Tissue Level of Organization

Transcription

1 4 O Tissue Level of Organization TISSUES U T L I N E 4.1 Epithelial Tissue a Characteristics of Epithelial Tissue b Functions of Epithelial Tissue c Specialized of Epithelial Tissue d Classification of Epithelial Tissue e Types of Epithelium f Glands Connective Tissue a Characteristics of Connective Tissue b Functions of Connective Tissue c Development of Connective Tissue d Classification of Connective Tissue Body Membranes Muscle Tissue a Classification of Muscle Tissue Nervous Tissue a Characteristics of Neurons Tissue Change and Aging a Tissue Change b Tissue Aging 113 MODULE 3: TISSUES

2 Chapter Four Tissue Level of Organization 81 The human body is composed of trillions of cells, which are organized into more complex units called tissues. Tissues are groups of similar cells and extracellular products that carry out a common function, such as providing protection or facilitating body movement. The study of tissues and their relationships within organs is called histology. There are four principal types of tissues in the body: epithelial tissue, connective tissue, muscle tissue, and nervous tissue. Immediately following the connective tissue discussion, section 4.3 (Body Membranes) has been inserted because these structures are composed of an epithelial sheet and an underlying connective tissue layer. Tissues are formed from the three primary germ layers (ectoderm, mesoderm, and endoderm). The four tissue types vary in terms of the structure and function of their specialized cells, as well as the presence of an extracellular matrix (ma triks; matrix = womb) that not only is produced by the cells but surrounds them. The extracellular matrix is composed of varying amounts of water, protein fibers, and dissolved macromolecules. Its consistency ranges from fluid to quite solid. Epithelial, muscle, and nervous tissues have relatively little matrix between their cells. In contrast, connective tissue types contain varying amounts of extracellular matrix that differ in the volume of space occupied, the relative amounts of the extracellular matrix components, and the consistency (fluid to solid) of the extracellular matrix. As we examine each of the four classes of tissues in this chapter, it may help you to refer to table 4.1, which summarizes their characteristics and functions. This chapter is a transition between chapter 2, which investigated the nature of cells, and later chapters, which examine tissue interactions in organs and organ systems. 4.1 Epithelial Tissue Learning Objectives: 1. Identify the structure and function of each type of epithelial tissue. 2. Explain where each type of epithelial tissue is found in the body. 3. Describe the specialized features of an epithelium. 4. Classify exocrine glands. Epithelial (ep-i-the le -a l; epi = upon, the le = nipple) tissue covers or lines every body surface and all body cavities; thus it forms both the external and internal lining of many organs, and it constitutes the majority of glands. An epithelium (pl., epithelia) is composed of one or more layers of closely packed cells between two compartments having different components. There is little to no extracellular matrix between epithelial cells; additionally, no blood vessels penetrate an epithelium. 4.1a Characteristics of Epithelial Tissue All epithelia exhibit several common characteristics: Cellularity. Epithelial tissue is composed almost entirely of cells. The cells of an epithelium are bound closely together by different types of intercellular junctions (discussed later). A minimal amount of extracellular matrix separates the cells in an epithelium. Polarity. Every epithelium has an apical (a p i-ka l) surface (free or top surface), which is exposed either to the external environment or to some internal body space, and lateral surfaces having intercellular junctions. Additionally, each epithelium has a basal (ba śa l) surface (fixed or bottom surface) where the epithelium is attached to the underlying connective tissue. Attachment. At the basal surface of an epithelium, the epithelial layer is bound to a thin basement membrane, a complex molecular structure produced by both the epithelium and the underlying connective tissue. Avascularity. All epithelial tissues lack blood vessels. Epithelial cells obtain nutrients either directly across the apical surface or by diffusion across the basal surface from the underlying connective tissue. Innervation. Epithelia are richly innervated to detect changes in the environment at a particular body or organ surface region. Most nervous tissue is in the underlying connective tissue. High regeneration capacity. Because epithelial cells have an apical surface that is exposed to the environment, they are frequently damaged or lost by abrasion. However, damaged or lost epithelial cells generally are replaced as fast as they Table 4.1 Tissue Types Type General Characteristics General Functions Primary Germ Layer Derivative Epithelial tissue Connective tissue Muscle tissue Nervous tissue Cellular, polar, attached, avascular, innervated, high regeneration capacity Diverse types; all contain cells, protein fibers, and ground substance Contractile; receives stimulation from nervous system and/or endocrine system Neurons: Excitable, high metabolic rate, extreme longevity, nonmitotic Glial cells: Nonexcitable, mitotic Covers surfaces; lines insides of organs and body cavities Protects, binds together, and supports organs Facilitates movement of skeleton or organ walls Neurons: Control activities, process information Glial cells: Support and protect neurons Ectoderm, mesoderm, endoderm Mesoderm Mesoderm Ectoderm Example Subtypes and Their s Simple columnar epithelium: Inner lining of digestive tract Stratified squamous epithelium: Epidermis of skin Transitional epithelium: Inner lining of urinary bladder Adipose connective tissue: Fat Dense regular connective tissue: Ligaments and tendons Dense irregular connective tissue: Dermis of skin Hyaline cartilage: Articular cartilage in some joints Fluid connective tissue: Blood Skeletal muscle: Muscles attached to bones Cardiac muscle: Muscle layer in heart Smooth muscle: Muscle layer in digestive tract Neurons: Brain and spinal cord Glial cells: Brain and spinal cord

