contrast, a true epithelial tissue is present only in a single layer of cells held together via occluding junctions called tight junctions, to create a selectively permeable barrier. this tissue covers all organismal surfaces that come in contact with the external environment such as the skin, the airways, and the digestive tract. it serves functions of protection, secretion, and absorption, and is separated from other tissues below by a basal lamina. the connective tissue and the muscular are derived from the mesoderm. the nervous tissue is derived from the ectoderm. epithelial tissues main article: epithelium the epithelial tissues are formed by cells that cover the organ surfaces, such as the surface of skin, the airways, surfaces of soft organs, the reproductive tract, and the inner lining of the digestive tract. the cells comprising an epithelial layer are linked via semi-permeable, tight junctions; hence, this tissue provides a barrier between the external environment and the organ it covers. in addition to this protective function, epithelial tissue may also be specialized to function in secretion, excretion and absorption. epithelial tissue helps to protect organs from microorganisms, injury, and fluid loss. functions of epithelial tissue: the principle function of epithelial tissues are covering and lining of free surface the cells of the body's surface form the outer layer of skin. inside the body, epithelial cells form the lining of the mouth and alimentary canal and protect these organs. epithelial tissues help in the elimination of waste. epithelial tissues secrete enzymes and/or hormones in the form of glands. some epithelial tissue perform secretory functions. they secrete a variety of substances including sweat, saliva, mucus, enzymes. there are many kinds of epithelium, and nomenclature is somewhat variable. most classification schemes combine a description of the cell-shape in the upper layer of the epithelium with a word denoting the number of layers: either simple (one layer of cells) or stratified (multiple layers of cells). however, other cellular features such as cilia may also be described in the classification system. some common kinds of epithelium are listed below: simple squamous (pavement) epithelium simple cuboidal epithelium simple columnar epithelium simple ciliated (pseudostratified) columnar epithelium simple glandular columnar epithelium stratified non-keratinized squamous epithelium stratified keratinized epithelium stratified transitional epithelium connective tissue main article: connective tissue connective tissues are made up of cells separated by non-living material, which is called an extracellular matrix. this matrix can be liquid or rigid. for example, blood contains plasma as its matrix and bone's matrix is rigid. connective tissue gives shape to organs and holds them in place. blood, bone, tendon, ligament, adipose, and areolar tissues are examples of connective tissues. one method of classifying connective tissues is to divide them into three types: fibrous connective tissue, skeletal connective tissue, and fluid connective tissue. muscle tissue main article: muscle tissue cross section through skeletal muscle and a small nerve at high magnification (h&e stain) muscle cells (myocytes) form the active contractile tissue of the body. muscle tissue functions to produce force and cause motion, either locomotion or movement within internal organs. muscle is formed of contractile filaments and is separated into three main types; smooth muscle, skeletal muscle and cardiac muscle. smooth muscle has no striations when examined microscopically. it contracts slowly but maintains contractibility over a wide range of stretch lengths. it is found in such organs as sea anemone tentacles and the body wall of sea cucumbers. skeletal muscle contracts rapidly but has a limited range of extension. it is found in the movement of appendages and jaws. obliquely striated muscle is intermediate between the other two. the filaments are staggered and this is the type of muscle found in earthworms that can extend slowly or make rapid contractions. in higher animals striated muscles occur in bundles attached to bone to provide movement and are often arranged in antagonistic sets. smooth muscle is found in the walls of the uterus, bladder, intestines, stomach, oesophagus, respiratory airways, and blood vessels. cardiac muscle is found only in the heart, allowing it to contract and pump blood round the body. nervous tissue main article: nervous tissue cells comprising the central nervous system and peripheral nervous system are classified as nervous (or neural) tissue. in the central nervous system, neural tissues form the brain and spinal cord. in the peripheral nervous system, neural tissues form the cranial nerves and spinal nerves, inclusive of the motor neurons. mineralized tissues mineralized tissues are biological tissues that incorporate minerals into soft matrices. such tissues may be found in both plants and animals, history xavier bichat (1771–1802) xavier bichat introduced word tissue into the study of anatomy by 1801. he was "the first to propose that tissue is a central element in human anatomy, and he considered organs as collections of often disparate tissues, rather than as entities in themselves". although he worked without a microscope, bichat distinguished 21 types of elementary tissues from which the organs of the human body are composed, a number later reduced by other authors.