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Chapter 4
The Study of Tissues: Histology
In This Chapter
Checking out the skin
Keeping things together with connective tissues
Flexing muscle tissues
Sending signals through nerve tissue
O
h, what tangled webs we weave! As the chapter title says, histology is the study of tis- sues, but you may be surprised to find out that the Greek histo doesn’t translate as “tissue” but instead as “web.” It’s a logical next step after reviewing the cell and cellular division to take a look at what happens when groups of similar cells “web” together to form tissues. The four different types of tissue in the body are as follows:
Epithelial, or skin, tissue (from the Greek epi– for “over” or “outer”)
Connective tissue Muscle tissue
Nerve tissue
In this chapter, you find a quick review of the basics of each of these types of tissues along with practice questions to test your knowledge of them.
Getting Under Your Skin
Perhaps because of its unique job of both protecting the outer body and lining internal organs, epithelial tissue comes in more varieties than any other tissue.
Epithelial tissues, which generally are arranged in sheets or tubes of tightly-packed cells, always have a free, or apical, surface that can be exposed to the air or to fluid. That free sur- face also can be covered by additional layers of epithelial tissue. But whether it’s layered or not, each epithelial cell has polarity (a top and a bottom), and all but one side of the cell is tucked snugly against neighboring cells. The apical side sometimes has cytoplasmic projec- tions such as cilia, hair-like growths that can move material over the cell’s surface, or microvilli, finger-like projections that increase the cell’s surface area for absorption. Opposite the apical side is the basal side (think basement), which typically attaches to some kind of connective tissue.
Epithelial tissue serves several key functions, including the following:
Protection: Skin protects vulnerable structures or tissues deeper in the body.
Barrier: Epithelial tissues prevent foreign materials from getting inside the body.
Sensation: Sensory nerve endings embedded in epithelial tissue connect the body with outside stimuli. Secretion: Epithelial tissue in glands can be specialized to secrete enzymes, hor- mones, and fluids.
Single-layer epithelial tissue is classified as simple. Tissue with more than one layer is called stratified. Epithelial tissues also can be classified according to shape: Squamous is a thin, flat cell; cuboidal is, as the name implies, equal in height and width and shaped like a cube; and columnar cells are taller than they are wide.
Following are the ten primary types of epithelial tissues:
Simple squamous epithelium: Looking a bit like rolling tundra, this flat layer of scale-like cells is useful in diffusion, secretion, or absorption. Each cell nucleus is centrally located and is round or oval. Simple squamous epithelium lines the lungs’ air sacs where oxygen and carbon dioxide are exchanged; forms blood fil- ters inside the kidneys; and lines the inner surface of the eardrum, known as the tympanic membrane.
Simple cuboidal epithelium: These cube-shaped cells, found in a single layer that looks like a microscopic mattress, have centrally located nuclei that usually are round. Found in the ovaries, kidneys, and some glands, this type of epithe- lium functions in secretion, absorption, and tube formation. Simple columnar epithelium: These densely packed cells are taller than they are wide, with nuclei located near the base of each cell. Found lining the digestive tract from the stomach to the anal canal, this type of epithelium functions in secretion and absorption.
Simple columnar ciliated epithelium: A close cousin to simple columnar epithe- lium, this type of tissue has hair-like cilia that can move mucus and other sub- stances across the cell. It’s found lining the small respiratory tubes. Pseudostratified columnar epithelium: Pay attention to the prefix pseudo – here, which means “false.” It may look multilayered because the cells’ nuclei are scat- tered at different levels, but it’s not. This type of epithelium is found in the sali- vary glands and some segments of the male reproductive system, including the urethra.
Pseudostratified columnar ciliated epithelium: Another variation on a theme, this tissue is nearly identical to pseudostratified columnar epithelium. The differ- ence is that this tissue’s free surface has cilia, making it ideal for lining air pas- sages because the cilia’s uniform waving action causes a thin layer of mucus to move in one direction — toward the throat and mouth — and trap dust particles. Stratified squamous epithelium: This tissue is the stuff you see everyday — your outer skin, or epidermis. This multilayered tissue has squamous cells on the outside plus deeper layers of cuboidal or columnar cells. Found in areas where the outer cell layer is constantly worn away, this type of epithelium regen- erates its surface layer with cells from lower layers.
Stratified cuboidal epithelium: This multilayered epithelium can be found in sweat glands, conjunctiva of the eye, and the male urethra. Its function is prima- rily protection. Stratified columnar epithelium: Also multilayered, this epithelium is found lining parts of the male urethra, excretory ducts of glands, and some small areas of the anal mucus membrane.
Stratified transitional epithelium: This epithelium is referred to as transitional because its cells can shape-shift from cubes to squamous-like flat surfaces and back again. Found lining the bladder, the cells flatten out to make room for urine.
48 Part I: Building Blocks of the Body
- – 19. Use the terms that follow to identify the epithelial tissues shown in Figure 4-1.
Illustration by Imagineering Media Services Inc.
a. Stratified squamous b. Simple columnar
c. Squamous d. Transitional stretched
e. Simple squamous f. Columnar
g. Pseudostratified h. Cuboidal
i. Transitional relaxed j. Simple cuboidal
Making a Connection: Connective Tissue
Connective tissues connect, support, and bind body structures together. Unlike other types of tissues, connective tissues are classified more by the stuff in which the cells lay — the extracellular matrix — than by the cells themselves. The cells that produce that matrix are scattered within it like chocolate chips in ice cream. The load-bearing
CELL SHAPES SIMPLE STRATIFIED
Figure 4-1: Epithelial tissues.
