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With its 50 trillion cells and thousands of organs, the human body may seem to be a structure of forbidding complexity. Fortunately for our health, longevity, and self-understanding, biologists of the past weren’t discouraged by this, but discovered patterns that made it more understandable. One such pattern is the fact that these trillions of cells belong to only 200 types or so, and they are organized into tissues that fall into just four primary categories—epithelial, connective, nervous, and muscular tissue—although there are at least 23 subtypes of these four. Organs derive their function not from their cells alone but from how the cells are organized into tissues. Cells are specialized for certain tasks: muscle contraction, defense, enzyme secretion, and so forth. No one cell type can carry out all of the body’s vital functions. Cells therefore work together at certain tasks and form tissues that carry out a particular function, such as nerve signaling or nutrient digestion. An organ is a structure with discrete boundaries that is composed of two or more tissue types. The study of tissues and how they are arranged into organs is called histology,1 or microscopic anatomy—the subject of this chapter. Here we study the four tissue classes; the variations within each class; how to recognize tissue types microscopically and relate their microscopic anatomy to their function; how tissues are arranged to form an organ; how tissues change as they grow, shrink, or change from one tissue type to another over the life of the individual; and modes of tissue degeneration and death. Histology bridges the gap between the cytology of the preceding chapters and the organ system approach of the chapters that follow. A tissue is a group of similar cells and cell products that arise from the same region of the embryo and work together to perform a specific structural or physiological role in an organ. The four primary tissues—epithelial, connective, nervous, and muscular, differ in the types and functions of their cells, the characteristics of the matrix (extracellular material) that surrounds the cells, and the relative amount of space occupied by the cells and matrix. In muscle and epithelium, the cells are so close together that the matrix is scarcely visible, but in most connective tissues, the matrix occupies much more space than the cells do. TABLE 5.1 The Four Primary Tissue Classes Type Definition Representative Locations Epithelial Tissue composed of layers of closely spaced cells that cover organ surfaces, form glands, and serve for protection, secretion, and absorption Epidermis Inner lining of digestive tract Liver and other glands Connective Tissue with usually more matrix than cell volume, often specialized to support and protect organs and to bind other tissues and organs to each other Tendons and ligaments Cartilage and bone Blood Nervous Tissue containing excitable cells specialized for rapid transmission of coded information to other cells. The matrix is composed of fibrous proteins and, usually, a clear gel variously known as ground substance, tissue fluid, extracellular fluid (ECF), or interstitial2 fluid. The ground substance contains water, gases, minerals, nutrients, wastes, hormones, and other chemicals. This is the medium from which all cells obtain their oxygen, nutrients, and other needs, and into which cells release metabolic wastes, hormones, and other products. In cartilage and bone, the matrix is a rubbery or stony material rather than a gel or fluid. In summary, a tissue is composed of cells and matrix, and the matrix is composed of fibers and ground substance. Embryonic Tissues Human development begins with a single cell, the fertilized egg, which soon divides to produce scores of identical, smaller cells. The first tissues appear when these cells start to organize themselves into layers—first two, and soon three strata called the primary germ layers, which give rise to all of the body’s mature tissues. The three primary germ layers are called ectoderm, mesoderm, and endoderm. The ectoderm3 is an outer layer that gives rise to the epidermis and nervous system. The innermost layer, the endoderm,4 gives rise to the mucous membranes of the digestive and respiratory tracts and to the digestive glands, among other things. Between these two is the mesoderm,5 a layer of more loosely organized cells. Mesoderm eventually turns to a gelatinous tissue called mesenchyme, composed of fine, wispy collagen (protein) fibers and branching mesenchymal cells embedded in ground substance. Mesenchyme gives rise to cardiac muscle, bone, and blood, among other tissues. Most organs are composed of tissues derived from two or more primary germ layers. The rest of this chapter concerns the “mature” tissues that exist from infancy through adulthood. Interpreting Tissue Sections In your study of histology, you may be presented with various tissue preparations mounted on microscope slides. Most such preparations are thin slices called histological sections. The best anatomical insight depends on an ability to deduce the three-dimensional structure of an organ from these two-dimensional sections. This ability, in turn, depends on an awareness of how tissues are prepared for study. Three-Dimensional Interpretation of Two-Dimensional Images. A boiled egg. Grazing sections (top left and right) would miss the yolk, just as a tissue section may miss a nucleus or other structure. Elbow macaroni, which resembles many curved ducts and tubules. A section far from the bend would give the impression of two separate tubules; a section near the bend would show two interconnected lumina (cavities); and a section still farther down could miss the lumen completely. A coiled gland in three dimensions and as it would look in a vertical tissue section of a tissue such as the lining of the uterus. Histologists use a variety of techniques for preserving, sectioning (slicing), and staining tissues to show their structural details as clearly as possible. Tissue specimens are preserved in a fixative—a chemical such as formalin that prevents decay. After fixation, most tissues are cut into sections typically only one or two cells thick. Sectioning is necessary to allow the light of a microscope to pass through and so the image is not confused by too many layers of overlapping cells. The sections are then mounted on slides and artificially colored with histological stains to enhance detail. If they’re not stained, most tissue sections appear a hazy gray. With stains that bind to different components of a tissue, however, you may see pink cytoplasm; violet nuclei; and blue, green, or golden-brown protein fibers, depending on the stain used. Sectioning a tissue reduces a three-dimensional structure to a series of two-dimensional slices. You must keep this in mind and try to translate the microscopic image into a mental image of the whole structure. Like the boiled egg and elbow macaroni, an object may look quite different when it is cut at various levels, or planes of section. A coiled tube, such as a gland of the uterus, is often broken up into multiple portions since it meanders in and out of the plane of section. An experienced viewer, however, recognizes that the separated pieces are parts of a single tube winding its way to the organ surface. Note that a grazing slice through a boiled egg might miss the yolk, just as a tissue section might miss the nucleus of a cell even though it was present. Many anatomical structures are longer on one axis than another—the humerus and esophagus, for example. A tissue cut on its long axis is called a longitudinal section (l.s.), and one cut perpendicular to this is a cross section (c.s.). A section cut on a slant between a longitudinal and cross section is an oblique section. shows how certain organs look when sectioned on each of these planes. Three Planes of Section. A bone and blood vessel are used to relate a two-dimensional sectioned appearance to a three-dimensional structure. (a) Longitudinal sections. (b) Cross sections. (c) Oblique sections. ? Would you classify the egg sections in the previous figure as longitudinal, cross, or oblique sections? How would the egg look if sectioned in the other two planes? Not all histological preparations are sections. Liquid tissues such as blood and soft tissues such as spinal cord may be prepared as smears, in which the tissue is rubbed or spread across the slide rather than sliced. Some membranes and cobwebby tissues, like the areolar tissue in figure 5.14, are sometimes mounted as spreads, in which the tissue is laid out on the slide, like placing a small square of tissue paper or a tuft of lint on a sheet of glass. BEFORE YOU GO ON Answer the following questions to test your understanding of the preceding section: . Classify each of the following into one of the four primary tissue classes: the skin surface, fat, the spinal cord, most heart tissue, bone, tendons, blood, and the inner lining of the stomach. . What are tissues composed of in addition to cells? . What embryonic germ layer gives rise to nervous tissue? To the liver? To muscle? . What is the term for a thin, stained slice of tissue mounted on a microscope slide?