3 82 Chapter Four Tissue Level of Organization are lost because epithelia have a high regeneration capacity. The continual replacement occurs through the mitotic divisions of the deepest epithelial cells (called stem cells), which are found within the epithelium near its base. 4.1b Functions of Epithelial Tissue Epithelia may have several functions, although no single epithelium performs all of them. These functions include: Physical protection. Epithelial tissues protect both exposed and internal surfaces from dehydration, abrasion, and destruction by physical, chemical, or biological agents. Selective permeability. All epithelial cells act as gatekeepers, in that they regulate the movement of materials into and out of certain regions of the body. All substances that enter or leave the body must pass through an epithelium. Sometimes an epithelium exhibits a range of permeability; that is, it may be relatively impermeable to some substances, while at the same time promoting and assisting the passage of other molecules by absorption or secretion. The structure and characteristics of an epithelium may change as a result of applied pressure or stress; for example, walking around without shoes may increase the thickness of calluses on the bottom of the feet, which could alter or reduce the movement of materials across the epithelium. Secretions. Some epithelial cells, called exocrine glands, are specialized to produce secretions. Individual gland cells may be scattered among other cell types in an epithelium, or a large group of epithelial secretory cells may form a gland to produce specific secretions. Sensations. Epithelial tissues contain some nerve endings to detect changes in the external environment at their surface. These sensory nerve endings and those in the underlying connective tissue continuously supply information to the nervous system concerning touch, pressure, temperature, and pain. For example, receptors in the epithelium of the skin respond to pressure by stimulating adjacent sensory nerves. Additionally, several organs contain a specialized epithelium, called a neuroepithelium, that houses specific cells responsible for the senses of sight, taste, smell, hearing, and equilibrium. 1 WHAT DO YOU THINK? Why do you think epithelial tissue does not contain blood vessels? Can you think of an epithelial function that could be compromised if blood vessels were running through the tissue? 4.1c Specialized of Epithelial Tissue Because epithelial tissues are located at all free surfaces in the body, they exhibit distinct structural specializations. An epithelium rests on a layer of connective tissue and adheres firmly to it which secures the epithelium in place and prevents it from tearing. Between the epithelium and the underlying connective tissue is a thin extracellular layer called the basement membrane. The basement membrane can be seen as a single layer beneath epithelium using the light microscope (figure 4.1a). However, it consists of three layers when observed using an electron microscope: the lamina lucida, the lamina densa, and the reticular lamina. The two laminae closest to the epithelium contain collagen fibers as well as specific proteins and carbohydrates some of which are secreted by the epithelial cells. Cells in the underlying connective tissue secrete the reticular lamina, which contains protein fibers and carbohydrates. Together, these components of the basement membrane strengthen the attachment and form a selective molecular barrier between the epithelium and the underlying connective tissue. The basement membrane has the following functions: Providing physical support for the epithelium Anchoring the epithelium to the connective tissue Acting as a barrier to regulate the movement of large molecules between the epithelium and the underlying connective tissue Intercellular Junctions Epithelial cells are strongly bound together by specialized connections in the plasma membranes of their lateral surfaces called intercellular junctions. There are four types of junctions: tight junctions, adhering junctions, desmosomes, and gap junctions (figure 4.1b). Each of these types of junctions has a specialized structure. Tight Junctions A tight junction, also called a zonula (zo ńu la ) occludens ( occluding belt ), encircles epithelial cells near their apical surface and completely attaches each cell to its neighbors. Plasma membrane proteins among neighboring cells fuse, so the apical surfaces of the cells are tightly connected everywhere around the cell. This seals off the intercellular space and prevents substances from passing between the epithelial cells. The tight junction forces almost all materials to move through, rather than between, the epithelial cells in order to cross the epithelium. Thus, epithelial cells control whatever enters and leaves the body by moving across the epithelium. For example, in the small intestine, tight junctions prevent digestive enzymes that degrade molecules from moving between epithelial cells into underlying connective tissue. Adhering Junctions An adhering junction, also called a zonula adherens ( adhesion belt ), is formed completely around the cell. This type of junction occurs when extensive zones of microfilaments extend from the cytoplasm into the plasma membrane, forming a supporting and strengthening belt within the plasma membrane that completely encircles the cell immediately adjacent to all of its neighbors. Typically, adhering junctions are located deep to the tight junctions; the anchoring of the microfilament proteins within this belt provides the only means of junctional support for the apical surface of the cell. The ultra-strong tight junctions are needed only near the apical surface and not along the entire length of the cell. Once neighboring cells are fused together by the tight junctions near the apical surface, the adhering junctions support the apical surface and provide for a small space between neighboring cells in the direction of the basal surface. Thus, the junction affords a passageway between cells for materials that have already passed through the apical surface of the epithelial cell and can then exit through the membranes on the lateral surface and continue their journey toward the basement membrane. Desmosomes A desmosome (dez ḿo -so m; desmos = a band, soma = body), also called a macula adherens ( adhering spot ), is like a button or snap between adjacent epithelial cells. Each cell contributes half of the complete desmosome. It is a small region that holds cells together and provides resistance to mechanical stress at a single point, but it does not totally encircle the cell. In contrast to tight junctions, which encircle the cell to secure it to its neighbors everywhere around its periphery, the desmosome attaches a cell to its neighbors only at potential stress points. The neighboring cells are separated by a small space that is spanned by a fine web of protein filaments.

