Lesson 5 Types of Tissues.pdf

6 E X E R C I S E Classification of Tissues Learning Outcomes Go to Mastering A&P™ > Study...

6 E X E R C I S E Classification of Tissues Learning Outcomes Go to Mastering A&P™ > Study Area to improve your performance ▶▶ Name the four primary tissue types in the human body, and state a general in A&P Lab. function of each. ▶▶ Name the major subcategories of the primary tissue types, and identify the tissues of each subcategory microscopically or in an appropriate image. ▶▶ State the locations of the various tissues in the body. ▶▶ List the general function and structural characteristics of each of the tissues studied. Instructors may assign these and other Pre-Lab Pre-Lab Quiz Quiz questions using Mastering A&P™ > Lab Tools > Practice Anatomy Laboratory > Histology 1. Epithelial tissues can be classified according to cell shape. Instructors may assign new __________________ epithelial cells are scalelike and flattened. Building Vocabulary coaching a. Columnar c. Squamous activities, Pre-Lab Quiz questions, Art b. Cuboidal d. Transitional Labeling activities, Practice Anatomy Lab Practical questions (PAL), and more 2. All connective tissue is derived from an embryonic tissue known as: using the Mastering A&P™ Item a. cartilage c. mesenchyme Library. b. ground substance d. reticular 3. All the following are examples of connective tissue except: a. bones c. neurons Materials b. ligaments d. tendons ▶▶ Compound microscope 4. Circle the correct underlined term. Of the two major cell types found ▶▶ Immersion oil in nervous tissue, neurons / neuroglial cells are highly specialized to ▶▶ Prepared slides of simple squamous, generate and conduct electrical signals. simple cuboidal, simple columnar, 5. How many basic types of muscle tissue are there? _________________ stratified squamous (nonkeratinized), stratified cuboidal, stratified columnar, pseudostratified ciliated columnar, and T transitional epithelium he human body is organized into structural levels of organization. The simplest ▶▶ Prepared slides of mesenchyme; of level is the chemical level, where atoms combine to form molecules. Molecules adipose, areolar, reticular, and dense form organelles, the functional units of cells. The cellular level is the functional (both regular and irregular connective unit of life. In humans and other multicellular organisms, cells function together to tissues); of hyaline and elastic cartilage; maintain homeostasis in the body. of fibrocartilage; of bone (x.s.); and of Groups of cells that are similar in structure and function are called tissues. blood The four primary tissue types—epithelium, connective tissue, nervous tissue, and ▶▶ Prepared slide of nervous tissue (spinal muscle—have distinctive structures, patterns, and functions. The four primary tis- sues are further divided into subcategories, as described shortly. cord smear) To perform specific body functions, the tissues are organized into organs such ▶▶ Prepared slides of skeletal, cardiac, as the heart, kidneys, and lungs. Most organs contain several representatives of the and smooth muscle (l.s.) 65  66 Exercise 6 Chemicals Organelles Cells Tissues Organs Organ systems Figure 6.1 Levels of structural organization. primary tissues, and the arrangement of these tissues deter- The next level of organization is the organ system level, mines the organ’s structure and function. Thus histology, the where organs work together. Figure 6.1 summarizes the struc- study of tissues, complements a study of gross anatomy and tural level of organization in the body from the simplest to the provides the structural basis for a study of organ physiology. most complex. 6 Epithelial Tissue Epithelial tissue, or an epithelium, is a sheet of cells that covers Based on cell shape, epithelia are classified into three a body surface or lines a body cavity. It occurs in the body as categories: (1) covering and lining epithelium and (2) glandular epithelium. Epithelial functions include protection, absorption, filtra- Squamous (scalelike) tion, excretion, secretion, and sensory reception. For example, Cuboidal (cubelike) the epithelium covering the body surface protects against bac- Columnar (column-shaped) terial invasion and chemical damage. Epithelium specialized to absorb substances lines the stomach and small intestine. In The terms denoting shape and arrangement of the epithelial the kidney tubules, the epithelium absorbs, secretes, and filters. cells are combined to describe