Embryonic stem cell research is advancing fast. In 2005, for instance, Korean researchers established eleven stem cell lines that are an exact genetic match to donor patients affected by diseases. As Section 13.5 explains, however, this research has opponents as well as proponents. Do adult stem cells hold the same potential? Adults retain patches of stem cells in bone marrow, fat, and other tissues. Duke University researchers experimenting with stem cells from liposuctioned fat have already prodded descendant cells to become bone, cartilage, or nerve cells. Before such lineages can be used for treating diseases and injuries, researchers must learn how to control the steps from stem cells to normal—and stable—differentiated cells. In this respect, adult stem cells are not as versatile as the embryonic ones. They form fewer kinds of tissues. Also, the lineages now being maintained in culture for long periods of time tend to lose the capacity to differentiate. Stem cell research is a fitting introduction to this unit, which deals with animal anatomy (how the body is put together) and physiology (how it works in the environment). These cells invite you to reflect on who we are, where we came from, and where medical research may be taking us. In this chapter, you will start with the four basic types of animal tissues—epithelial, connective, muscle, and nervous tissues. A tissue, remember, is a community of cells and intercellular substances that carry out one or more tasks, such as muscle tissue contraction. An organ is a structural unit of two or more tissues organized in proportions and patterns necessary to carry out specific tasks. Your heart is an organ that has certain proportions and arrangements of all four types of tissues. In organ systems, two or more Hundreds of millions of years ago, in the forerunners of animals, cells started to interact as part of functional units called tissues. Today, animal tissues arise from the tiny ball of cells that form by way of divisions of a fertilized egg. The cells in that ball are not yet committed to being a particular kind of cell; they are undifferentiated (Figure 33.1). However, they are the body’s first stem cells. Cell lineages descended from them will differentiate and give rise to all of the body’s specialized tissues and organs. In the laboratory, embryonic stem cells divide again and again. Their descendants often can be coaxed to develop into the specialized cells of blood, muscle, nerve, and other tissues. Why coax them to do so? Stem cells might one day help mend tissue damage resulting from injury or diseases, including Parkinson’s disease, leukemia, and heart attacks. Actor Christopher Reeve, known for his motion picture role as Superman, stoked public awareness of the potential for stem cell research. In a 1995 showjumping competition, Reeve was thrown from his horse and landed on his head. The fall paralyzed him; he could not even breathe on his own. Medical advances kept him alive and gave him hope. He became an advocate for the disabled and a champion of human embryonic stem cell research (Figure 33.2). Where did the laboratory stocks of embryonic stem cell lineages come from? They were established from cells removed from aborted embryos, cells that formed by in vitro fertilization but were not used, and cells in the blood of discarded umbilical cords. Such cells have already shown potential for repaired spinal cord injuries. In laboratory rats, dividing and differentiating cells replaced damaged tissues, and the rats regained some use of their legs. 33 ANIMAL TISSUES AND ORGAN SYSTEMS Open or Close the Stem Cell Factories? Figure 33.1 Left, moment before the union of two cells— sperm and egg. Right, after fertilization, a tiny ball of stem cells, not yet differentiated with a capacity to give rise to all of the human body’s specialized cells, tissues, and organ systems. Watch the video online! 15997_33_c33_p558_571.qxd 10/4/05 2:38 PM Page 558
Transcript
Embryonic stem cell research is advancing fast. In 2005,for instance, Korean researchers established eleven stemcell lines that are an exact genetic match to donor patientsaffected by diseases. As Section 13.5 explains, however, thisresearch has opponents as well as proponents. Do adultstem cells hold the same potential? Adults retain patches of stem cells in bone marrow, fat, and other tissues. DukeUniversity researchers experimenting with stem cells fromliposuctioned fat have already prodded descendant cells tobecome bone, cartilage, or nerve cells.
Before such lineages can be used for treating diseasesand injuries, researchers must learn how to control the stepsfrom stem cells to normal—and stable—differentiated cells.In this respect, adult stem cells are not as versatile as theembryonic ones. They form fewer kinds of tissues. Also, thelineages now being maintained in culture for long periodsof time tend to lose the capacity to differentiate.
Stem cell research is a fitting introduction to this unit,which deals with animal anatomy (how the body is puttogether) and physiology (how it works in the environment).These cells invite you to reflect on who we are, where wecame from, and where medical research may be taking us.
