The first type of tissue that we should be aware of is the epithelial tissue. The epithelial cells are found throughout the body and are typically found as a tissue that lines the body. In their function as a barrier tissue, these cells will be attached to these tissues by a layer of connective tissue described as the basal membrane. The description of epithelium is based on the shape of the cell or based on the number of layers of cells contained in the tissue, figure 9.
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The identification that is based on layers of epithelium is given by the number of layers of cells between the basal membrane of the tissue and the outermost cells. It is generally divided into either being a single layer of cells or appearing to have more than a single layer of cells. In which we discuss these cells as being: simple (only 1 cell layer of epithelium),
stratified (more than 1 cell layer of epithelium), or pseudostratified (appears to have multiple layers of cells, but all cells contact the basal membrane). The layer identification is then combined with the general shape of the epithelial cell to give the complete identification. The shapes are described as: squamous (flat elongated cells associated with lining of tissues and organs, primary cell of the epidermis), cuboidal (cube shaped cells that are associated with absorbing materials but may also be involved with secretory functions of glands), or columnar (column shaped cells that are associated with secreting and absorbing materials from the extracellular spaces). Associated with the epithelial cells is identification of the cell based on the presence or absence of ciliated projections. Ciliated epithelia are typically columnar (or cuboidal) that use the cilia to establish a “brush border” within the tissue. These brush borders are used to either increase total surface area for interaction between substances with the epithelial cells or assist with the movement of materials along surface of tissue.
Within the columnar cell types there is a specialized label given based on “layers” of cells. The pseudostratified subtypes that appear to have more than one cell layer, but in actuality all cells make contact with the basal membrane. There are also some specialized columnar cells. One are secretory cells that are identified as “goblet” and the prototypical cell used for most tissues involved with secretions within lumens of the body. The other developmentally will specialize and derive into the neurons in our body.
Within the types of epithelium are specialized secretory cells (cells that secrete materials into the extracellular (extrastitial) fluids. These “glandular tissues” are described using 3 general classifications. There are merocrine glands, which release only secretions from the cell into ducts or onto tissues. There are apocrine glands that release small parts of the cell that are “squeezed off” from the cell into the ducts for secretion. Lastly there are holocrine glands that release entire cells into the ducts for secretion. Unlike the other glands, holocrine glandular tissues undergo high levels of cellular division prior to secretion occurring. These glands will comprise three distinct types of epithelial cellular arrangements. One type is referred to as goblet cells that are single secretory cells associated with ciliated cells (cells with cilia projections to move secretions along the outer surface of the epithelial cells). This type of a secreting gland is found along the lumens (open tubes) of the body where materials need to be moved along the surface of the lumen, ingested into the body, or removed from the body. Another type are the multiple cell glands that are associated with the secretory tissues of the body and involved with major excretions of fluids and materials from the body onto the surface of the body, into a lumen for transport to another tissue in the body. The last type is the serosa gland, where epithelial cells secrete a mucous coating that allows for a decrease in the friction between two interacting surfaces of tissues within the body.
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The glandular tissue (not to be confused with the glands of the body) forms through invagination of epithelial cells within the underlying connective tissue layers primarily during the fetal developmental stages. The process forms eight (8) general shapes that the glandular tissue can assume. From which there are two (2) general categories that we classify these 8 shapes, either simple or compound. The indication of simple and compound describes the complexity of the collection of secretory cells contained within the gland, figure 11. The general classifications are: Simple Tubular, Simple Coiled Tubular, Simple Branched Tubular, Simple Alveolar, Simple Branched Alveolar, Compound Tubular, Compound Alveolar and Compound Tubuloalveolar. Every gland that is used will be one of these general shapes, regardless of the location or the function of the gland.
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