4 Chapter Four Tissue Level of Organization 83 Apical (free) surface Lateral surface Epithelium Basal surface Connective tissue Blood vessel (a) Epithelium connective tissue junction Tight junction Membrane protein Plasma membrane Adhering junction Microfilament Intercellular space Adjacent plasma membranes (b) Types of intercellular junctions Intercellular space Hemidesmosome Desmosome Protein filaments Protein plaque Intermediate filaments Plasma membrane Gap junction Pore Connexon Figure 4.1 Polarity and Intercellular Junctions in an Epithelium. An epithelium exhibits polarity and has intercellular junctions only on the lateral surfaces of its individual cells. (a) The apical surface is the cell's free surface exposed to a body cavity, an organ lumen, or the exterior of the body. The basal surface of the cell adheres to the underlying connective tissue by a basement membrane. Hemidesmosomes help anchor cells to the basement membrane in some epithelia. (b) The lateral surfaces of the cell contain intercellular junctions. Types of intercellular junctions are tight junctions, adhering junctions, desmosomes, and gap junctions. These filaments anchor into a thickened protein plaque located at the internal surface of the plasma membrane. On the cytoplasmic side of each plaque, intermediate filaments of the cytoskeleton penetrate the plaque to extend throughout the cell the support and strength supplied between the cells by the desmosome. The basal cells of some epithelial tissue exhibit structures called hemidesmosomes, half-desmosomes that anchor them to the underlying basement membrane. Gap Junctions A gap junction is formed across the intercellular gap between neighboring cells. This gap (about 2 nanometers in length) is bridged by structures called connexons (kon-neks ón). Each connexon consists of six transmembrane proteins, arranged in a circular fashion to form a tiny, fluid-filled tunnel or pore. Gap junctions provide a direct passageway for small molecules traveling between neighboring cells. Ions, glucose, amino acids, and other small solutes can pass directly from the cytoplasm of one cell into the neighboring cell through these channels. The flow of ions between cells coordinates such cellular activities as the beating of cilia. Gap junctions are also seen in certain types of muscle tissue, where they help coordinate contraction activities WHAT DID YOU LEARN? Describe the basement membrane, its origins, and its functions. Which intercellular junction ensures that epithelial cells act as gatekeepers? What type of intercellular junction provides resistance to mechanical stress at a single point?

5 84 Chapter Four Tissue Level of Organization Apical surface Nucleus Figure 4.2 Classification of Epithelia. Two criteria are used to classify epithelia: the number of cell layers and the shape of the cell at the apical surface. (a) An epithelium is simple if it is one cell layer thick, and stratified if it has two or more layers of cells. (b) Epithelial cell shapes include squamous (thin, flattened cells), cuboidal (cells about as tall as they are wide), and columnar (cells taller than they are wide). Lateral surface Apical surface Lateral surface Simple epithelium Basal surface Nucleus Nucleus Basal surface Squamous cell Cuboidal cell Stratified epithelium (a) Epithelium classified by layers Columnar cell (b) Epithelium classified by shapes 4.1d Classification of Epithelial Tissue The body contains many different kinds of epithelia, and the classification of each type is indicated by a two-part name. The first part of the name refers to the number of epithelial cell layers, and the second part describes the shape of the cells at the apical surface of the epithelium. Classification by Number of Cell Layers Epithelia may be classified based on number of cell layers as either simple or stratified (figure 4.2a). A simple epithelium is one cell layer thick, and all of these epithelial cells are in direct contact with the basement membrane. Often, the apical surface is covered by a thin layer of fluid or mucus to prevent desiccation and help protect the cells from abrasion or friction. A simple epithelium is found in areas where stress is minimal and where filtration, absorption, or secretion is the primary function. Such locations include the linings of the air sacs in the lungs, intestines, and blood vessels. A stratified epithelium contains two or more layers of epithelial cells. Only the cells in the deepest (basal) layer are in contact with the basement membrane. A stratified epithelium resembles a brick wall, where the bricks in contact with the ground represent the basal layer and the bricks at the top of the wall represent the apical layer. The multiple cell layers of a stratified epithelium make it strong and capable of resisting stress and protecting underlying tissue. In contrast to a simple epithelium, a stratified epithelium is found in areas likely to be subjected to abrasive activities or mechanical stresses, where two or more layers of cells are better able to resist this wear and tear (e.g., the internal lining of the esophagus, pharynx, or vagina). Cells in the basal layer continuously regenerate as the cells in the more superficial layer are lost due to abrasion or stress. Finally, a pseudostratified (soo do -strat i-fı d; pseudes = false, stratum = layer) epithelium looks layered (stratified) because the cells nuclei are distributed at different levels between the apical and basal surfaces. But although all of these epithelial cells are attached to the basement membrane, some of them do not reach its apical surface. Those cells that do reach the apical surface often bear cilia to move mucus along the surface. This so-called ciliated pseudostratified epithelium lines the nasal cavity and the respiratory passageways. Classification by Cell Shape Epithelia are also classified by the shape of the cell at the apical surface. In a simple epithelium, all of the cells display the same shape. However, in a stratified epithelium there is usually a difference in cell shape between the basal layer and the apical layer. Figure 4.2b shows the three common cell shapes observed in epithelia: squamous, cuboidal, and columnar. (Note that the cells in this figure all appear hexagonal when looking at their apical surface, or en face ; thus these terms describe the cells shapes when viewed laterally, or from the side.) Squamous (skwa ḿu s; squamosus = scaly) cells are flat, wide, and somewhat irregular in shape. The nucleus looks like a flattened disc. The cells are arranged like irregular, flattened floor tiles. Cuboidal (ku -boy da l; kybos = cube, eidos = resemblance) cells are about as tall as they are wide. The cells do not resemble perfect cubes, because they do not have squared edges. The cell nucleus is spherical and located within the center of the cell. Columnar (kol u m ńa r; columna = column) cells are slender and taller than they are wide. The cells look like a group of hexagonal columns aligned next to each other. Each cell nucleus is oval and usually oriented lengthwise and located in the basal region of the cell. Another shape that occurs in epithelial cells is called transitional (tran-zish u n-a l; transitio = to go across). These cells can readily change their shape or appearance depending upon how stretched the epithelium becomes. They are found where the epithelium cycles between distended and relaxed states, such as in the lining of the bladder, which fills with urine and is later emptied. When the transitional epithelium is in a relaxed state, the cells are described as polyhedral, which means many-sided and reflects the ranges in shape that are possible in this type of epithelium. When transitional epithelium is stretched, the surface cells resemble squamous cells.