the epithelium fully. Stratified Secretion is a specialty of glandular epithelium. epithelia are named according to the cells at the apical surface The following characteristics distinguish epithelial tissues of the epithelial sheet, not those resting on the basement from other types: membrane. Polarity. The membranes always have one free surface, Apical surface called the apical surface, and typically that surface is signifi- cantly different from the basal surface. Specialized contacts. Cells fit closely together to form membranes, or sheets of cells, and are bound together by specialized junctions. Supported by connective tissue. The cells are attached to and supported by an adhesive basement membrane, which is an acellular material secreted partly by the epithelial cells Apical surface (basal lamina) and connective tissue cells (reticular lamina) that lie next to each other. Avascular but innervated. Epithelial tissues are supplied by nerves but have no blood supply of their own (are avascu- Basal surface Basal surface lar). Instead they depend on diffusion of nutrients from the Simple Stratified underlying connective tissue. (a) Regeneration. If well nourished, epithelial cells can easily divide to regenerate the tissue. This is an important char- acteristic because many epithelia are subjected to a good deal of abrasion. The covering and lining epithelia are classified according to two criteria—arrangement or relative number of layers and cell shape (Figure 6.2). On the basis of arrangement, epithelia are classified as follows: Simple epithelia consist of one layer of cells attached to the basement membrane. Stratified epithelia consist of two or more layers of cells. Squamous Cuboidal Columnar (b) Figure 6.2 Classification of epithelia. (a) Classification based on number of cell layers. (b) Classification based on cell shape. Instructors may assign this figure as an Art Labeling Activity using Mastering A&P™  Classification of Tissues 67 There are, in addition, two less easily categorized types of Epithelial cells forming glands are highly specialized to epithelia. remove materials from the blood and to manufacture them into new materials, which they then secrete. There are two types of Pseudostratified epithelium is actually a simple columnar glands, endocrine and exocrine. Endocrine glands lose their sur- epithelium (one layer of cells), but because its cells vary face connection (duct) as they develop; thus they are referred to in height and the nuclei lie at different levels above the as ductless glands. They secrete hormones into the extracellular basement membrane, it gives the false appearance of being fluid, and from there the hormones enter the blood or the lym- stratified. This epithelium is often ciliated. phatic vessels that weave through the glands. Exocrine glands Transitional epithelium is a rather peculiar stratified squa- retain their ducts, and their secretions empty through these mous epithelium formed of rounded, or “plump,” cells with ducts either to the body surface or into body cavities. The exo- the ability to slide over one another to allow the organ to be crine glands include the sweat and oil glands, liver, and pancreas. 6 stretched. Transitional epithelium is found only in urinary The most common types of epithelia, their characteris- system organs subjected to stretch, such as the bladder. tic locations in the body, and their functions are described in The superficial cells are flattened (like true squamous cells) Figure 6.3. when the organ is full and rounded when the organ is empty. Activity 1 Examining Epithelial Tissue Under the Microscope Obtain slides of simple squamous, simple cuboidal, simple to move substances along the cell surface), and microvilli, which columnar, stratified squamous (nonkeratinized), pseudostrati- increase the surface area for absorption. Also be alert for goblet fied ciliated columnar, stratified cuboidal, stratified columnar, cells, which secrete lubricating mucus. Compare your observa- and transitional epithelia. Examine each carefully, and notice tions with the descriptions and photomicrographs in Figure 6.3. how the epithelial cells fit closely together to form intact sheets While working, check the questions in the Review Sheet at of cells, a necessity for a tissue that forms linings or the coverings the end of this exercise. A number of the questions there refer of membranes. Scan each epithelial type for modifications for to some of the observations you