The body of humans and most other animals is made up of four different kinds of tissues. A tissue is defined as a group of similar cells organized to perform a specific function. Our body is made out of tissue, not tissue. The only animals which lack true tissue structures are sponges. The four types of tissue are epithelial tissue, connective tissue, muscle tissue, and nervous tissue. Epithelial tissue is distinct from connective tissue because epithelial tissues have dense populations of cells directly connected to each other. Epithelial cells will always be connected to neighboring cells using cell junctions. These epithelial tissues, also called epithelium, are made up of sheets of cells directly connected to each other. These sheets of cells may be one to many layers thick. This epithelial tissue is used to line the surfaces of our body. This includes both our outer and inner surfaces. The outer layers of our skin are definitely composed of epithelium, along with the inner lining of our digestive and respiratory systems. In fact, many of our internal organs and blood vessels have a lining of epithelial tissue. Connective tissue includes a large variety of tissues that are unified under this title because they are composed of a sparse population of cells, suspended in a uniform extracellular matrix. Extracellular means that this network of protein fibers and other material are found outside of the cells. Each type of connective tissue has a different extracellular matrix and performs a different function in the body. This includes loose connective tissue, fibrous or dense connective tissue, adipose tissue, cartilage, bone, and even blood. Lose connective tissue is the most widespread of the connective tissues and connects epithelial tissue to their underlying tissues. Lose connective tissue is also what holds some of the internal organs in place within the body cavity. Fibrous connective tissue, also called dense connective tissue, is a very strong type of tissue. It is used to attach muscles to bones in the form of tendons and to attach bones to bones in the form of ligaments. The extracellular matrix of fibrous connective tissue is composed of many fibers of collagen protein. This protein is very strong, allowing it to serve its function. Adipose tissue, also called fat tissue. The size of these cells may change, depending on the amount of lipid stored within them. The adipose tissue functions as an energy stockpile for the body, as well as padding and thermally insulating it. Cartilage is a connective tissue with a rubbery extracellular matrix. Our outer ear and the end of our nose is made of cartilage. But cartilage can also be found between the bones of our vertebral column, and at the ends of our bones, at the joints. This cartilage is used as a physical shock absorber, as well as a way to reduce friction at movable joints. Bone is also considered a connective tissue. The extracellular matrix of bones consists of rubbery protein fibers, which are hardened by deposits of calcium salts. The bones of our body provide support and protection, as well as allowing the body to move. The final, and probably the most surprising of the connective tissues, is blood. Blood is composed of a collection of different types of cells suspended in a liquid extracellular matrix, called the plasma. The blood serves as the circulating fluid for our cardiovascular system, and aids in gas exchange, along with the transportation of many important chemicals within the body. Muscles are the next type of tissue, and are found in three different varieties within the vertebrate body. Skeletal muscles are linear cylindrical cells, which are responsible for the voluntary movement of the body. They are attached to the bones of the skeleton by tendons. When observed under the microscope, these muscles appear striated or striped, because the contracting proteins within these muscles are very organized, exhibiting alternating light and dark bands. The cardiac muscle is found only in the heart, and is responsible for the heartbeat. This muscle also appears striated under the microscope, but unlike skeletal muscles, the cells are highly branched, and connected to several other cardiac muscles in the general area. Smooth muscle cells are named so, because they do not have any obvious visible striations, when viewed under a light microscope. They still contract, but it's just that their contractal apparatus is not as orderly. The smooth muscle is responsible for much of the involuntary contractions within our internal organs. Smooth muscle is often found within the walls of internal organs, and the larger blood vessels. Nervous tissue is the last category. This tissue includes neurons and supporting cells. The function of neurons is to both receive and send electrical impulses, called action potentials. These neurons establish an electrical communication system within the body. These are the four categories of tissues that make up the organs and other structures of our body. Now, time for some questions. A tissue is a group of blank, organized to perform a specific function. That atoms, molecules, cells, or organs. Connective tissue cells are directly connected to each other, true or false. When we have conscious, intentional control over a group of muscles, they are said to be voluntary, involuntary, viable, invisible, or violet.