6 Chapter Four Tissue Level of Organization 85 Study Tip! In your anatomy lab, you may be asked to identify a particular type of epithelium under the microscope. This can be a daunting task, especially for a student who has never examined tissues under the microscope before. Ask the following questions to help identify each type of epithelium: 1. Is the epithelium one layer or many layers thick? If it is one layer thick, you are looking at some type of simple epithelium. If it is many layers thick, you are looking at some type of stratified epithelium or one of the unusual types of epithelium (such as pseudostratified or transitional). 2. What is the shape of the cells? If the cells (or at least the apical layer of cells) are flattened, you are looking at some type of squamous epithelium. Your answer to question 1 gives you the first part of the epithelium s name (e.g., simple). Your answer to question 2 gives you the second part of the epithelium s name (e.g., squamous). Put these answers together, and you will have the name of the tissue (simple squamous epithelium, in this case). 4.1e Types of Epithelium Using the classification system just described, epithelium can be broken down into the primary types shown in table 4.2. In this section, we describe the characteristics of these types of epithelium and show how each appears under the microscope. Simple Squamous Epithelium A simple squamous epithelium consists of a single layer of flattened cells (table 4.3a). When viewed en face, the irregularly shaped cells display a spherical to oval nucleus, and they appear tightly bound together in a mosaic-like pattern. Each squamous cell resembles a fried egg, with the nucleus representing the yolk. This epithelium is extremely delicate and highly specialized to allow rapid movement of molecules across its surface by diffusion, osmosis, or filtration. Simple squamous epithelium is found only in protected regions where moist surfaces reduce friction and abrasion. For example, in the lining of the lung air sacs (alveoli), the thin epithelium is well suited for the exchange of oxygen and carbon dioxide between the blood and inhaled air. This type of epithelium is also found lining the lumen (inside space) of blood vessel walls, where it allows for rapid exchange of nutrients and waste between the blood and the interstitial fluid surrounding the blood vessels. Simple squamous epithelia that line closed internal body cavities and all circulatory structures have special names. The simple squamous epithelium that lines the lumen of the blood and lymphatic vessels and the heart and its chambers is termed endothelium (en-do -the -le -u m; endon = within, thele = nipple). Mesothelium (mez-o -the le -u m; mesos = middle) is the simple squamous epithelium of the serous membrane (discussed in chapter 1) that lines the internal walls of the pericardial, pleural, and peritoneal cavities as well as the external surfaces of the organs within those cavities. Mesothelium gets its name from the primary germ layer mesoderm, from which it is derived. Simple Cuboidal Epithelium A simple cuboidal epithelium consists of a single layer of cells that are as tall as they are wide (table 4.3b). A spherical nucleus is located in the center of the cell. A simple cuboidal epithelium functions primarily to absorb fluids and other substances across its apical membrane and to secrete specific molecules. It forms the walls of kidney tubules, where it participates in the reabsorption of nutrients, ions, and water that are filtered out of the blood plasma. It also forms the ducts of exocrine glands, which secrete materials. Simple cuboidal epithelium covers the surface of the ovary and also lines the follicles of the thyroid gland. Table 4.2 Type SIMPLE EPITHELIUM Simple squamous Simple cuboidal Simple columnar, nonciliated Simple columnar, ciliated STRATIFIED EPITHELIUM Stratified squamous, keratinized Stratified squamous, nonkeratinized Stratified cuboidal Stratified columnar OTHER TYPES OF EPITHELIUM Pseudostratified columnar Transitional Types of Epithelium One cell layer thick; all cells are tightly bound; all cells attach directly to the basement membrane One layer of flattened cells One layer of cells about as tall as they are wide One layer of nonciliated cells that are taller than they are wide; cells may contain microvilli One layer of ciliated cells that are taller than they are wide Two or more cell layers thick; only the deepest layer directly attaches to the basement membrane Many layers thick; cells in surface layers are dead, flat, and filled with the protein keratin Many layers thick; no keratin in cells; surface layers are alive, flat, and moist Two or more layers of cells; apical layer of cells is cuboidal-shaped Two or more layers of cells; cells in apical layer are columnar-shaped Cell layers vary, from single to many One layer of cells of varying heights; all cells attach to basement membrane; ciliated form contains cilia and goblet cells; nonciliated form lacks cilia and goblet cells Multiple layers of polyhedral cells (when tissue is relaxed) or flattened cells (when tissue is distended); some cells may be binucleated

7 86 Chapter Four Tissue Level of Organization Table 4.3 Simple Epithelia Amnion Kidney tubules Simple squamous cell Lumen of kidney tubule Simple cuboidal cell LM 400x LM 1000x Simple squamous cell Lumen of kidney tubule Simple cuboidal cell (a) Simple Squamous Epithelium (b) Simple Cuboidal Epithelium Single layer of thin, flat, irregularly shaped cells resembling floor tiles; the single nucleus of each cell bulges at its center Single layer of cells about as tall as they are wide; spherical, centrally located nucleus Function Rapid diffusion, filtration, and some secretion in serous membranes Function Absorption and secretion Air sacs in lungs (alveoli); lining of heart chambers and lumen of blood vessels (endothelium); serous membranes of body cavities (mesothelium) Thyroid gland follicles; kidney tubules; ducts and secretory regions of most glands; surface of ovary Simple Columnar Epithelium A simple columnar epithelium is composed of a single layer of tall, narrow cells. The nucleus is oval and located within the basal region of the cell. Active movement of molecules occurs across this type of epithelium by either absorption or secretion. Simple columnar epithelium has two forms; one type has no cilia, while the apical surface of the other type is lined with cilia. Nonciliated simple columnar epithelium often contains microvilli and a scattering of unicellular glands called goblet cells (table 4.3c). Recall that microvilli are tiny, cytoplasmic projections

