Biological tissue is an aggregation of interconnected, morphologically, and functionally similar cells, and associated intercellular matter, that together perform one or more specific functions within an organism. Organs are usually composed of several tissues.
Cells work together harmoniously in a tissue to perform a function(s), such as epithelial tissue in the stomach producing the enzyme pepsin to help with digestion, or muscle tissue providing movement. Each cell not only performs actions for its own maintenance, self-preservation, and self-strengthening, but also performs specific actions that contribute to the larger entity, the tissue and the body. The body, on the other hand, supports the individual cell by providing food, oxygen, and other necessary materials, and by transporting away toxic waste materials. Each cell actively depends on the other cells in the body to perform their functions and thus keep the body in proper functioning order. The harmony seen between cells in a tissue reflects upon human society, where ideally individuals would contribute to their families (as cells to tissues), their families to their communities, their communities to their societies, their societies to their nations, and their nations to the world, and in turn each would be benefited by those larger entities.
The classical tools for studying tissues are tissue stains and optical microscopes, though developments in electron microscopy, immunofluorescence (using fluorescent dyes), and frozen sections have all added to the sum of knowledge in the last couple of decades. With these tools, the classical appearances of the tissues can be examined in health and disease, enabling considerable refinement of clinical diagnosis and prognosis.
The study of tissue is known as histology, or, in connection with disease, histopathology.
Despite the fact that each cell in the body has the same DNA, they may have vastly different structures and functions.
Types of animal tissue
There are four basic types of tissue in the body of all animals, including the human body and lower multicellular organisms, such as insects. These compose all the organs, structures, and other contents.
- Epithelium. Epithelium is a tissue that covers organs and surfaces of the bodies of animals, including both outside surfaces (the skin) and inside cavities and lumen (interior of a vessel, such as the small central space in an artery or vein through which blood flows). Epithelial cells are found lining the insides of the lungs, the gastrointestinal tract, the reproductive and urinary tracts, and make up the exocrine and endocrine glands. Epithelial cells are close together, with very little intercellular matter, and generally free of blood vessels. Functions of epithelial cells include protection, secretion, absorption, transcellular transport, sensation detection, and selective permeability.
- Connective tissue. As the name suggests, connective tissue holds everything together. It is largely a category of exclusion rather than one with a precise definition. Areolar (or loose) connective tissue holds organs and epithelia in place; adipose tissue is used for cushioning, thermal insulation, lubrication, and energy storage; dense connective tissue forms ligaments and tendons; and reticular connective tissue forms a soft skeleton to support the lymphoid organs (lymph nodes, bone marrow, and spleen.) Blood, bone, and cartilage are specialized connective tissues. In the circulatory system of humans, cells of connective tissue (blood) are separated by an inorganic material (plasma). Plasma is the extracellular matrix that includes everything, but the red and white blood cells. Blood and lymph also may be placed separately as "vascular tissue."
- Muscle tissue. Muscle cells contain contractile filaments that move past each other and change the size of the cell. Muscle tissue also is separated into three distinct categories: visceral or smooth muscle, which is found in the inner linings of organs; skeletal muscle, which is found attached to bone in order for mobility to take place; and cardiac muscle, which is found in the heart.
- Nervous tissue. Nervous tissue comprises cells forming the brain, spinal cord, and peripheral nervous system. The function of the nervous tissue is communication between parts of the body. It is composed of neurons, which transmit impulses, and the neuroglia, which assist propagation of the nerve impulse as well as provide nutrients to the neuron. All nervous tissue of an organism together makes up its nervous system.
Types of plant tissue
Plant tissues are categorized broadly into four tissue systems: the dermal tissue (or epidermis), ground tissue (or fundamental tissue), vascular tissue, and meristematic tissue.
- Dermal or protective tissue. These form the boundary between the plant and the external world. Epidermis and periderm are dermal tissues in vascular plants. The epidermis is the outer, single-layered group of cells covering a plant, especially the leaf and young tissues of a vascular plant, including stems and roots. The epidermis serves several functions: protection against water loss, regulation of gas exchange, secretion of metabolic compounds, and (especially in roots) absorption of water and mineral nutrients. The epidermis of most leaves shows dorsoventral anatomy: the upper (adaxial) and lower (abaxial) surfaces have somewhat different construction and may serve different functions. Cork is an external, secondary tissue impermeable to water and gases. In smaller stems and on typically non woody plants, sometimes a secondary covering forms called the periderm.
- Vascular tissue. The two primary components of vascular tissue are the xylem and phloem. These two types of transport tissues conduct fluids and nutrients internally in vascular plants (all plants except mosses and their relatives). Xylem is the primary water-conducting tissue and phloem is the living tissue that carries sugar and organic nutrients throughout the plant.
- Ground tissue. Ground tissue, or fundamental tissue, makes up most of the plant body and is less differentiated than other tissues. It develops from ground tissue meristem. Ground tissue manufactures nutrients by photosynthesis and stores reserve nutrients. The types of ground tissue found in plants consists of three simple tissues: parenchyma, collenchyma, and sclerenchyma (have lost their protoplasm in mature stage, i.e. are "dead"). Parenchyma is the most common ground tissue. It forms the cortex and pith of stems, the cortex of roots, the mesophyll (photosynthetic cells), the pulp of fruits, the endosperm of seeds, and the photosynthetic areas of a leaf.
- Meristematic tissue. These cells differentiate into all the other cell types. A meristem is a tissue in plants consisting of undifferentiated cells (meristematic cells) and found in zones of the plant where growth can take place—the roots and shoots. Differentiated plant cells generally cannot divide or produce cells of a different type. Therefore, cell division in the meristem is required to provide new cells for expansion and differentiation of tissues and initiation of new organs, providing the basic structure of the plant body. The most general form of meristem is the apical meristem (also called terminal meristem). These are found in buds at the tips of shoots and at the root tip; they are responsible for shoot and root growth respectively. Procambium develops into primary xylem and primary phloem. Vascular cambium, which produces secondary xylem and secondary phloem, a process that may continue throughout the life of the plant, is what gives rise to wood in plants. Cork cambium gives rise to the bark of a tree.
- Larkin, M. 1992. Coping with connective tissue diseases. FDA Consumer, 26: 28.
- Raven, Peter H., Ray F. Evert, and Susan E. Eichhorn. 1986. Biology of Plants, 4th ed. New York: Worth Publishers. ISBN 0-87901-315-X.
- Ross, M. H., G. I. Kaye, and W. Pawlina. 2003. Histology: A Text and Atlas. Philadelphia, Pa: Lippincott Williams & Wilkins. ISBN 0683302426.
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