Epithelial tissue is made of closely-packed cells arranged in flat sheets. Epithelia form the surface of the skin, line the various cavities and tubes of the body, and cover the internal organs. Subsets of Epithelia
Epithelia that form the interface between the internal and external environments. Skin as well as the lining of the mouth and nasal cavity. These are derived from ectoderm. Inner lining of the GI tract, lungs, urinary bladder, exocrine glands, vagina and more. These are derived from endoderm. The apical surface of these epithelial cells is exposed to the "external environment", the lumen of the organ or the air. [View example]
Mesothelia. These are derived from mesoderm.
pleura — the outer covering of the lungs and the inner lining of the thoracic (chest) cavity. peritoneum — the outer covering of all the abdominal organs and the inner lining of the abdominal cavity. pericardium — the outer lining of the heart.
Endothelia. The inner lining of the heart, all blood and lymphatic vessels — derived from mesoderm. The basolateral surface of all epithelia is exposed to the internal environment (ECF). The entire sheet of epithelial cells is attached to a layer of extracellular matrix that is called the basement membrane or, better (because it is not a membrane in the biological sense), the basal lamina. [View example] View showing relationship between the apical and basolateral surfaces of epithelial cells and how they maintain their distinction. The function of epithelia always reflects the fact that they are boundaries between masses of cells and a cavity or space. Some examples:
The epithelium of the skin protects the underlying tissues from mechanical damage
invasion by bacteria
The columnar epithelium of the intestine
secretes digestive enzymes into the intestine;
absorbs the products of digestion from it.
An epithelium also lines our air passages and the alveoli of the lungs. It secretes mucus which keeps it from drying out and traps inhaled dust particles. Most of its cells have cilia on their apical surface that propel the mucus with its load of foreign matter back up to the throat. 2. Muscle
Three kinds of muscle are found in vertebrates:
Skeletal muscle is made of long fibers whose contraction provides the force of locomotion and other voluntary body movements. Smooth muscle lines the walls of the hollow structures of the body, such as the intestine, urinary bladder, uterus, and blood vessels. Its contraction, which is involuntary, reduces the size of these hollow organs. The heart is made of cardiac muscle.
Link to page devoted to the structure and properties of the three kinds of muscles. 3. Connective
The cells of connective tissue are embedded in a great amount of extracellular material. This matrix is secreted by the cells. It consists of protein fibers embedded in an amorphous mixture of protein-polysaccharide ("proteoglycan") molecules.
Supporting connective tissue
Gives strength, support, and protection to the soft parts of the body. cartilage. Example: the outer ear
bone. The matrix of bone contains collagen fibers and mineral deposits. The most abundant mineral is calcium phosphate, although magnesium, carbonate, and fluoride ions are also present. [More on bone]
Dense connective tissue
Often called fibrous connective tissue.
Tendons connect muscle to bone. [View] The matrix is principally Type I collagen, and the fibers are all oriented parallel to each other. Tendons are strong but not elastic. Ligaments attach one bone to another. They contain both collagen and also the protein elastin. Elastin permits ligaments to be stretched.
Loose connective tissue
It is distributed throughout the body. It serves as a packing and binding material for most of our organs. Sheets of loose connective tissue that bind muscles and other structures together are called fascia....