8 Chapter Four Tissue Level of Organization 87 Mucosa of small intestine Uterine tube Cilia Goblet cell Microvilli (brush border) Nonciliated simple columnar cell Simple columnar epithelial cell LM 400x LM 100x Cilia Goblet cell Microvilli (brush border) Nonciliated simple columnar cell Simple columnar epithelial cell (c) Nonciliated Simple Columnar Epithelium (d) Ciliated Simple Columnar Epithelium Single layer of tall, narrow cells; oval-shaped nucleus in basal region of cell; nucleus oriented lengthwise in cell; apical regions of cells have microvilli; may contain goblet cells that secrete mucin Single layer of tall, narrow, ciliated cells; ovalshaped nucleus oriented lengthwise in the basal region of the cell; goblet cells may be present Function Absorption and secretion; secretion of mucin Function Secretion of mucin and movement of mucus along apical surface of epithelium by action of cilia; oocyte movement through uterine tube Lining of most of digestive tract; lining of stomach does not contain goblet cells Lining of uterine tubes and larger bronchioles of respiratory tract on the apical surface of the cell that increase the surface area for secretion and absorption. You cannot distinguish individual microvilli under the microscope; rather, the microvilli collectively appear as a darkened, fuzzy structure known as a brush border. Goblet cells secrete mucin (mu śin; mucus = mucus), a glycoprotein that upon hydration (being mixed with water) forms mucus for lubrication. Nonciliated simple columnar epithelium lines most of the digestive tract, from the stomach to the anal canal. In ciliated simple columnar epithelium, cilia project from the apical surfaces of the cells (table 4.3d). Mucus covers these apical

9 88 Chapter Four Tissue Level of Organization Study Tip! If you are having trouble distinguishing cilia from microvilli, recall that cilia appear under the light microscope like fine hairs extending from the apical surface of the cell, while microvilli are extensive folds of the plasma membrane that appear as a fuzzy, darkened brush border at the apical surface. surfaces and is moved along by the beating of the cilia. Goblet cells typically are interspersed throughout this epithelium. This type of epithelium lines the luminal (internal) surface of the uterine tubes, where it helps move an oocyte from the ovary to the uterus. A ciliated simple columnar epithelium is also present in the bronchioles (smaller air tubes) of the lung. Stratified Squamous Epithelium A stratified squamous epithelium has multiple cell layers, and only the deepest layer of cells is in direct contact with the basement membrane. While the cells in the basal layers have a varied shape often described as polyhedral, the superficial cells at the apical surface display a flattened, squamous shape. Thus, stratified squamous epithelium is so named because of its multiple cell layers and the shape of its most superficial cells. This epithelium is adapted to protect underlying tissues from damage due to activities that are abrasive and cause friction. Stem cells in the basal layer continuously divide to produce a new stem cell and a committed cell that gradually moves toward the surface to replace the cells lost during protective activities. This type of epithelium exists in two forms: nonkeratinized and keratinized. The cells in nonkeratinized stratified squamous epithelium remain alive all the way to its apical surface, and they are kept moist with secretions such as saliva or mucus. Keratin, a fibrous intracellular protein, is not present within the cells. Thus, because all of the cells are still alive, the flattened nuclei characteristic of squamous cells are visible even in the most superficial cells (table 4.4a). Nonkeratinized stratified squamous epithelium lines the oral cavity (mouth), part of the pharynx (throat), the esophagus, the vagina, and the anus. In keratinized (ker a -ti-nı zd; keras = horn) stratified squamous epithelium, the apical surface is composed of layers of cells that are dead; these cells lack nuclei and all organelles and are filled with tough, protective keratin. It is obvious that the superficial cells lack nuclei when they are viewed under the microscope (table 4.4b). New committed cells produced in the basal region of the epithelium migrate toward the apical surface. During their migration, they fill with keratin, lose their organelles and nuclei, and die. However, the keratin in these dead cells makes them very strong. Thus, there is a tradeoff with the appearance of keratin, in that the tissue becomes very strong, but the cells must die as a result. The epidermis (outer layer) of the skin consists of keratinized stratified squamous epithelium. Stratified Cuboidal Epithelium A stratified cuboidal epithelium contains two or more layers of cells, and the apical cells tend to be cuboidal in shape (table 4.4c). This type of epithelium forms the walls of the larger ducts of most exocrine glands, such as the sweat glands in the skin. Although the function of stratified cuboidal epithelium is mainly protective, it also serves to strengthen the wall of gland ducts. Stratified Columnar Epithelium A stratified columnar epithelium is relatively rare in the body. It consists of two or more layers of cells, but only the apical surface cells are columnar in shape (table 4.4d). This type of epithelium is found in the large ducts of salivary glands and in the membranous segment of the male urethra. Pseudostratified Columnar Epithelium Pseudostratified columnar epithelium is so named because upon first glance, it appears to consist of multiple layers of cells. However, this epithelium is not really stratified, because all of its cells are in direct contact with the basement membrane. It may look stratified, but it is actually pseudostratified due to the fact that the nuclei are scattered at different distances from the basal surface but not all of the cells reach the apical surface (table 4.5a). The columnar cells within this epithelium always reach the apical surface; the shorter cells are stem cells that give rise to the columnar cells. There are two forms of pseudostratified columnar epithelium: Pseudostratified ciliated columnar epithelium has cilia on its apical surface, whereas pseudostratified nonciliated columnar epithelium lacks cilia. Both types of this epithelium perform protective functions. The ciliated form houses goblet cells, which secrete mucin that forms mucus. This mucus traps foreign particles and is moved along the apical surface by the beating of the cilia. Pseudostratified ciliated columnar epithelium lines much of the larger portions of the respiratory tract, including the nasal cavity, part of the pharynx (throat), the larynx (voice box), the trachea, and the bronchi. The cilia in this epithelium help propel dust particles and foreign materials away from the lungs and to the nose and mouth. In contrast, the nonciliated form of this epithelium has no goblet cells. It is a rare epithelium that occurs primarily in part of the male urethra and the epididymis. Transitional Epithelium A transitional epithelium varies in appearance, depending on whether it is in a relaxed or a stretched state (table 4.5b). In a relaxed state, the basal cells appear almost cuboidal, and the apical cells are large and rounded. During stretching, the transitional epithelium thins, and the apical cells continue to flatten, becoming almost squamous. In this distended state, it may be difficult to distinguish a transitional epithelium from a squamous epithelium. However, one distinguishing feature of transitional epithelium is the presence of a handful of binucleated (double-nucleus-containing) cells. This epithelium lines the urinary bladder, an organ that changes shape as it fills with urine. It also lines the ureters and the proximal part of the urethra. Transitional epithelium permits stretching and ensures that toxic urine does not seep into the underlying tissues and structures of these organs. 2 WHAT DO YOU THINK? What types of epithelium are well suited for protection?