are asked to make during your specific functions, such as cilia (motile cell projections that help microscopic study. (a) Simple squamous epithelium Description: Single layer of flattened cells with disc-shaped central nuclei and sparse cytoplasm; the simplest of the epithelia. Air sacs of lung tissue Nuclei of squamous epithelial Function: Allows materials to pass by cells diffusion and filtration in sites where protection is not important; secretes lubricating substances in serosae. Location: Kidney glomeruli; air sacs of lungs; lining of heart, blood vessels, and lymphatic vessels; lining of ventral body cavity (serosae). Photomicrograph: Simple squamous epithelium forming part of the alveolar (air sac) walls (1403). Figure 6.3 Epithelial tissues. Simple epithelia (a). ➤  68 Exercise 6 (b) Simple cuboidal epithelium Description: Single layer of cubelike cells with large, spherical central nuclei. Simple cuboidal epithelial 6 cells Lumen Function: Secretion and absorption. Basement membrane Location: Kidney tubules; ducts and secretory portions of small glands; ovary surface. Connective tissue Photomicrograph: Simple cuboidal epithelium in kidney tubules (4303). (c) Simple columnar epithelium Description: Single layer of tall cells with round to oval nuclei; some cells bear cilia; layer may contain mucus-secreting unicellular glands (goblet cells). Goblet cells Mucus secretion Function: Absorption; secretion of mucus, enzymes, and other substances; ciliated type propels mucus (or reproductive cells) by ciliary action. Location: Nonciliated type lines most of the digestive tract (stomach to rectum), Microvilli gallbladder, and excretory ducts of some (brush border) glands; ciliated variety lines small bronchi, uterine tubes, and some regions of the uterus. Photomicrograph: Simple columnar epithelium containing goblet cells from the small intestine (6403). Figure 6.3 (continued) Epithelial tissues. Simple epithelia (b) and (c).  Classification of Tissues 69 (d) Pseudostratified columnar epithelium Description: Single layer of cells of differing heights, some not reaching the free surface; nuclei seen at different levels; may contain mucus-secreting goblet cells and bear cilia. Mucus of 6 goblet cell Cilia Function: Secretes substances, particularly mucus; propulsion of mucus by ciliary action. Pseudo- Location: Nonciliated type in male’s stratified sperm-carrying ducts and ducts of large epithelial glands; ciliated variety lines the trachea, most layer of the upper respiratory tract. Basement Trachea membrane Photomicrograph: Pseudostratified ciliated columnar epithelium lining the human trachea (5303). (e) Stratified squamous epithelium Description: Thick membrane composed of several cell layers; basal cells are cuboidal or columnar and metabolically active; surface cells are flattened (squamous); in the keratinized type, the surface cells are full of keratin and dead; basal cells are active in mitosis and produce the cells of the more superficial layers. Stratified squamous epithelium Function: Protects underlying tissues in areas subjected to abrasion. Location: Nonkeratinized type forms the Nuclei moist linings of the esophagus, mouth, and vagina; keratinized variety forms the Basement epidermis of the skin, a dry membrane. membrane Connective tissue Photomicrograph: Stratified squamous epithelium lining the esophagus (2803). ➤ Figure 6.3 (continued) Stratified epithelia (d) and (e).  70 Exercise 6 (f) Stratified cuboidal epithelium Description: Generally two layers of cubelike cells. Basement membrane 6 Cuboidal Function: Protection. epithelial cells Location: Largest ducts of sweat glands, mammary glands, and salivary glands. Lumen of duct Photomicrograph: Stratified cuboidal epithelium forming a salivary gland duct (2903). (g) Stratified columnar epithelium Description: Several cell layers; basal cells usually cuboidal; Basement superficial cells elongated membrane and columnar. Stratified columnar epithelium Function: Protection; secretion. Location: Rare in the body; small amounts in male urethra and in large ducts of some glands. Connective tissue Urethra Photomicrograph: Stratified columnar epithelium lining of the male urethra (2903). Figure 6.3 (continued) Epithelial tissues. Stratified epithelia (f) and (g).  Classification of Tissues 71 (h) Transitional epithelium Description: Resembles both stratified squamous and stratified cuboidal; basal cells cuboidal or columnar; surface cells dome shaped or squamouslike, depending on degree of organ stretch. 