Tissues are complex structures made of highly specialized cells. They don't form overnight. In fact, an organism begins to develop its tissues early on in embryo development following what's known as gastrulation. After gastrulation, these cell layers called germ layers develop. These are kind of like embryonic tissues and there are 3 types: the outer layer or the ectoderm, the internal cells which are called the mesoderm, and the innermost cells, the endoderm. And you can see those depicted in the gastrula here. And, if you don't remember what a gastrula is, I recommend you go back and check out our video on development that covers this whole process of blastulation and gastrulation and talks about how you get to a weird ball of cells like that.
Now, adult tissues come from these tissues and there are 4 types of adult tissues. You have nervous tissue which will come from the ectoderm, muscle tissue which comes from the mesoderm, connective tissue which also comes from the mesoderm, and epithelial tissue that comes from both the endoderm and the ectoderm depending on what type of epithelium you're dealing with. You can see some examples of the final products of these germ layers and what they will wind up as right here.
With that, let's turn the page.