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Classification of Tissues - Histology
Histology is the study of tissues. Tissues are groups of cells that are similar in structure and function. There are four tissue types associated with animals such as humans. They are:
Epithelial tissue,
Connective tissue (CT),
Muscle tissue, and
Nerve tissue
Each of these tissue categories has distinctive cell shapes and functions.
(I) EPITHELIAL TISSUE
Epithelial tissues are avascular and they are supported by a structure known as the basement membrane. The basement membrane is amorphous, and consists of a combination of secretions from epithelial cells as well as CT cells. The portion of the membrane that is composed of epithelial secretions is termed the basal lamina. The portion of the membrane composed of CT secretions is called the reticular lamina. Epithelial tissues have multiple functions, including covering and lining, secretion, absorption, protection, filtration, excretion, and sensory reception.
Epithelial cells can also form glands. There are two types of glands in the human body: endocrine and exocrine.
Endocrine glands are "ductless glands". In other words, they lack ducts for passage of materials from their internal structure to the epithelial surface. The endocrine glands of the human produce and release regulatory chemicals called hormones. Exocrine glands do have ducts, and they release their products directly to the epithelial surface. Glands such as sweat and oil glands are examples of exocrine glands.
We classify the covering and lining epithelia based upon (1) cell shape and (2) the number of layers. There are three basic epithelial shapes, plus several specializations. The three basic shapes of epithelial cells are squamous, cuboidal, and columnar. All three types of cells look the same in whole mount: six-sided. In order to distinguish among the three cell types you must view them in cross-section or longitudinal section. Epithelial cells may form single layers (simple epithelium), multiple layers (stratified epithelium), or appear to be multiple layers when, in fact, they are only a single layer (pseudostratified epithelium).
There are two important types of epithelia that are less easily categorized. They are (1) pseudostratified and (2) transitional. Pseudostratified is a special form of columnar epithelium in which the cells vary in height as well as the placement of their nuclei. Because their heights are asymmetrical and their nuclei are located at various levels, they appear to be "stacked". In fact, each cell in the so-called "stack" does touch the basement membrane. Therefore, there is no stacking, just an irregular single (simple epithelium) layer of cells. Pseudostratified epithelium tends to be ciliated.
Transitional epithelium consists of modified stratified squamous epithelium cells that are distensible and can slide over one another. These cells can stretch (flatten out) to accommodate a change in volume, and then return to their original (plump) shape. They are located in the kidneys, ureters, and the urethra.
(II) CONNECTIVE TISSUE (CT)
CT is the most abundant and variable of the human tissues. It is designed for supporting, protecting, and binding the tissues of the body. Most CT's are well vascularized (except for tendons, ligaments, and cartilage). Connective tissues all consist of three elements: (1) CT cells, (2) matrix produced by the CT cells, and (3) fibers produced by the CT cells. There are many types of cells associated with CT, but the generic "CT precursor" is the fibroblast.
Matrix is produced by the CT cells, then released from the cells. It is noncellular and nonliving. It provides extracellular support for the CT cells that produced it. There are several types of fibers in CT. The three major types are collagen, elastic, and reticular.
(III) MUSCLE TISSUE
Muscle tissue is responsible for locomotion and for the movements of the various parts of the body with respect to one another. There are three categories of muscle tissue: skeletal, cardiac, and smooth muscle. Another term for skeletal muscle is striated muscle. The term visceral is often used as a synonymon for smooth muscle.
Skeletal muscles are associated primarily with the bones of the body and help provide movement from place to place. Skeletal muscle cells are long and tapered, with many nuclei. They are so long and thin that each cell is referred to as a fiber. Skeletal muscle is voluntary.
Cardiac muscle is associated only with the heart. It forms the bulk of the heart wall. The muscle of the heart wall is referred to as the myocardium of the heart. This tissue has striations like skeletal muscle, but also has a distinct specialization: the presence of clusters of gap junctions and desmosomes referred to as the intercalated discs. Cardiac muscle is "ropy" in appearance, whereas skeletal muscle is tightly organized. Cardiac muscle is involuntary.
Smooth muscle is located in the walls of blood vessels and internal organs. It is also found in the walls of ducts associated with the digestive, respiratory, reproductive, and urinary systems. Smooth muscle cells are usually long and tapered, therefore they are often said to be "spindle-shaped". These cells have a single nucleus. Smooth muscle is involuntary.
(IV) NERVE TISSUE
Nerve tissue consists of two cell types: neurons and neuroglial cells. Neuroglia are also called "glial" cells. The neurons are the cells that do the job we associate with the nervous system. They are electrically excitable and can receive, process and transmit electrical impulses. Neuroglial cells support, protect, nourish, and defend the neurons of the NS.
Research has recently revealed that glial cells also play an active role in the process of nerve impulse conduction, as well as being critical players in the formation and maintenance of memory sequences. Half the volume of the central nervous system (CNS) consists of glial cells.
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