Human Anatomy and Tissues Help
A tissue is an aggregation of similar cells that perform a specific set of functions. The body is composed of over 25 kinds of tissues, classified as epithelial tissue, connective tissue, muscle tissue, and nervous tissue.
Epithelial tissue covers body and organ surfaces, lines body cavities and lumina, and forms various glands. It functions in protection, absorption, excretion, and secretion. The outer surface of epithelium is exposed either to the outside of the body or to a lumen or cavity within the body. The deep inner surface is bound by a basement membrane. Epithelial tissue is avascular (without blood vessels) and composed of tightly packed cells. Epithelium is classified by
- the number of layers of cells: a single layer is referred to as simple epithelium, multilayered epithelium is called stratified; and
- by the shape of the cells: squamous (flat), cuboidal, or columnar.
The stratified squamous epithelium of the epidermal layer of the skin contains the protein keratin, which functions to waterproof the skin. Transitional epithelium is similar to nonkeratinized stratified squamous epithelium, except that the surface cells of the former are large and round rather than flat, and they may have two nuclei. Transitional epithelium is specialized to permit distension of the ureters and urinary bladder.
During development, certain epithelial cells invade the underlying connective tissue and form specialized secretory accumulations called glands. Exocrine glands retain a connection to the surface in the form of a duct. The three types of exocrine glands are merocrine, apocrine, and holocrine glands. Endocrine glands lack ducts and secrete their products (hormones) directly into the bloodstream.
One of the most important components of connective tissue is the matrix, a bed of secreted organic material of varying composition that binds widely separated cells of a tissue. Connective tissue supports and binds other tissues, stores nutrients, and/or manufactures protective and regulatory materials.
By contracting, muscle tissue moves materials through the body, enables movement of one part of the body with respect to another, and allows locomotion. Muscle cells, also called muscle fibers, are elongated in the direction of contraction, and movement is accomplished through the shortening of the fibers in response to a stimulus. In addition to the contractile properties of muscle, all muscle fibers are irritable, responding to nervous stimuli, extendible and elastic. There are three types of muscle tissue in the body: smooth, cardiac, and skeletal.
Metabolism within cells releases heat as an end product. Muscles account for nearly one-half of the body weight, and even the fibers of resting muscles are in a continuous state of fiber activity (tonus). Thus, muscles are major heat sources. Maintaining a high body temperature is of homeostatic value in providing optimal conditions for metabolism.
Nervous tissue consists of mainly two types of cells: neurons and neuroglia. Neurons, or nerve cells, are highly specialized to conduct impulses, called action potentials. Neuroglia primarily function to support and assist neurons. Neuroglia are about five times as abundant as neurons and they have mitotic capabilities throughout life. Neurons have branched dendrites that extend from the surface of the cell body to provide a large surface area for receiving stimuli and conducting impulses to the cell body. The elongated axon conducts the impulse away from the cell body to another neuron or to an organ that responds to the impulse. There are six types of neuroglia. Four are found in the central nervous system (CNS): astocytes, ependymal cells, oligodentrocytes, and microglia. The remaining two, ganglionic gliocytes (satellite cells) and neurolemmocytes (Schwann cells) are located in the peripheral nervous system (PNS). Neurolemmocytes support the axon by ensheathing it with a lipid-protein substance, myelin. This myelin sheath aids in the conduction of nerve impulses and promotes regeneration of a damaged neuron.
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