Okay. So now let's talk about cell-to-cell interaction. Cell-to-cell interactions are super important for proper development because the cells have to respond to signals from either other cells or their environment so that they properly develop into whatever they're supposed to develop into. The cells are constantly like, "Yay, give me information so I know what I'm to be doing." Now, during very early development, I'm talking about a single cell or a couple of cells, there's not a lot of information going on. Right? If you just have one other cell to interact with, it's not going to give you a lot of information.
And if you're a single cell, then how are you supposed to respond? So during very early development, the different regions of that single cell, such as which portion is closest to the nucleus, which portion is closest to the plasma membrane, etc., can respond differently to the same molecule. The two examples I want to give you of this are actually an asymmetric egg and a symmetric egg. If the egg is asymmetric, what that means is that there's a bunch of different proteins and molecules being expressed either inside the egg or surrounding the egg, and those molecules are not equally distributed. Some of them sit maybe closer to the posterior end, and some of them sit closer to the anterior end. Because of that, they're not equally distributed. We say that the egg is asymmetric because those molecules are actually concentrated in one region of the cell.
These concentrations of molecules result in different developmental occurrences. An example of this is fruit fly eggs have cells called nurse cells that surround the embryo, the eggs. Nurse cells express bicoid, but they are only present on one side of the cell. So when those cells are expressing bicoid, they're pumping that bicoid into the egg, or into the zygote once the embryo once it's formed, that means that bicoid is only going to be present at the anterior end and so that itself is an asymmetric egg. The alternative to that is the symmetric egg. In the symmetric egg, every molecule within it is expressed equally throughout the cell. There's no kind of concentration gradient that exists in that cell. It's all symmetrical. So the difference that determines that, actual, you know, which side becomes the interior, which becomes posterior, what develops where, is actually controlled through the sperm.
Because the sperm, you have this full egg, and the sperm can only enter in one location. So where the sperm enters actually will determine whether it is going to be anterior or posterior. This happens in C. Elegans, which, if you remember, are the worms. So if I back up for a second, here we have the two examples of this. We have the asymmetric egg and the symmetric egg, and these eggs are expressing some kind of black molecule. The black molecule in the asymmetric egg is going to be asymmetrically distributed or not distributed equally, and you can see that it's concentrated on one side of the organism. This concentration will determine the asymmetry of the egg, and it also determines the development of that region of the cell or the embryo. For the symmetric egg, the molecules are actually distributed equally. There's no kind of concentration gradient or anything that's going to lead to the development of one region of the cell differently from other regions of the cell. The thing that determines asymmetry in this situation is actually the sperm. It can only enter into one region; it enters here, and that will determine which end is anterior and which end is posterior. If the sperm entered here, it would do the same. It could enter here, it would do the same. There's nothing different about any of these locations, but it's just now that the sperm's there, there's more genetic material, there's extra proteins, and that determines the asymmetry, not the egg itself. So, with that, let's now move on.
