During the course of development, cells also interact with each other. Now, we've talked a lot about cell interactions in the cell signaling lesson. But during development, there are really two main types of interactions that are happening: paracrine, where cells emit chemical messengers that are picked up by the receptors of another nearby cell, and juxtacrine, which occurs when the ligand bound to the surface of one cell interacts with the receptor on another cell. In these ways, cells influence their neighbors to move around, divide, differentiate, and also die, topics which we'll discuss shortly. Differentiating cells have this special way of influencing their neighbors to also differentiate or to behave in a certain way—these are social control mechanisms, which we will talk more about later.
Regarding cells moving, this phenomenon is primarily observed in animals; it is not seen in plants, a topic we'll cover in more detail when we specifically talk about animal and plant development. It's important to note that cells actually have to move around during development to create specialized tissues and form specialized structures. Plant cells, on the other hand, are very good at expanding their size and changing, which results in alterations in the shape and form of the plant. Plant cells are also kind of moving around, just not in the same way that animal cells literally detach from one point and migrate to another part of the embryo. In plant cells, the cells will just expand their size and somewhat alter the shape of the plant as a result. We can see an example of this cell movement in the figure right here; this structure rearranges into this structure, and it does so by these individual cells that we see here actually moving around to form this configuration seen right here.
I previously mentioned dying as an important aspect of development, and indeed, we have discussed apoptosis, a particular type of programmed cell death, before in our discussions on cell signaling. It turns out that programmed cell death is incredibly important to development, and a great example of that is in the development of digits on your fingers. During the course of animal development, cells that make up the webbing between your fingers—similar to the webbed toes of a duck—undergo apoptosis and die out, resulting in separated digits. So, cell death is actually a very important part of life, believe it or not.
Alright, let's flip the page.