In this chapter, you will start with the four basic types ofanimal tissues—epithelial, connective, muscle, and nervoustissues. A tissue, remember, is a community of cells andintercellular substances that carry out one or more tasks,such as muscle tissue contraction. An organ is a structuralunit of two or more tissues organized in proportions andpatterns necessary to carry out specific tasks. Your heart isan organ that has certain proportions and arrangements ofall four types of tissues. In organ systems, two or more
Hundreds of millions of years ago, in the forerunners ofanimals, cells started to interact as part of functional unitscalled tissues. Today, animal tissues arise from the tiny ball of cells that form by way of divisions of a fertilized egg. Thecells in that ball are not yet committed to being a particularkind of cell; they are undifferentiated (Figure 33.1). However,they are the body’s first stem cells. Cell lineages descendedfrom them will differentiate and give rise to all of the body’sspecialized tissues and organs.
In the laboratory, embryonic stem cells divide again andagain. Their descendants often can be coaxed to developinto the specialized cells of blood, muscle, nerve, and othertissues. Why coax them to do so? Stem cells might one dayhelp mend tissue damage resulting from injury or diseases,including Parkinson’s disease, leukemia, and heart attacks.
Actor Christopher Reeve, known for his motion picturerole as Superman, stoked public awareness of the potentialfor stem cell research. In a 1995 showjumping competition,Reeve was thrown from his horse and landed on his head.The fall paralyzed him; he could not even breathe on hisown. Medical advances kept him alive and gave him hope.He became an advocate for the disabled and a champion of human embryonic stem cell research (Figure 33.2).
Where did the laboratory stocks of embryonic stem celllineages come from? They were established from cellsremoved from aborted embryos, cells that formed by in vitrofertilization but were not used, and cells in the blood ofdiscarded umbilical cords. Such cells have already shownpotential for repaired spinal cord injuries. In laboratory rats, dividing and differentiating cells replaced damagedtissues, and the rats regained some use of their legs.
33 ANIMAL TISSUES ANDORGAN SYSTEMS
Open or Close the Stem Cell Factories?
Figure 33.1 Left,moment before theunion of two cells—sperm and egg. Right,after fertilization, a tinyball of stem cells, notyet differentiated with a capacity to give riseto all of the humanbody’s specializedcells, tissues, and organ systems.
organs and other body components interact physically,chemically, or both in a common task, as when a beatingheart forces blood through interconnected vessels.
Throughout this unit, you will come across examples of a concept outlined in Chapter 28. Cells, tissues, and organsinteract smoothly when the body’s internal environment isbeing maintained within a range that individual cells cantolerate. That state, recall, is called homeostasis. In mostkinds of animals, blood and interstitial fluid make up theinternal environment. It is worth a reminder: Whether youare considering the body of a flatworm or salmon, a bird or human, you will see that it must do the following:
1. Function in ways that ensure homeostasis.
2. Acquire and distribute raw materials to individualcells and dispose of wastes.
3. Protect tissues against injury or attack.
4. Reproduce and, in many species, nourish and protectoffspring through early growth and development.
Figure 33.2 Actor Christopher Reeve before and after a spinal cord injury left him paralyzed. He died in 2004.
How Would You Vote?Many researchers believe that embryonic stem cell studieswould greatly benefit medical science. Others object to theuse of any cells from human embryos. Should researchers be allowed to start embryonic stem cell lines from humanembryos that were frozen but were never used for in vitrofertilization? See BiologyNow for details, then vote online.
Links to Earlier ConceptsWith this unit, you have now arrived at the tissue and organsystem levels of biological organization for animals (Section1.1). This chapter expands on the nature of multicelled bodyplans (26.1, 28.1–28.3). It builds on your knowledge of theorigin and evolution of animal tissues (25.1, 25.2). You maywish to refresh your understanding of cell junctions (4.9),membrane transport proteins (5.2, 5.4), aerobic respiration(8.1), and energy conversion pathways (8.6).
Key Concepts
BASIC TYPES OF ANIMAL TISSUESEpithelial, connective, muscle, and nervous tissues are the basic categories of tissues in nearly all animals.
Epithelia line the body surface and its internal cavities and tubes. They have protective and secretory functions.
Connective tissues bind, support, strengthen, protect,and insulate other tissues. They include soft connectivetissues, cartilage, bone, blood, and adipose tissue.
Muscle tissues help move the body and its parts. Thethree kinds are skeletal, cardiac, and smooth muscle tissue.
