Tissues
Tissues are structured groups of cells that are specialized to perform a common function necessary for an organism's survival. Every complex animal or plant (multicellular organism) has tissue that organizes and helps to regulate the organism's responses to the environment. The study of tissues is called histology, a term introduced by C. F. Heusinger (1792-1883) in 1822.
Animal tissues include epithelial (skin and digestive), muscular, nervous, connective, and vascular (blood) types. Though it was obvious to early scientists that organisms consist of many types of body parts, they did not possess the technology (such as microscopes) to examine bodily components closely. More importantly, general anatomy was at first a descriptive discipline, based upon obvious characteristics such as the location of organs; no scientist made an effort to classify components according to function or structure. In the 1700s, the term "tissue," from the French tisser, began to be used in anatomical science in recognition of the fine textures that differentiated various bodily components.
Xavier Bichat (1771-1802) was perhaps the first biologist to fully consider the body of an organism with reference to tissues. Bichat, extending the work of Phillippe Pinel (1745-1826), made careful observations--without a microscope--and found that organs were built up out of different types of simpler structures, and each of these simpler structures could occur in more than one organ. Bichat observed that, because each of these tissues has specific properties, it is also subject to specific diseases. So instead of a descriptive order, he suggested adopting a systematic order for anatomy. Bichat enumerated 21 tissues (or systems) in the human body, based on what he saw with his naked eye during his many postmortem examinations, distinguishing these different tissues by their composition and the arrangement of their fibers.
Histology began to take on its modern form when Theodor Schwann introduced cell theory in 1839. It was then that tissues began to be understood not as the basic building blocks of living things, but as unique systems of cells with distinct embryological origins.
Today doctors are exploring new ways to help patients through tissue transplants. Tissue is obtained from deceased or surgical donors, then processed into a freeze-dried or frozen form. The processed tissue can be used in orthopedic, neurological, plastic, cardiovascular, or oral reconstructive surgery. For example, skin is used as a temporary covering to reduce pain and lower the risk of infection in burn patients, heart valves help children born with heart problems or adults with heart disease, bone is used to repair or replace tissue after a serious injury or bone disease, tendons and ligaments are used to replace or strengthen damaged tissue in injured joints, and corneas help restore sight in people with certain kinds of vision loss.
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