Hi. In this video, we're going to talk about development, how organisms start out as that single-celled zygote and progress or develop into these complex multicellular creatures that have specialized tissues, organs, bone structures, and all these crazy functions. How does that happen? Well, it turns out that certain processes are fundamental to development across all organisms. Chief among these, of course, is cell division. How does a single cell turn into a multicellular thing? Well, cell division, obviously, right? And you guys might recall from our discussion of cell division, that mitosis is a highly regulated process. And right here, we see a chart of the cell cycle or a figure of the cell cycle and these black bars in it represent the various checkpoints in mitosis. The control mechanisms, the gates if you will, that regulate the mitotic process. Now, in addition to these checkpoints, we also talked about social control which is how cells' neighbors can regulate their division. So, in the course of development, the timing and location of cell division is incredibly tightly regulated, as it's crucial for proper development. So, the systems involved in this are chiefly these mitotic control mechanisms and also these social control mechanisms. Neighbors influencing each other to divide or to halt division. Now, in addition to cell division, cell differentiation is another crucial process. Cell differentiation is how one cell becomes or one undifferentiated cell can become a specialized cell, like a neuron, for example. The cells lining your stomach and the cells in your brain, those neurons, are very different looking cells. But they all come from the same place. And that is namely, stem cells. These undifferentiated cells that through differentiation, give rise to specialized types of cells. And we call that process cell differentiation and we call the ultimate end of it cell fate. It's the destiny of the cell. What will this cell become? That is the cell fate, right? Is this stem cell to become a neuron or is it to become a cell of the intestine, an epithelial cell? Well, that's determined by its fate and we'll talk more about how cell fate is determined later on. Now, in plants, stem cells are actually located within the plant and remain there and continue to develop throughout the entire life of the plant and we call these meristems and actually, plants have multiple meristems. Basically, anywhere you're going to get new growth you have meristems. So, 2 pretty obvious places are the roots, right? The roots need to continue to grow so you have root meristems and also, the shoot. Right? Plants continue to grow upward, they branch out, send out new leaves. You have these shoot meristems and we'll talk more about meristems when we cover plant development specifically. Now, animals also use stem cells. They use stem cells or I should say, we use stem cells to repair our wounds, replace cells, and also to create the cells of the immune system which have to be developed in a very specialized fashion to match a specific immune function. So, animals actually keep a supply of stem cells in their bodies. However, unlike plants, we don't have carte blanche with our stem cells. We can't just continue to produce anything willy-nilly. We use our stem cells for very specific things. And here in this figure, you can see an example of the fertilization event which will lead to the formation of a zygote and how over the course of development, you will arise or you will give rise to specialized cells that will function in the circulatory system or the nervous system like neurons that we're talking about or the immune system as we're also talking about.
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