Nervous tissue provides local and long-distance lines of communication among cells. It consists of neurons andneuroglia. Sections 33.1–33.4
INTRODUCING ANIMAL ORGAN SYSTEMSVertebrate organ systems compartmentalize the tasks ofsurvival and reproduction for the body as a whole; theyshow a division of labor. Different systems arise fromectoderm, mesoderm, and endoderm, the primary tissuelayers that form in the early embryo. Section 33.5
CASE STUDY: AN INTEGUMENTARY SYSTEMHuman skin is an example of an organ system. It includesepithelial layers, connective tissue, adipose tissue, glands,blood vessels, and sensory receptors. It helps protect thebody from injury and some pathogens, conserve water andcontrol internal temperature, excrete certain wastes, anddetect some external stimuli. Section 33.6
TYPE: Simple squamous DESCRIPTION: Friction-reducing slick, single layer of flattened cells COMMON LOCATIONS: Lining of blood and lymph vessels, heart; air sacs of lungs; peritoneum FUNCTION: Diffusion, filtration, secretion of lubricants
TYPE: Simple cuboidal DESCRIPTION: Single layer of squarish cells COMMON LOCATIONS: Ducts, secretory part of small glands; retina; kidney tubules; ovaries, testes; bronchioles FUNCTION: Secretion, absorption
TYPE: Simple columnar DESCRIPTION: Single layer of tall cells; free surface may have cilia, mucus- secreting glandular cells, microvilli COMMON L OCATIONS: Glands, ducts; gut; parts of uterus; small bronchiFUNCTION: Secretion, absorption; ciliated types move substances
560 Unit VI How Animals Work
GENERAL CHARACTERISTICS
Epithelium (plural, epithelia) is a sheetlike tissue ofcells that are close together, with little extracellularmaterial between them. One free surface is exposed tothe outside environment or to some body fluid (Figure33.3). At the opposite surface, epithelial cell secretionsform a basement membrane that incorporates manyadhesion proteins, including integrins and cadherins.The proteins anchor epithelium to other tissues.
Most epithelial cells have a squamous (flattened),cuboidal, or columnar shape, as in Figure 33.3. Certaintypes are highly specialized for absorbing substances.Others are highly specialized for secreting products;they release them at their free surface. Secretion is notthe same as excretion, which refers to a concentrationand removal of substances of no use to the body.
In simple epithelium, cells form a layer that is onlyone cell thick. In stratified epithelium, the cells formtwo or more layers. The outer layer of your own skinis mostly stratified squamous epithelium.
GLANDULAR EPITHELIUM
Gland cells occur only in epithelia. Such cells secreteproducts, unrelated to their own metabolism, that are
Epithelial Tissue
Recall, from Section 25.1, that ectodermis the first layer of cells to form in the embryos of nearly allanimals. Epithelia arises from this primary tissue layer.
Figure 33.3 Characteristics ofepithelium, an animal tissue with a free surface exposed to a bodyfluid or the outside environment.
(a) Section near the surface of stratified squamous epithelium. It consists of multiple layers andcells that become flattened nearthe free surface. (b) In all epithelia,the opposite surface rests on abasement membrane that anchorsit to an underlying tissue.
(c) Light micrographs of threesimple epithelia, showing the three most common shapes of cells in this tissue. c
Strands (rows ofproteins) runningparallel with thefree surface of thetissue; they blockleaking betweenadjoining cells.
Adjoining cells arewelded together at a mass of proteins,which is anchoredunder the plasmamembrane by tuftsof intermediatefilaments of thecytoskeleton.
Cylindrical arrays ofproteins span theplasma membrane of adjoining cells. They pair up as openchannels for signalsbetween cells.
Tight junction
Gap junction
Adhering junction
Chapter 33 Animal Tissues and Organ Systems 561
to be used elsewhere. In most animals, gland cells areconcentrated in glands, which are saclike, secretoryorgans that open to the free epithelial surface.
Exocrine glands secrete many substances, such asoils, mucus, saliva, tears, milk, digestive enzymes, andearwax. They have ducts or tubes that open onto thefree epithelial surface (Figures 33.3c and 33.4).
Endocrine glands have no ducts; they secrete theirproducts, hormones, directly into interstitial fluid. Thehormone molecules typically diffuse into neighboringblood capillaries, and the circulatory system transportsthem to target cells in tissues that are typically somedistance away (Chapter 36).