10 Chapter Four Tissue Level of Organization 89 Table 4.4 Stratified Epithelia Vagina Epidermis of skin Squamous epithelial cell Keratinized stratified squamous epithelial cells Nonkeratinized stratified squamous epithelium Living stratified squamous epithelial cells Connective tissue LM 125x Connective tissue LM 100x Squamous epithelial cell Keratinized stratified squamous epithelial cells Nonkeratinized stratified squamous epithelium Living stratified squamous epithelial cells Connective tissue Connective tissue (a) Nonkeratinized Stratified Squamous Epithelium Multiple layers of cells; basal cells typically are cuboidal or polyhedral, while apical (superficial) cells are squamous; surface cells are alive and kept moist (b) Keratinized Stratified Squamous Epithelium Multiple layers of cells; basal cells typically are cuboidal or polyhedral, while apical (superficial) cells are squamous; more superficial cells are dead and filled with the protein keratin Function Protection of underlying tissue Function Protection of underlying tissue Lining of oral cavity, part of pharynx, esophagus, vagina, and anus Epidermis of skin (continued on next page)

11 90 Chapter Four Tissue Level of Organization Table 4.4 Stratified Epithelia (continued) Duct of sweat gland Male urethra LM 100x Cuboidal cell Stratified cuboidal epithelium Columnar cell Stratified columnar epithelium Connective tissue LM 500x Cuboidal cell Stratified cuboidal epithelium Columnar cell Stratified columnar epithelium Connective tissue (c) Stratified Cuboidal Epithelium Two or more layers of cells; cells at the apical surface are cuboidal (d) Stratified Columnar Epithelium Function Protection and secretion Function Protection and secretion Found in large ducts in most exocrine glands and in some parts of the male urethra Two or more layers of cells; cells at the apical surface are columnar Rare; found in large ducts of some exocrine glands and in some regions of the male urethra

12 Chapter Four Tissue Level of Organization 91 Table 4.5 Other Epithelia Nasal cavity lining Urinary bladder lining Goblet cell Cilia Pseudostratified ciliated columnar epithelium Columnar cell Transitional epithelium (relaxed) Binucleated epithelial cell Basal cell Connective tissue LM 600x Connective tissue LM 180x Goblet cell Cilia Pseudostratified ciliated columnar epithelium Columnar cell Transitional epithelium (relaxed) Binucleated epithelial cell Basal cell Connective tissue Connective tissue (a) Pseudostratified Columnar Epithelium (b) Transitional Epithelium Single layer of cells with varying heights that appears multilayered; all cells connect to the basement membrane, but not all cells reach the apical surface. Ciliated form has goblet cells and cilia (shown); nonciliated form lacks goblet cells and cilia Epithelial appearance varies, depending on whether the tissue is stretched or relaxed; shape of cells at apical surface changes; some cells may be binucleated Function Protection; ciliated form also involved in secretion of mucin and movement of mucus across surface by ciliary action Function Distention and relaxation to accommodate urine volume changes in bladder, ureters, and urethra Ciliated form lines most of respiratory tract, including nasal cavity, part of pharynx, larynx, trachea, bronchi. Nonciliated form is rare; lines epididymis and part of male urethra Lining of urinary bladder, ureters, and part of urethra