6 Transitional epithelium Function: Stretches readily and permits distension of urinary organ by contained urine. Location: Lines the ureters, urinary bladder, and part of the urethra. Basement membrane Connective tissue Photomicrograph: Transitional epithelium lining the urinary bladder, relaxed state (3653); note the bulbous, or rounded, appearance of the cells at the surface; these cells flatten and become elongated when the bladder is filled with urine. Figure 6.3 (continued) Stratified epithelia (h). Connective Tissue Connective tissue is found in all parts of the body as discrete body. For example, bones are composed of connective tissue structures or as part of various body organs. It is the most abun- (bone, or osseous tissue), and they protect and support other dant and widely distributed of the tissue types. body tissues and organs. The ligaments and tendons (dense There are four main types of adult connective tissue. These regular connective tissue) bind the bones together or connect are connective tissue proper, cartilage, bone, and blood. Con- skeletal muscles to bones. nective tissue proper has two subclasses: loose connective Areolar connective tissue (Figure 6.4, p. 72) is a soft pack- tissues (areolar, adipose, and reticular) and dense connective aging material that cushions and protects body organs. Adipose tissues (dense regular, dense irregular, and elastic). Connec- (fat) tissue provides insulation for the body tissues and a source tive tissues perform a variety of functions, but they primarily of stored energy. protect, support, insulate, and bind together other tissues of the  72 Exercise 6 Cell types Extracellular matrix Ground substance Macrophage Fibers Collagen fiber Elastic fiber Reticular fiber 6 Fibroblast Lymphocyte Fat cell Capillary Mast cell Figure 6.4 Areolar connective tissue: A model connective tissue. This tissue underlies epithelia and surrounds capillaries. Note the various cell types and Neutrophil the three classes of fibers (collagen, reticular, elastic) embedded in the ground substance. Instructors may assign this figure as an Art Labeling Activity using Mastering A&P™ The characteristics of connective tissue include the reside in cavities in the matrix called lacunae. The fibers, which following: provide support, include collagen (white) fibers, elastic (yel- low) fibers, and reticular (fine collagen) fibers. Of these, the Common origin. All connective tissues are derived from collagen fibers are most abundant. embryonic tissue (mesenchyme). The connective tissues have a common structural plan Degrees of vascularity. Many types of connective tissue seen best in areolar connective tissue (Figure 6.4). Since all have a rich blood supply. Exceptions include cartilage, other connective tissues are variations of areolar, it is consid- which is avascular, and dense connective tissue, which is ered the model, or prototype, of the connective tissues. Notice poorly vascularized. that areolar tissue has all three varieties of fibers, but they Extracellular matrix. There is a great deal of noncellular, are sparsely arranged in its transparent gel-like ground sub- nonliving meterial (matrix) between the cells of connec- stance (Figure 6.4). The cell type that secretes its matrix is the tive tissue. The composition and amount of matrix vary for fibroblast, but a wide variety of other cells (including phago- connective tissues. cytic cells, such as macrophages, and certain white blood cells and mast cells that act in the inflammatory response) are pres- The extracellular matrix has two components—ground sub- ent as well. The more durable connective tissues, such as bone, stance and fibers. The ground substance is composed chiefly cartilage, and the dense connective tissues, characteristically of interstitial fluid, cell adhesion proteins, and proteoglycans. have a firm ground substance and many more fibers. Depending on its specific composition, the ground substance Figure 6.5 lists the general characteristics, location, and may be liquid, semisolid, gel-like, or very hard. When the matrix function of some of the connective tissues found in the body. is firm, as in cartilage and bone, the connective tissue cells  Classification of Tissues 73 Activity 2 Examining Connective Tissue Under the Microscope Obtain prepared slides of mesenchyme; of adipose, areolar, histamine, which makes capillaries more permeable during reticular, dense regular, elastic, and dense