Introduction:    A tissue is a grouping of similar cells with a common purpose or function.  All cells have a variety of organelles that perform functions for the cell and these are often increased or decreased in proportion to the function of the tissue.  This enhancement of organelles that accompany specialization is in keeping with the structure-function relationship that underpins the understanding of tissue activity.  However, cooperating colonies of cells also brings a level of interaction and possibilities that can not be done at the cellular level.  For example, a single cell cannot form a membrane simply because of physical limitations of size.  Thus, membranes become a tissue function rather than a cellular one.  By extension, cell-cell junctions where margins contact each other become important in forming the continuity of this tissue.  Also, material placed between cells and tieing each to the next becomes important as well.  As you can see, when looking at tissues of any sort, one must consider more than the cellular elements alone.  The result is greater than would be predicted from the sum of the contributing cellular parts!  This is an important principle and should be considered in understanding the contribution of inter-relationships of levels of body organization.  After all, injuries increase with age partly because cell function is not what it once was, but also because tissues are not functioning at their optimum either.

    An interesting barrier is placed before the student trying to learn tissue basics.  Four major groupings of tissues are recognized based on their function, but identification is by structure.  With time, this makes sense, but it does make learning somewhat confusing to the student.  The four major tissue types are listed below:

Mitosis Stages - Tissues are multiple cells.  Thus, these, and all cells, must have arisen from the original zygote cell through the process of mitosis.  Although the embryonic tissues are quite different from those we recognize in this section, the process continues throughout life to produce repair and replacement cells.  Mistakes made during mitosis, or unrepaired mutations or damage to genetic material, will be faithfully duplicated for the remainder our lives and live on as sub-functioning tissue from then on.  While this is not intended to be an example of histological structure-function, it is useful to include at this point.

Epithelium- Surface tissues that form barriers to control exchange.  One side is not attached; the other side is buried in connective tissue binding it to the tissues beneath.  Surfaces may be external such as the skin or lining of the mouth, or internal, such as the lining of true body cavities or the linings of blood vessels.

Connective Tissue - Tissues which bind the body together, support it, allow for movement, distribute nutrients and conduct wastes, defend the body, store nutrients, insulate and protect the body, etc.  A very diverse grouping of functions.  If you were filling out a form, it would say "other _______ ".

Muscle- Tissue with the ability to develop force through shortening.  Interestingly, this tissue can also sense some forms of stimuli  and communicate through its mass much as nervous tissue can.

Nervous - Neurons and supportive cells that permit sensing, integration (sorting of information), and communication.