Tissue is a cellular organizational level intermediate between cells and a complete organism. Hence, a tissue is an ensemble of cells, not necessarily identical, but from the same origin, that together carry out a specific function. Organs are then formed by the functional grouping together of multiple tissues.
The study of tissue is known as histology or, in connection with disease, histopathology.
The classical tools for studying tissues are the paraffin block in which tissue is embedded and then sectioned, the histological stain, and the optical microscope. In the last couple of decades, developments in electron microscopy, immunofluorescence, and the use of frozen tissue sections have enhanced the detail that can be observed in tissues. With these tools, the classical appearances of tissues can be examined in health and disease, enabling considerable refinement of clinical diagnosis and prognosis.
Animal tissues
Based on morphology, animal tissues can be grouped into four basic types. Multiple tissue types comprise organs and body structures. While all animal can generally be considered to contain the four tissue types, the manifestation of these tissues can differ depending on the type of organism. For example, the origin of the cells comprising a particular tissue type may differ developmentally for different classifications of animals. The epithelium in all animals is derived from the ectoderm and endoderm with a small contribution from the mesoderm which forms the endothelium. By contrast, a true connective tissue is present only in triploblastic animals; diploblastic animals such as sponges, jellyfish and coral in effect do not have connective tissue but rather an acellular gel known as the mesoglea separates different tissues types in these asnimals. The mesoglea serves the function of a connective tissue but is derived from ectoderm.
Epithelium is 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. Endothelium, which comprises the vasculature, is a specialized type of epithelium.
Connective tissue is comprised of cells separated by non-living material, which is called extracellular matrix. Connective tissue holds other tissues together such as in the formation of organs, and has the ability to stretch and contract passively. Bone and blood are examples of specialized connective tissues.
Muscle cells form the active contractile tissue of the body known as muscle tissue. Muscle tissue functions to produce force and cause motion, either locomotion or movement within internal organs. Muscle tissue 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 providing for gross movement; and cardiac muscle which is found in the heart, allowing it to contract and pump blood throughout an organism.
Cells comprising the central nervous system and peripheral nervous system are classified as neural tissue. In the central nervous system, neural tissue forms the brain, cranial nerves and spinal cord and, in the peripheral nervous system, peripheral nerves inclusive of the motor neurons.
Plant tissues
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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.
See also
References
- Raven, Peter H., Evert, Ray F., & Eichhorn, Susan E. (1986). Biology of Plants (4th ed.). New York: Worth Publishers. ISBN 0-87901-315-X.
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