irregular connective inflammation and allergies. tissue; of hyaline and elastic cartilage and fibrocartilage; of In adipose tissue, locate a “signet ring” cell, a fat cell in osseous connective tissue (bone); and of blood. Compare your which the nucleus can be seen pushed to one side by the large, observations with the views illustrated in Figure 6.5. fat-filled vacuole that appears to be a large empty space. Also Distinguish the living cells from the matrix. Pay particular notice how little matrix there is in adipose (fat) tissue. Distin- 6 attention to the denseness and arrangement of the matrix. For guish the living cells from the matrix in the dense connective example, notice how the matrix of the dense regular and dense tissue, bone, and hyaline cartilage preparations. irregular connective tissues, respectively making up tendons Scan the blood slide at low and then high power to examine and the dermis of the skin, is packed with collagen fibers. Note the general shape of the red blood cells. Then, switch to the oil also that in the regular variety (tendon), the fibers are all run- immersion lens for a closer look at the various types of white ning in the same direction, whereas in the dermis they appear blood cells. How does the matrix of blood differ from all other to be running in many directions. connective tissues? While examining the areolar connective tissue, notice how much empty space there appears to be (areol = small empty ______________________________________________________ space), and distinguish the collagen fibers from the coiled elas- tic fibers. Identify the starlike fibroblasts. Also, try to locate a ______________________________________________________ mast cell, which has large, darkly staining granules in its cyto- plasm (mast = stuffed full of granules). This cell type releases (a) Embryonic connective tissue: mesenchyme Description: Embryonic connective tissue; gel-like ground substance containing fibers; star-shaped mesenchymal cells. Fibers Function: Gives rise to all other connective tissue types. Ground substance Location: Primarily in embryo. Mesenchymal cell Photomicrograph: Mesenchymal tissue, an embryonic connective tissue (6273); the clear-appearing background is the fluid ground substance of the matrix; notice the fine, sparse fibers. Figure 6.5 Connective tissues. Embryonic connective tissue (a). ➤  74 Exercise 6 (b) Connective tissue proper: loose connective tissue, areolar Description: Gel-like matrix with all three fiber types; cells: fibroblasts, macrophages, mast cells, and some white blood cells. Collagen fibers Function: Wraps and cushions organs; its macrophages phagocytize bacteria; plays 6 important role in inflammation; holds and Fibroblast conveys tissue fluid. nuclei Location: Widely distributed under epithelia of body, e.g., forms lamina propria of mucous membranes; packages organs; surrounds capillaries. Elastic fibers Epithelium Lamina Photomicrograph: Areolar connective tissue, a soft packaging propria tissue of the body (3653). (c) Connective tissue proper: loose connective tissue, adipose Description: Matrix as in areolar, but very sparse; closely packed adipocytes, or fat cells, have nucleus pushed to the side by large fat droplet. Function: Provides reserve fuel; insulates against heat loss; supports and protects organs. Vacuole containing fat droplet Location: Under skin; around kidneys and eyeballs; within abdomen; in breasts. Nuclei of fat cells Photomicrograph: Adipose tissue from the subcutaneous layer under the skin (1103). Figure 6.5 (continued) Connective tissues. Connective tissue proper (b) and (c).  Classification of Tissues 75 (d) Connective tissue proper: loose connective tissue, reticular Description: Network of reticular fibers in a typical loose ground substance; reticular cells lie on the network. Function: Fibers form a soft internal skeleton (stroma) that supports other cell types, including white blood cells, mast cells, and 6 macrophages. White blood cell (lymphocyte) Location: Lymphoid organs (lymph nodes, bone marrow, and spleen). Reticular fibers Spleen Photomicrograph: Dark-staining network of reticular connective tissue fibers forming the internal skeleton of the spleen (3503). (e) Connective tissue proper: dense regular connective tissue Description: Primarily parallel collagen fibers; a few elastic fibers; major cell type is the fibroblast. Function: Attaches muscles to bones or to other muscles; attaches bones to bones; withstands great tensile