13 92 Chapter Four Tissue Level of Organization Microvilli Secretory vesicles containing mucin Figure 4.3 Goblet Cell: A Unicellular Exocrine Gland. (a) Photomicrograph and (b) diagram of a goblet cell in the small intestine. Rough ER Mitochondria Golgi apparatus Nucleus TEM 30,000x (a) (b) 4.1f Glands As epithelial tissue develops in the embryo, small invaginations from this epithelium into the underlying connective tissue give rise to specialized secretory structures called glands. Glands are either individual cells or multicellular organs composed predominantly of epithelial tissue. Glands perform a secretory function by producing substances either for use elsewhere in the body or for elimination from the body. Glandular secretions include mucin, hormones, enzymes, and waste products. Endocrine and Exocrine Glands Glands are classified as either endocrine or exocrine, depending upon whether they have a duct connecting the secretory cells to the surface of an epithelium. Endocrine (en do -krin; endon = within, krino = to separate) glands lack ducts and secrete their products directly into the interstitial fluid and bloodstream. The secretions of endocrine glands, called hormones, act as chemical messengers to influence cell activities elsewhere in the body. Endocrine glands are discussed in depth in chapter 20. Exocrine (ek śo -krin; exo = outside) glands typically originate from an invagination of epithelium that burrows into the deeper connective tissues. These glands usually maintain their contact with the epithelial surface by means of a duct, an epithelium-lined tube through which secretions of the gland are discharged onto the epithelial surface. This duct may secrete materials onto the surface of the skin (e.g., sweat from sweat glands or milk from mammary glands) or onto an epithelial surface lining an internal passageway (e.g., enzymes from the pancreas into the small intestine or saliva from the salivary glands into the oral cavity). Exocrine Gland An exocrine gland may be unicellular or multicellular. A unicellular exocrine gland is an individual exocrine cell located within an epithelium that is predominantly nonsecretory. Unicellular exocrine glands typically do not contain a duct, and they are located close to the surface of the epithelium in which they reside. The most common type of unicellular exocrine gland is the goblet cell (figure 4.3). For example, the respiratory tract is lined mainly by pseudostratified ciliated columnar epithelium, which also contains some mucin-secreting goblet cells. Mucus then coats the inner surface of the respiratory passageway to cover and protect its lining and to help warm, humidify, and cleanse the inhaled air before it reaches the gas exchange surfaces in the lungs. Multicellular exocrine glands are composed of numerous cells that work together to produce a secretion and secrete it onto the surface of an epithelium. A multicellular exocrine gland consists of acini (as i-nı ; sing., as i-nu s; acinus = grape), sacs that produce the secretion, and one or more smaller ducts, which merge to eventually form a larger duct that transports the secretion to the epithelial surface (figure 4.4). Acini are the secretory portions, while ducts are the conducting portions of these glands. Most multicellular exocrine glands are enclosed within a fibrous capsule. Extensions of this capsule, called septa or trabeculae, partition the gland internally into compartments called lobes. Further subdivisions of the septa within each lobe form microscopic lobules (lob u l). The septa contain ducts, blood vessels, and nerves supplying the gland. The connective tissue framework of the gland is called the stroma. The stroma supports and organizes the parenchyma (pa -reng ki-ma ), the functional cells of the gland that produce and secrete the gland products. These cells are usually simple cuboidal or columnar epithelial cells. Multicellular exocrine glands are found in the mammary glands, pancreas, and salivary glands. Classification of Exocrine Glands Multicellular exocrine glands may be classified according to three criteria: (1) form and structure (morphology), which is considered an anatomic classification; (2) type of secretion;

14 Chapter Four Tissue Level of Organization 93 Stroma Septum Capsule Parenchyma and (3) method of secretion. The latter two are considered physiologic classifications. Duct (a) Lobe Lobules (within lobe) Secretory vesicles Secretory acini Form and Based on the structure and complexity of their ducts, exocrine glands are considered either simple or compound. Simple glands have a single, unbranched duct; compound glands exhibit branched ducts. Exocrine glands are also classified according to the shape or organization of their secretory portions. If the secretory portion and the duct are of uniform diameter, the gland is called tubular. If the secretory cells form an expanded sac, the gland is called acinar (as i-nar). Finally, a gland with both secretory tubules and secretory acini is called a tubuloacinar gland. Figure 4.5 shows the several types of exocrine glands as classified by morphology. Figure 4.4 (b) Acinus (secretory portion) Duct (conducting portion) General of Exocrine Glands. (a) Exocrine glands have a connection called a duct that leads to an organ or body surface. Inside the gland, the duct branches repeatedly, following the connective tissue septa, until its finest divisions end on secretory acini. (b) The acinus is the secretory portion of the gland, and the duct is the conducting portion. Secretion Types Exocrine glands are classified by the nature of their secretions as serous glands, mucous glands, or mixed glands. Serous (se r u s; serum = whey) glands produce and secrete a nonviscous, watery fluid, such as sweat, milk, tears, or digestive juices. This fluid carries wastes (sweat) to the surface of the skin, nutrients (milk) to a nursing infant, or digestive enzymes from the pancreas to the lumen of the small intestine. Mucous (mu ḱu s) glands secrete mucin, which forms mucus when mixed with water. Mucous glands are found in such places as the roof of the oral cavity and the surface of the tongue. Mixed glands, such as the two pairs of salivary glands inferior to the oral cavity, contain both serous and mucous cells, and produce a mixture of the two types of secretions. Duct Secretory portion Simple tubular Simple branched tubular Simple coiled tubular Simple acinar Simple branched acinar (a) Simple glands Duct Secretory portions Compound tubular Compound acinar Compound tubuloacinar (b) Compound glands Figure 4.5 Structural Classification of Multicellular Exocrine Glands. (a) Simple glands have unbranched ducts, whereas (b) compound glands have ducts that branch. These glands also exhibit different forms: Tubular glands have secretory cells in a space with a uniform diameter, acinar glands have secretory cells arranged in saclike acini, and tubuloacinar glands have secretory cells in both types of regions.

