ORGANIZATION OF CELLS INTO TISSUES

 

>- Cells with a similar embryonic origin or function are often organized into larger functional units called tissues, and these tissues in turn associate with other, dissimilar tissues to form the various organs of the body.

>- Connective tissue is the most abundant tissue of the body. It is found in a variety of forms, ranging from solid bone to blood cells that circulate in the vascular system.

>- Epithelial tissue forms sheets that cover the body's outer surface, lines internal surfaces, and forms glandular tissue. It is supported by a basement membrane, is avascular, and must receive nourishment from capillaries in supporting connective tissues.

>- Muscle tissue contains actin and myosin filaments that allow it to contract and provide locomotion and movement of skeletal structures (skeletal muscle), pumping of blood through the heart (cardiac muscle), and contraction of blood vessels and visceral organs (smooth muscle).

>- Nervous tissue provides the means for controlling body function and for sensing and moving about in the external environment. It consists of two major types of cells: the neurons, which function in communication, and the neuroglial cells, which support the neurons.

let cells forms the lining of most of the upper respiratory tract. All of the tall cells reaching the surface of this type of epithelium are either ciliated cells or mucus-producing goblet cells. The basal cells that do not reach the surface serve as stem cells for ciliated and goblet cells. Transitional epithelium is a stratified epithelium characterized by cells that can change shape and become thinner when the tissue is stretched. Such tissue can be stretched without pulling the superficial cells apart. Transitional epithelium is well adapted for the lining of organs that are constantly chang­ing their volume, such as the urinary bladder.

Glandular Epithelium. Glandular epithelial tissue is formed by cells specialized to produce a fluid secretion. This process is usually accompanied by the intracellular syn­thesis of macromolecules. The chemical nature of these macromolecules is variable. The macromolecules typically are stored in the cells in small, membrane-bound vesicles called secretory granules. For example, glandular epithelia can synthesize, store, and secrete proteins (e.g., insulin), lipids (e.g., adrenocortical hormones, secretions of the se­baceous glands), and complexes of carbohydrates and pro­teins (e.g., saliva). Less common are secretions such as those produced by the sweat glands, which require mini­mal synthetic activity.

All glandular cells arise from surface epithelia by means of cell proliferation and invasion of the underlying connec­tive tissue, and all release their contents or secretions into the extracellular compartment. Exocrine glands, such as the sweat glands and lactating mammary glands, retain their

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UNIT II Cell Function and Growth

Simple cuboidal

Simple squamous

Simple columnar

Transitional

Pseudostratified columnar ciliated

Stratified squamous

FIGURE4-22 Representation of the various epithelial tissue types.

connection with the surface epithelium from which they originated. This connection takes the form of epithelium-lined tubular ducts through which the secretions pass to reach the surface. Exocrine glands are often classified ac­cording to the way secretory products are released by their cells. In holocrine-type cells (e.g., sebaceous glands), the glandular cell ruptures, releasing its entire contents into the duct system. New generations of cells are replaced by mitosis of basal cells. Merocrine- or eccrine-type glands (e.g., salivary glands, exocrine glands of the pancreas) re­lease their glandular products by exocytosis. In apocrine secretions (e.g., mammary glands, certain sweat glands), the apical portion of the cell, along with small portions of the cytoplasm, is pinched off the glandular cells. Endocrine glands are epithelial structures that have had their con­nection with the surface obliterated during development. These glands are ductless and produce secretions (i.e., hor­mones) that move directly into the bloodstream.