. Rather, some cells use only parts of your genome, and other cells use other parts of your genome. No cell in your body uses the whole thing.

So, your question really might be rephrased: how do the cells know which set of genes to use?

For that, you have to go all the way back to the moment of conception. The point at which the sperm enters creates some damage on the way in, which we call a pole. The egg cell starts depositing chemicals in the area in response to the damage. 

As the cell starts to divide, some new cells grow near that pole and are being influenced by those chemicals that were dispatched in response. The influence is that it starts turning certain genes on and off: specifically, they start activating genes that start creating proteins. Some of those proteins will diffuse to farther reaches of the cell, but most of them stay local.  The farther away, the weaker the effect.

These chemicals, in turn, will end up turning other genes on and off. This creates multiple layers in the developing embryo (3 layers, to be exact). Each layer only has the capability of forming certain types of cells. 

How each cell comes to "know" its function (i.e., has certain genes turned on, or "expressed," if we use better terminology here) starts to become a little different at this point, as a few different processes are in play, and not all organs form at the same time. It would be a different explanation for each one.

Within a few weeks, most (not all, but most) of the cells have differentiated before starting to divide. For example, cells that form the heart walls are already cardiac muscle cells: they're just splitting into more cardiac muscle cells to complete the process. Once something becomes one kind of cell, it isn't going to be able to switch to something else. It's fixed in that particular role. 

By the time you're born, your "nose cells" can only split into more nose cells because they're fixed in the role. You can't grow a "kidney cell" there because the genes for the kidney aren't active in the nose cells, and if they split, you'll just have two cells that don't have the kidney genes active. As such, you can't grow a nose in your kidneys or another hand on your ear.