stress when pulling Collagen force is applied in one direction. fibers Location: Tendons, most ligaments, aponeuroses. Nuclei of fibroblasts Shoulder joint Ligament Photomicrograph: Dense regular connective tissue from a tendon (5903). Tendon Figure 6.5 (continued) Connective tissue proper (d) and (e). ➤  76 Exercise 6 (f) Connective tissue proper: elastic connective tissue Description: Dense regular connective tissue containing a high proportion of elastic fibers. Function: Allows recoil of tissue following stretching; maintains pulsatile flow of blood through arteries; aids passive recoil of lungs 6 following inspiration. Elastic fibers Location: Walls of large arteries; within certain ligaments associated with the vertebral column; within the walls of the bronchial tubes. Aorta Heart Photomicrograph: Elastic connective tissue in the wall of the aorta (2503). (g) Connective tissue proper: dense irregular connective tissue Description: Primarily irregularly arranged collagen fibers; some elastic fibers; major cell type is the fibroblast. Nuclei of Function: Able to withstand tension exerted fibroblasts in many directions; provides structural strength. Location: Fibrous capsules of organs and of joints; dermis of the skin; submucosa of digestive tract. Collagen fibers Fibrous joint capsule Photomicrograph: Dense irregular connective tissue from the dermis of the skin (2103). Figure 6.5 (continued) Connective tissues. Connective tissue proper (f) and (g).  Classification of Tissues 77 (h) Cartilage: hyaline Description: Amorphous but firm matrix; collagen fibers form an imperceptible network; chondroblasts produce the matrix and, when mature (chondrocytes), lie in lacunae. Chondrocyte Function: Supports and reinforces; serves as in lacuna resilient cushion; resists compressive stress. 6 Location: Forms most of the embryonic skeleton; covers the ends of long bones in joint cavities; forms costal cartilages of the ribs; cartilages of the nose, trachea, and larynx. Matrix Costal cartilages Photomicrograph: Hyaline cartilage from a costal cartilage of a rib (4703). (i) Cartilage: elastic Description: Similar to hyaline cartilage, but more elastic fibers in matrix. Function: Maintains the shape of a structure while allowing great flexibility. Chondrocyte in lacuna Location: Supports the external ear Matrix (auricle); epiglottis. Photomicrograph: Elastic cartilage from the human ear auricle; forms the flexible skeleton of the ear (5103). Figure 6.5 (continued) Cartilage (h) and (i). ➤  78 Exercise 6 (j) Cartilage: fibrocartilage Description: Matrix similar to but less firm than matrix in hyaline cartilage; thick collagen fibers predominate. Function: Tensile strength with the ability to absorb compressive shock. 6 Location: Intervertebral discs; pubic symphysis; discs of knee joint. Chondrocytes in lacunae Intervertebral discs Collagen fiber Photomicrograph: Fibrocartilage of an intervertebral disc (1603). Special staining produced the blue color. (k) Bones (osseous tissue) Description: Hard, calcified matrix containing many collagen fibers; osteocytes lie in lacunae. Very well vascularized. Central Function: Bone supports and protects canal (by enclosing); provides levers for the muscles to act on; stores calcium and other minerals Osteocytes and fat; marrow inside bones is the site for in lacunae blood cell formation (hematopoiesis). Lamella Location: Bones. Photomicrograph: Cross-sectional view of bone (1753). Figure 6.5 (continued) Connective tissues. Cartilage (j) and bone (k).  Classification of Tissues 79 (l) Blood Description: Red and white blood cells in a fluid matrix (plasma). Plasma Neutrophil Function: Transport of respiratory gases, nutrients, wastes, and other substances. 6 Red blood cells Location: Contained within blood vessels. Lymphocyte Photomicrograph: Smear of human blood (10003); shown are two white blood cells (neutrophil and lymphocyte) surrounded by red blood cells. Figure 6.5 (continued) Blood (l). Nervous Tissue Activity 3 Nervous tissue is made up of two major cell populations. The Examining Nervous Tissue neuroglia are special supporting cells that protect, support, and Under the Microscope insulate the more delicate neurons. The neurons are highly spe- cialized to receive stimuli (excitability) and to generate electrical Obtain a prepared slide of a spinal cord smear. Locate a neuron signals that may be sent to all parts of the body (conductivity). and compare it to Figure 6.6, p. 80. Keep the light dim—this The structure of neurons is markedly different from that of will help you see the cellular extensions of the neurons. (See all other body cells. They have a nucleus-containing cell body, also Figure 15.2 in Exercise 15.) and their cytoplasm is drawn out into long extensions (cell pro- cesses)—sometimes as long as 1 m (about 3 feet), which allows a single neuron to conduct an electrical signal over relatively long distances. (More detail about the anatomy of the different classes of neurons and neuroglia appears in Exercise 15.)  