Tissue

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.

The study of tissue is known as histology, or, in connection with disease, histopathology.

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.

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.

Plant tissues

Examples of tissue in other multicellular organisms are vascular tissue in plants, such as xylem and phloem.

Plant tissues are categorized broadly into three tissue systems: the epidermis, the ground tissue, and the vascular tissue. Together they are often referred to as biomass.

• Dermal or protective tissue Epidermis - These cells form the outer surface of the leaves and of the young plant body. 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. Epidermis and periderm are the dermal tissues in vascular plants. The epidermis forms the boundary between the plant and the external world. 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. (The periderm replaces the dermal layer and acts as a covering much like the corky bark, it too is made up of mostly dead tissue. The skin on the potato is a periderm.)

• Vascular tissue - The two primary components of vascular tissue are the xylem and phloem. These two types of transport tissues conduct fluid 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. Ground tissue manufactures nutrients by photosynthesis and stores reserve nutrients. The types of ground tissue found in plants develops from ground tissue meristem and consists of three simple tissues:

• Parenchyma (have retained their protoplasm)
• Collenchyma (have retained their protoplasm)
• Sclerenchyma (have lost their protoplasm in mature stage, i.e. are 'dead')

Parenchyma is the most common ground tissue, it forms e.g. 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. Vascular cambium - produces secondary xylem and secondary phloem, this is a process which may continue throughout the life of the plant. This is what gives rise to wood in plants. Such plants are called arborescent. This does not occur in plants which do not go through secondary growth (known as herbaceous plants).

Cork cambium - gives rise to the bark of a tree.

See also

• Cell differentiation

References

ISBN links support NWE through referral fees

• Larkin, M. 1992. Coping with connective tissue diseases. FDA Consumer 26: 28.
• Raven, Peter H., Evert, Ray F., & Eichhorn, Susan E. (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.

Biological tissue

Animals : Epithelium - Connective - Muscular - Nervous

Plants : Dermal - Vascular - Ground - Meristematic

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