Interpreting Tissue Sections

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 very thin slices called histological sections. These sections are typically only one or two cells thick, to allow the light of a microscope to pass through and to reduce the confusion of the image that would result from many layers of overlapping cells. They are mounted on slides and arti-

2inter = between + stit = to stand

4endo = inner

5meso = middle

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Chapter 5 Histology 159

Table 5.1 The Four Primary Tissue Classes

Type

Definition

Representative Locations

Four Primary Tissue ClassesBoiled Egg Cut Section Histology

Figure 5.1 Three-Dimensional Interpretation of Two-Dimensional Images. (a) A boiled egg. Note that grazing sections (far left and right) would miss the yolk, just as a tissue section may miss a nucleus or other structure. (b) 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. (c) A coiled gland in three dimensions and as it would look in a vertical tissue section.

Figure 5.1 Three-Dimensional Interpretation of Two-Dimensional Images. (a) A boiled egg. Note that grazing sections (far left and right) would miss the yolk, just as a tissue section may miss a nucleus or other structure. (b) 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. (c) A coiled gland in three dimensions and as it would look in a vertical tissue section.

ficially colored with histological stains to bring out detail. If they were not stained, most tissues would appear very pale 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 two-dimensional slice. 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 in figure 5.1, 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 (fig. 5.1c), is often broken up into multiple portions since it meanders in and out of the plane of section. An experienced viewer, however, would recognize 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

Saladin: Anatomy & I 5. Histology I Text I I © The McGraw-Hill

Physiology: The Unity of Companies, 2003 Form and Function, Third Edition

160 Part One Organization of the Body

160 Part One Organization of the Body

Oblique Section Anatomy

Figure 5.2 Longitudinal, Cross, and Oblique Sections. Note the effect of the plane of section on the two-dimensional appearance of elongated structures such as bones and blood vessels. 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?

Figure 5.2 Longitudinal, Cross, and Oblique Sections. Note the effect of the plane of section on the two-dimensional appearance of elongated structures such as bones and blood vessels. 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?

egg might miss the yolk, just as a tissue section might miss the nucleus of a cell or an egg in the ovary, even though these structures were present.

Many anatomical structures are significantly longer in one direction than another—the humerus and esophagus, for example. A tissue cut in the long direction is called a longitudinal section (l.s.), and one cut perpendicular to this is a cross section (c.s. or x.s.), or transverse section (t.s.). A section cut at an angle between a longitudinal and cross section is an oblique section. Figure 5.2 shows how certain organs look when sectioned on each of these planes.

Before You Go On

Answer the following questions to test your understanding of the preceding section:

1. Classify each of the following into one of the four primary tissue classes: the skin surface, fat, the spinal cord, most heart tissue, bones, tendons, blood, and the inner lining of the stomach.

2. What are tissues composed of in addition to cells?

3. What embryonic germ layer gives rise to nervous tissue? To the liver? To muscle?

4. What is the term for a thin, stained slice of tissue mounted on a microscope slide?

Epithelial Tissue

Objectives

When you have completed this section, you should be able to ' describe the properties that distinguish epithelium from other tissue classes; ' list and classify eight types of epithelium, distinguish them from each other, and state where each type can be found in the body;

' explain how the structural differences between epithelia relate to their functional differences; and ' visually recognize each epithelial type from specimens or photographs.

Epithelial6 tissue consists of a flat sheet of closely adhering cells, one or more cells thick, with the upper surface usually exposed to the environment or to an internal space in the body. Epithelium covers the body surface, lines body cavities, forms the external and internal linings of many organs, and constitutes most gland tissue. The extracellular material is so thin it is not visible with the light microscope, and epithelia allow no room for blood vessels. They do, however, almost always lie on a layer of loose connective tissue and depend on its blood vessels for nourishment and waste removal.

Between an epithelium and the underlying connective tissue is a layer called the basement membrane, usually too thin to be visible with the light microscope. It contains collagen, adhesive glycoproteins called laminin and fibronectin, and a large protein-carbohydrate complex called heparan sulfate. It gradually blends with collage-nous and reticular fibers on the connective tissue side. The basement membrane serves to anchor an epithelium to the connective tissue below it. The surface of an epithelial cell that faces the basement membrane is its basal surface, and the one that faces away from the basement membrane is the apical surface.

Epithelia are classified into two broad categories— simple and stratified—with four types in each category. In a simple epithelium, every cell touches the basement membrane, whereas in a stratified epithelium, some cells rest on top of other cells and do not contact the basement membrane (fig. 5.3).

Saladin: Anatomy & I 5. Histology I Text I I © The McGraw-Hill

Physiology: The Unity of Companies, 2003 Form and Function, Third Edition

Simple epithelium

Simple epithelium membrane membrane

Stratified epithelium

Stratified epithelium

Figure 5.3 Comparison of Simple and Stratified Epithelia.

Figure 5.3 Comparison of Simple and Stratified Epithelia.

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Responses

  • stephanie
    What is the term for a thin, stained slice of tissue mounted on a microscope slide?
    8 years ago
  • Eustorgio
    Which of the following terms describes a thin slice of tissue mounted on a microscope slide?
    6 years ago

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