80 Exercise 6 Nervous tissue Description: Neurons are branching cells; cell processes that may be quite long extend from the nucleus-containing cell body; also contributing to nervous Nuclei of tissue are nonexcitable supporting cells. supporting cells Cell body Neuron processes 6 Axon Dendrites Cell body of a neuron Function: Neurons transmit electrical signals from sensory receptors and to effectors (muscles and glands); supporting cells support and protect neurons. Neuron Location: Brain, spinal processes cord, and nerves. Photomicrograph: Neurons (3703). Figure 6.6 Nervous tissue. Muscle Tissue Muscle tissue (Figure 6.7) is highly specialized to contract and vessels). Typically it has two layers that run at right angles to produces most types of body movement. The three basic types each other; consequently its contraction can constrict or dilate of muscle tissue are described briefly here. the lumen (cavity) of an organ and propel substances. Smooth Skeletal muscle, the flesh of the body, is attached to the muscle cells are quite different in appearance from those of skeleton. It is under voluntary control (consciously controlled), skeletal or cardiac muscle. No striations are visible, and the and its contraction moves the limbs and other external body uninucleate smooth muscle cells are spindle-shaped (tapered parts. The cells of skeletal muscles are long, cylindrical, non- at the ends, like a candle). Like cardiac muscle, smooth muscle branching, and multinucleate (several nuclei per cell), with the is under involuntary control. nuclei pushed to the periphery of the cells; they have obvious striations (stripes). Cardiac muscle is found only in the heart. As it contracts, the heart acts as a pump, propelling the blood into the blood Activity 4 vessels. Cardiac muscle, like skeletal muscle, has striations, but Examining Muscle Tissue cardiac cells are branching uninucleate cells that interdigitate Under the Microscope (fit together) at junctions called intercalated discs. These struc- tural modifications allow the cardiac muscle to act as a unit. Obtain and examine prepared slides of skeletal, cardiac, and Cardiac muscle is under involuntary control, which means that smooth muscle. Notice their similarities and dissimilarities in we cannot voluntarily or consciously control the operation of your observations and in the illustrations and photomicro- the heart. graphs in Figure 6.7. Smooth muscle is found mainly in the walls of hollow organs (digestive and urinary tract organs, uterus, blood  Classification of Tissues 81 (a) Skeletal muscle Description: Long, cylindrical, multinucleate cells; obvious striations. Part of muscle Function: Voluntary movement; locomotion; fiber (cell) 6 manipulation of the environment; facial expression; voluntary control. Nuclei Location: In skeletal muscles attached to bones or occasionally to skin. Striations Photomicrograph: Skeletal muscle (approx. 5503). Notice the obvious banding pattern and the fact that these large cells are multinucleate. (b) Cardiac muscle Description: Branching, striated, generally uninucleate cells that interdigitate at specialized junctions called intercalated discs. Nucleus Intercalated discs Function: As it contracts, cardiac muscle propels blood into the circulation; involuntary control. Striations Location: The walls of the heart. Photomicrograph: Cardiac muscle (7753); notice the striations, branching of cells, and the intercalated discs. Figure 6.7 Muscle tissues. Skeletal muscle (a) and cardiac muscle (b). ➤  82 Exercise 6 (c) Smooth muscle Description: Spindle-shaped cells with central nuclei; no striations; cells arranged closely to form sheets. 6 Function: Propels substances (foodstuffs, urine) or a baby along internal passageways; involuntary control. Smooth muscle cell Nucleus Location: Mostly in the walls of hollow organs. Photomicrograph: Smooth muscle cells (2653). Figure 6.7 (continued) Muscle Tissues. Smooth muscle (c).