Porth's Essentials of Pathophysiology, 4e

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Cell Structure and Function

C h a p t e r 1

Cell Junctions The junctions between tissue cells are important in govern- ing the shape of the body, transmitting mechanical stresses fromone cell to another, and creating pathways for commu- nication. Cell junctions occur at many points in cell-to-cell contact, but they are particularly plentiful and important in epithelial tissue. These specialized junctions enable cells to form barriers to the movement of water, solutes, and cells from one body compartment to the next. Three basic types of intercellular junctions are observed: tight junctions, adhering junctions, and gap junctions (Fig. 1-20). Tight or occluding junctions (i.e., zonula occludens), which are found in epithelial tissue, seal the surface membranes of adjacent cells together. This type of inter- cellular junction prevents fluids and materials such as macromolecules present in the intestinal contents from entering the intercellular space. Adhering junctions represent sites of strong adhesion between cells. The primary role of adhering junctions may be that of preventing cell separation. Adhering junc- tions are not restricted to epithelial tissue; they provide adherence between adjacent cardiac muscle cells as well. Adhering junctions are found as continuous, beltlike adhesive junctions (i.e., zonula adherens) or scattered, spotlike adhesive junctions, called desmosomes (i.e., macula adherens). A special feature of the adhesion belt junction is that it provides an anchoring site to the cell membrane for actin filaments. In epithelial desmosomes, bundles of keratin intermediate filaments (i.e., tonofila- ments) are anchored to the junction on the cytoplasmic area of the cell membrane. A primary disease of desmo- somes is pemphigus, which is caused by antibody binding to the desmosome proteins and the resulting separation of neighboring cells. Affected persons have skin and mucous membrane blistering. Hemidesmosomes , which resemble a half desmosome, are another type of junction. They are found at the base of epithelial cells and help attach the epithelial cell to the underlying connective tissue. Gap or nexus junctions involve the close adherence of adjoining cell membranes with the formation of channels that connect the cytoplasm of the two cells. Because they are low-resistance channels, gap junctions are impor- tant in cell-to-cell conduction of electrical signals (e.g., between cells in sheets of smooth muscle or between adjacent cardiac muscle cells, where they function as electrical synapses). Gap junctions also enable ions and small molecules to pass directly from one cell to another. Extracellular Matrix Tissues are not made up solely of cells. A large part of their volume is made up of an extracellular matrix. This matrix is composed of a variety of proteins and polysac- charides (i.e., polymers made up of many sugar mono- mers). These proteins and polysaccharides are secreted locally and are organized into a supporting meshwork in close association with the cells that produced them. The amount and composition of the matrix vary with the different tissues and their function. In bone, for example, the matrix is more plentiful than the cells that surround it; in the brain, the cells are much more abun- dant and the matrix is only a minor constituent.

areas where neurogenesis occurs from neural stem cells/ progenitor cells throughout life. Embryonic development of the nervous system and the structure and functions of the nervous system are discussed more fully in Chapter 34. Structurally, nervous tissue consists of two cell types: nerve cells or neurons and supporting cells or neuroglia. Most nerve cells consist of three parts: the soma or cell body, dendrites, and the axon. The cytoplasm-filled den- drites, which are multiple elongated processes, receive and carry stimuli from the environment, sensory epithe- lial cells, and other neurons to the cell. The axon, which is a single cytoplasm-filled process, is specialized for gen- erating and conducting nerve impulses away from the cell body to other nerve cells, muscle cells, and glandular cells. Neurons can be classified as afferent and efferent neurons according to their function. Afferent or sensory neurons carry information toward the central nervous system; they are involved in the reception of sensory information from the external environment and from within the body. Efferent or motor neurons carry infor- mation away from the central nervous system (CNS); they are needed for control of muscle fibers and endo- crine and exocrine glands. Communication between neurons and effector organs, such as muscle cells, occurs at specialized structures called synapses . At the synapse, chemical messengers (i.e., neu- rotransmitters) alter the membrane potential to conduct impulses from one nerve to another or from a neuron to an effector cell. In addition, electrical synapses exist in which nerve cells are linked through gap junctions that permit the passage of ions from one cell to another. Neuroglia ( glia means “glue”) are the cells that sup- port neurons, form myelin, and have trophic and phago- cytic functions. Four types of neuroglia are found in the CNS: astrocytes, oligodendrocytes, microglia, and epen- dymal cells. Astrocytes are the most abundant of the neuroglia. They have many long processes that surround blood vessels in the CNS. They provide structural sup- port for the neurons, and their extensions form a sealed barrier that protects the CNS. In last decade, investiga- tions have established important roles for astrocytes in the response of the brain to injury, calcium ion-based excitation, CNS metabolism, neural stem cell source, blood–brain barrier maintenance, and numerous dis- eases and pathological conditions. The oligodendrocytes provide myelination of neuronal processes in the CNS. The microglia are phagocytic cells that represent the mononuclear phagocytic system in the nervous system. Ependymal cells line the cavities of the brain and spinal cord and are in contact with the cerebrospinal fluid. In the peripheral nervous system, supporting cells consist of the Schwann and satellite cells. The Schwann cells provide myelination of the axons and dendrites, and the satellite cells enclose and protect the dorsal root ganglia and autonomic ganglion cells. ExtracellularTissue Components The discussion thus far has focused on the cellular com- ponents of the different tissue types. Within tissues, cells are held together by cell junctions, and adhesion mol- ecules form intercellular contacts.