15 94 Chapter Four Tissue Level of Organization Secretory contents Disintegrating cells with contents becoming the secretion Secretions Secretory vesicle Nucleus of secretory cell Nucleus Secretory vesicles releasing their contents via exocytosis Cells dividing Pinching off of apical portion of secretory cell (a) Merocrine gland (b) Holocrine gland (c) Apocrine gland Figure 4.6 Modes of Exocrine Secretion. Exocrine glands use different processes to release their secretory products. (a) Merocrine glands secrete products by means of exocytosis at the apical surface of the secretory cells. (b) Holocrine gland secretion is produced through the destruction of the secretory cell. Lost cells are replaced by cell division at the base of the gland. (c) Apocrine gland secretion occurs with the decapitation of the apical surface of the cell and the subsequent release of secretory product and some cellular fragments. Secretion Methods Glands also can be classified by their mechanism of discharging secretory product as merocrine glands, holocrine glands, or apocrine glands (figure 4.6). Merocrine (mer -o -krin; meros = share) glands package their secretions in structures called secretory vesicles. The secretory vesicles travel to the apical surface of the glandular cell and release their secretion by exocytosis. The glandular cells remain intact and are not damaged in any way by producing the secretion. Lacrimal (tear) glands, salivary glands, some sweat glands, the exocrine glands of the pancreas, and the gastric glands of the stomach are examples of merocrine glands. Some merocrine glands are also called eccrine glands, to denote a type of sweat gland in the skin that is not connected to a hair follicle (see chapter 5). Holocrine (ho l o -krin; holos = whole) glands are formed from cells that accumulate a product and then the entire cell disintegrates. Thus, a holocrine secretion is a mixture of cell fragments and the product the cell synthesized prior to its destruction. The ruptured, dead cells are continuously replaced by other epithelial cells undergoing mitosis. Without this regenerative capacity, holocrine glands would quickly lose all of their cells during their secretory activities. Holocrine secretions tend to be more viscous than merocrine secretions. The oil-producing glands (sebaceous glands) in the skin are an example of holocrine glands. (So the oily secretion you feel on your skin is actually composed of ruptured, dead cells!) Apocrine (ap o -krin; apo = away from or off) glands are composed of cells that accumulate their secretory products within the apical portion of their cytoplasm. The secretion follows as this apical portion decapitates. The apical portion of the cytoplasm begins to pinch off into the lumen of the gland for the secretory product to be transported to the skin surface. Mammary glands and ceruminous glands are apocrine glands. Study Tip! The hol part of holocrine sounds like the word whole. Holocrine gland secretions are produced when the whole cell ruptures, dies, and becomes the secretion. The apo part of apocrine sounds like a part. Secretions produced when a part of the cell is pinched off come from apocrine glands WHAT DID YOU LEARN? What two main characteristics are used to classify epithelial tissues? Why is one epithelium referred to as pseudostratified? What are the two basic parts of a multicellular exocrine gland? Why is epithelial cell regeneration important to the continued functioning of a holocrine gland?

16 Chapter Four Tissue Level of Organization Connective Tissue Learning Objectives: 1. Describe the structure and function of connective tissue. 2. Identify the characteristics of embryonic connective tissue. 3. Compare connective tissue proper, supporting connective tissue, and fluid connective tissue. 4. Explain where each type of connective tissue is found in the body. Connective tissue is the most diverse, abundant, widely distributed, and microscopically variable of the tissues. Connective tissue is designed to support, protect, and bind organs. As its name implies, it is the glue that binds body structures together. The diversity of connective tissue is obvious when examining some of its types. Connective tissue includes the fibrous tendons and ligaments, body fat, the cartilage that connects the ends of ribs to the sternum, the bones of the skeleton, and the blood. 4.2a Characteristics of Connective Tissue Although the types of connective tissue are diverse, all of them share three basic components: cells, protein fibers, and ground substance (figure 4.7). Their diversity is due to varying proportions of these components as well as to differences in the types and amounts of protein fibers. Cells Each type of connective tissue contains specific types of cells. For example, connective tissue proper contains fibroblasts, fat contains adipocytes, cartilage contains chondrocytes, and bone contains osteocytes. Most connective tissue cells are not in direct contact with each other, but are scattered throughout the tissue. This differs markedly from epithelial tissue, whose cells crowd closely together with little to no extracellular matrix surrounding them. Protein Fibers Most connective tissue contains protein fibers throughout. These fibers strengthen and support connective tissue. The type and abundance of these fibers indicate to what extent the particular connective tissue is responsible for strength and support. Three types of protein fibers are found in connective tissue: collagen fibers, which are strong and stretch-resistant; elastic fibers, which are flexible and resilient; and reticular fibers, which form an interwoven framework. Ground Substance Both the cells and the protein fibers reside within a material called ground substance. This nonliving material is produced by the connective tissue cells. It primarily consists of protein and carbohydrate molecules and variable amounts of water. The ground substance may be viscous (as in blood), semisolid (as in cartilage), or solid (as in bone). Together, the ground substance and the protein fibers form an extracellular matrix. Most connective tissues are composed primarily of an extracellular matrix, with relatively small proportions of cells. 4.2b Functions of Connective Tissue As a group, the many types of connective tissue perform a wide variety of functions, including the following: Physical protection. The bones of the cranium, sternum, and thoracic cage protect delicate organs, such as the brain, heart, and lungs; fat packed around the kidneys Ground substance Elastic fibers Extracellular matrix Figure 4.7 Connective Tissue Components and Organization. Connective tissue is composed of cells and an extracellular matrix of protein fibers and ground substance. Collagen fibers Reticular fibers Mesenchymal cell Blood vessel Protein fibers Macrophage Adipocyte (fat cell) Fibroblast