50 Part I: Building Blocks of the Body
strength of connective tissue comes from a fibrous protein called collagen. All connec- tive tissues contain a varying mix of collagen, elastic, and reticular fibers.
Following are the primary types of connective tissue:
Areolar, or loose, tissue: This tissue exists between and around almost every- thing in the body to bind structures together and fill space. It’s made up of wavy ribbons called collagenous protein fibers, cylindrical threads called elastic fibers, and amorphous ground substance, a semisolid gel. Various cells including lympho- cytes, fibroblasts, fat cells, and mast cells are scattered throughout the ground substance (see Figure 4-2). Dense regular connective tissue: Made up of parallel, densely packed bands or sheets of fibers (see Figure 4-2), this type of tissue is found in tendons as bundles of collagenous fibers attaching muscles to bone and in ligaments as bundles of elastic fibers extending from bone to bone, surrounding a joint, and anchoring organs. It usually resists force in just two directions.
Illustration by Imagineering Media Services Inc.
Dense irregular connective tissue: Also known as dense fibrous connective tissue, it consists of fibers that twist and weave around each other, forming a thick tissue that can withstand stresses applied from any direction. This tissue makes up the strong inner skin layer called the dermis as well as the outer cap- sule of organs like the kidney and the spleen. Adipose tissue: Composed of fat cells, this tissue forms padding around internal organs, reduces heat loss through the skin, and stores energy in fat molecules called triglycerides. Fat molecules fill the cells, forcing the nuclei against the cell membranes and giving them a ring-like shape. Adipose has an intracellular matrix rather than an extracellular matrix.
Reticular tissue: Literally translated as “web-like” or “net-like,” reticular tissue is made up of slender, branching reticular fibers with reticular cells overlaying them. Its intricate structure makes it a particularly good filter, which explains why it’s found inside the spleen, lymph nodes, and bone marrow.
Mast cell
Fibroblast
Fibers of Matrix Collagen fibers Nuclei of fibroblasts
Figure 4-2: Areolar tissue and dense regular connective tissue.
Chapter 4: The Study of Tissues: Histology 51
- The tissue covering the surface of articulating bones is a. Hyaline cartilage
b. Areolar c. Vascular tissue
d. Fibrocartilage
- Vascular connective tissue is a. Hyaline cartilage
b. Elastic tissue c. Blood
d. Bone
- Tissue containing lacunae with osteocytes is a. Elastic cartilage
b. Bone c. Hyaline cartilage
d. Blood
- Blood contains cells functional in clotting called a. Phagocytes
b. Erythrocytes c. Leukocytes
d. Thrombocytes
Flexing It: Muscle Tissue
Although we review how muscles work in Chapter 6, in histology you should know that muscle tissue is made up of fibers known as myocytes. The cytoplasm within the fibers is called sarcoplasm, and within that sarcoplasm are minute myofibrils that contain the protein filaments actin and myosin. These filaments slide past each other during a muscle contraction, shortening the fiber.
Following are the three types of muscle tissue (see Figure 4-3):
Smooth muscle tissue: This type of tissue contracts without conscious control. Made up of spindle-shaped fibers with large, centrally located nuclei, it’s found in the walls of internal organs, or viscera. Smooth muscle gets its name from the fact that, unlike other muscle tissue types, it is not striated. Cardiac muscle tissue: Also known as myocardium, cardiac muscle tissue is made of branching fibers, each with a central nucleus and alternating light and dark striations. Between the fibers are dark structures called intercalated discs. As with smooth muscle, cardiac muscle tissue contractions occur through the autonomic nervous system (involuntary control).
Skeletal, or striated, muscle tissue: Biceps, triceps, pecs — these are the mus- cles that bodybuilders focus on. As the name implies, skeletal muscles attach to the skeleton and are used throughout the central nervous system for movement. Muscle fibers are cylindrical with several nuclei in each cell (which makes them multinucleated ) and cross-striations throughout.
Chapter 4: The Study of Tissues: Histology 53
Illustration by Imagineering Media Services Inc.
- Which type of tissue is multinucleated?
a. Skeletal muscle tissue b. Cardiac muscle tissue
c. Smooth muscle tissue
- A tissue that has intercalated discs is a. Cardiac muscle
b. Skeletal muscle c. Smooth muscle
d. Striated muscle
- Skeletal muscle tissue has prominent lines across the fiber called a. Fibroblasts
b. Multinucleation c. Lacunae
d. Striations
- Smooth muscle tissue is found in the a. Heart
b. Urinary bladder c. Bicep
d. Deltoid
Getting the Signal Across: Nerve Tissue
There’s only one type of nerve tissue and only one primary type of cell in it: the neuron. Nerve tissue is unique in that it can both generate and conduct electrical sig- nals in the body. That process starts when sense receptors receive a stimulus that causes electrical impulses to be sent through finger-like cytoplasmic projections called
Nucleus
Smooth muscle cell
Nuclei
Intercalated disc
Nuclei
Muscle fiber
Figure 4-3: Muscle tissues: Smooth, cardiac, and skeletal.
54 Part I: Building Blocks of the Body
