Connective tissue plays a major support role in the animal body. It connects, separates, and cushions other tissues and is basically made up of some cells scattered within an extracellular matrix. Hopefully, you remember, extracellular matrix is an array of proteins and this gel-like substance called ground substance. This functions as a support structure outside of eukaryotic cells in order to kind of play the role that a cell wall might, for example. It has a lot of other properties too, but you can think of it as a sort of surrogate cell wall, providing structural support. Now, loose connective tissue is the most common type of connective tissue found in vertebrates. It helps hold organs in place and attaches epithelial tissue, a topic we are going to discuss in just a moment. The most notable type of loose connective tissue is adipose tissue or fat. Adipose tissue is mostly made up of these cells called adipocytes or fat cells. You can see an example of loose connective tissue here in these two images; both are loose connective tissues.
Another type of connective tissue is dense, or sometimes it is called fibrous connective tissue, and this tissue is dense with collagen fibers. Most notably, it makes up tendons, which connect muscle to bone—very important—and ligaments, which connect bone to bone—also very important. That's how your body moves. You can see an example of this fibrous connective tissue right here above my head. Now, you also can have supportive connective tissue, such as bone and cartilage. These tissues provide structural integrity. You can see examples of bone and cartilage here; this is bone, and right behind my head, you have some cartilage. These tissues form a hard extracellular matrix, which is what gives them that structural integrity.
Lastly, there's fluid connective tissue, which is essentially blood. Blood cells have a liquid extracellular matrix we call plasma. Here, you can also see a nice little example of how connective tissue functions with other tissues. The stuff stained in blue in this image is connective tissue, and the purple stuff that it is surrounding is a type of epithelial tissue. So, you can see how the connective tissue is supporting that epithelium.
Nervous tissue is very important. This is how I am thinking these thoughts and speaking to you right now. This tissue conducts electrical and chemical signals and is divided between the central and peripheral nervous systems, which we'll discuss when we explore the nervous system in general. The main type of cell that gets all the credit in the nervous system is our neurons. These receive and transmit the electrical signals. You can see a neuron here. It transmits these electrical signals by transporting ions across the membrane in what we call the action potential. We will talk about that again when we cover the nervous system. The main components of a neuron are the axon, this portion here, and the dendrites. The branch-like structures out here, that I have circled, are the dendrites.
The axon can be thought of as the wire; it is the structure that can be very long sometimes, and this is what transmits the electrical signal. The dendrites are the branch structures that receive signals and determine how the cell needs to respond. Now, the reason I talk about neurons as though they get all the attention but don't necessarily deserve it is because of these other cells in nervous tissue, glia. Glia do not get nearly enough credit; they are super important as support cells for neurons, essential to their survival. Neurons would not live without these glia, and they also help the neurons with their functioning. We'll get to the specifics of their roles when we talk about the nervous system, but let me just say that this beautiful astrocyte that my head was covering, this glia right here, is probably responsible for much more of the functions of the nervous system than it actually gets credit for. A lot of current research in neuroscience is showing that glia play a much more important role than they are initially given credit for, and they also are involved in signaling, albeit in a different way from neurons. So super important stuff. Don't write off glia. Alright. With that, let's turn the page.