Tissues of the World, Unite!
Two basic types of human tissue are connective tissue and muscle tissue. They have distinct forms and functions, but along with epithelial tissue and nervous tissue, they work together to make the human body function.
Connective Tissue
As the name implies, connective tissue joins other tissues together. It is composed of cells and molecules that function together for this adhesive process.
Connective Tissue Cells
Fibroblast is the principal cell of connective tissue. This cell deposits fibers that are found in all connective tissues: collagen, a protein resistant to stretching, thus giving the tissue tensile strength. These cells also produce elastic fibers that allow the tissues to rebound after being stretched. Macrophages are also found in connective tissues; they function as the vacuum cleaners of the body by removing pathogens and debris. Fat cells, or adipocytes, may also be present in connective tissue. These cells are mostly droplets of fats (lipids, cholesterol, or fatty acids) that are stored and released depending on the body’s available energy. When fuel for the body is abundant, materials are stored as fat. In times when fuel is scarce in the blood, fat is converted into a useable form of energy.
Connective Tissue Classification
Connective tissue is classified based on the percentage of cells to fibers and how tightly packed the fibers are within the connective tissue.
· Loose connective tissue consists of widely spaced fibers and many cells migrating within the open spaces. Areas where you find loose connective tissue include the tissue around large blood vessels, beneath the epithelium of the skin, and in the digestive and respiratory tracts.
· Dense irregular connective tissue has many more fibers and fewer cells than loose connective tissue and is classified based on the orientation of the fibers. In the dermis of the skin, the collagen fibers have a swirling and disorganized pattern, so the connective tissue is called dense irregular.
· Dense regular connective tissue is made up almost entirely of collagen or elastic fibers and contains few cells. Since the fibers are tightly packed and arranged parallel to one another, this tissue is classified as dense regular connective tissue. Ligaments and tendons that connect bone to bone or muscle to bone, respectively, are resistant to stretching and are made of dense regular connective tissue.
Muscle Tissue
Muscle not only moves the body, it moves materials through the body. All muscles have one job: to contract. Muscle contraction can only happen with the sliding action of two proteins, actin and myosin. The overlapping actin and myosin molecules slide toward each other, pulling each end of the cell and shortening the muscle.
Skeletal Muscle
Skeletal muscles are muscles attached to bones. This arrangement allows the muscles, which are the engines, to move the bones, which are the levers upon which actions can occur and work can be done.
During embryonic development, individual muscle cells fuse together to create long tubes of muscle cells, which contain many nuclei. These are called skeletal muscle fibers. All of the nuclei are pressed to the periphery of the cell membrane because the middle of each muscle fiber is filled with long columns of overlapping actin and myosin molecules. The repeating and overlapping nature of the actin and myosin give skeletal muscle cells a striated appearance.
Anatomy of a Word
sarcomere
The repeating units of actin and myosin, which are arranged in series like the links in a chain, are called sarcomeres. To contract, each sarcomere shortens a small amount. When added together, all sarcomeres shortening at the same time results in the entire muscle organ shortening by as much as a few centimeters.
Skeletal muscle is the only type of muscle in the human body under voluntary control. Picking up a glass or running a race could not occur without your conscious control.
Cardiac Muscle
Like skeletal muscle cells, cardiac muscle also contains overlapping actin and myosin arranged into sarcomeres, yielding a striated appearance. However, cardiac muscle is under subconscious control and is considered involuntary. While you can increase or decrease your heart rate by your level of activity (running can increase the rate, while lying down will decrease the rate), heart rate cannot be voluntarily controlled by thought itself.
Cardiac cells are branched at many different points, unlike linear tubes of skeletal muscle. These branch points link cardiac muscle cells together in interwoven layers (laminae). Laminae allow the 3-dimensional contraction of the heart (rather than the linear contraction of skeletal muscle).
Cardiac muscle features attachment points between cells, called intercalated disks. Intercalated disks allow the muscle cells to hold on to each other tightly while contracting. These disks also contain membrane tunnels called gap junctions, which allow the cytoplasm of one cell to flow unimpeded into the adjacent cell. Thus, muscle cells joined via gap junctions contract at the same time. The heart functions as a single unit although composed of many parts.
Smooth Muscle
Smooth muscle lacks the striated pattern of skeletal and cardiac muscle. This does not mean smooth muscle lacks actin and myosin. Instead, it lacks the sarcomeric arrangement of the contractile proteins. In smooth muscle, the overlapping actin and myosin are attached to points on the plasma membrane called dense bodies, which are scattered all over the surface of the cell. This 3-dimensional pattern causes the cell to collapse upon itself when contracted.
Like cardiac muscle, smooth muscle is involuntary and may be joined together with gap junctions to form belts or bands of smooth muscle tissue. These are found in surrounding hollow organs, such as the digestive tract, urinary tract, and blood vessels, and assist in the movement of materials such as urine. For example, the pyloric sphincter is a belt of muscle between the stomach and small intestine that regulates when and if material passes from the stomach to the intestines. Contraction of this smooth muscle belt causes a narrowing and even closing of the passageway and a relaxation leads to an open passage.