CELL JUNCTIONS
In epithelia, as in most tissues, cell junctions connectadjoining cells. Adhering junctions function like spotwelds and lock adjoining cells together (Figure 33.5a).They are profuse in skin and other tissues subjectedto ongoing abrasion. Collectively, tight junctions stopmost substances from leaking across a tissue. Rows ofproteins fuse each cell to its neighbors and form tightseals (Figure 33.5b). Dissolved substances must passthrough cells to get to the opposite surface. Membranetransport proteins are selective about which ions andmolecules can enter and leave the cells (Section 5.4).
Think about the stomach’s acidic fluid. If it were toleak across the stomach’s epithelial lining, it woulddigest proteins of your body instead of the proteinsyou eat. That is what happened in people with pepticulcers. The stomach lining was breached, typically bya bacterial infection, and strong acid leaked into theabdominal cavity (Section 21.1).
Gap junctions permit ions and small molecules topass freely from the cytoplasm of one cell to another(Figure 33.4c). These open communication channelsare most abundant in heart muscle and other tissues inwhich the action of cells must be swiftly coordinated.
Epithelia are sheetlike tissues that line the body’s surface and its cavities, ducts, and tubes. Epithelia have one freesurface exposed to the outside environment or a body fluid.Glands are secretory organs derived from epithelium. Celljunctions structurally and functionally link adjoining cells.
pigmentedcell
pore that opens at skin surface
mucousgland
poisongland
Figure 33.4 Frog glandular epithelium. This frog (Dendrobates) makes one of the mostlethal glandular secretions known. Natives of one Colombian tribe use its exocrine glandsecretion to poison tips of blowgun darts. Pigment-rich epithelial cells impart color to the skin. The skin of all poisonous frogs has distinctive colors and patterns that evolved as a clear warning signal. In essence, it says to predators, “Don’t even think about it.”
Figure 33.5 Animated! Examples of animal cell junctions.
irregular connective tissue is a component of skin. Itsupports intestinal muscles and also forms protectivecapsules around organs that do not stretch much.
Fibrous, regular connective tissue has orderly rowsof fibroblasts between parallel, tightly packed bundlesof fibers (Figure 33.6c). This organization helps keepthe tissue from being torn apart when placed undermechanical stress. Tendons and ligaments are mainlydense, regular connective tissue. All tendons connectskeletal muscle to bones; ligaments attach one bone toanother. In ligaments, elastic fibers in the tissue matrixfacilitate movements around joints.
SPECIALIZED CONNECTIVE TISSUES
Cartilage is a tissue of fine collagen fibers packed in arubbery, compression-resistant matrix. Specialized cellssecrete the rubbery material, chondrin, which in timeimprisons them (Figure 33.6d ). Sharks, recall, have askeleton of cartilage. Your own skeleton started out ascartilage, but bone tissue replaced most of it. Cartilagesupports the outer ear, nose, and throat. It protectsand cushions joints between limb bones, and betweenbones of the vertebral column. Unlike unspecializedconnective tissues, it has no blood vessels; substancesmove to and from cells by diffusion. Also unlike thosetissues, its cells do not divide often in adults.
Connective tissues have “connecting” roles in the body. They structurally or functionally support, bind, separate,and in one case insulate other tissues. They are the body’smost abundant and widely distributed tissues.
Connective tissues consist of cells scattered within anextracellular matrix of their own secretions. In all butone connective tissue (blood), fibroblasts are the maintype of cell. They make and secrete structural fibers ofcollagen and elastin into the matrix. Some tissues areclassified as soft (loose and dense connective tissues).Others are specialized (cartilage, bone tissue, adiposetissue, and blood). Each kind has characteristic types,proportions, and arrangements of components. Whiteblood cells patrol all of them.
SOFT CONNECTIVE TISSUES
Loose and dense connective tissues actually have thesame components but in different proportions. In looseconnective tissue, fibroblasts and fibers are dispersedwidely through the matrix. Figure 33.6a is an example.This tissue, the most common type in the vertebratebody, helps hold organs and epithelia in place.
In fibrous, irregular connective tissue, the matrix ispacked with many fibroblasts and collagen fibers thatare positioned every which way (Figure 33.6b). Dense,
Figure 33.6 Characteristics of connective tissue.
TYPE: Loose connective tissueDESCRIPTION: Fibers, fibroblasts,other cells loosely arranged inextensive ground substance COMMON LOCATIONS: Beneathskin and most epitheliaFUNCTION: Elasticity, diffusion
TYPE: Fibrous, irregular connective tissueDESCRIPTION: Collagen fibers,fibroblasts occupy most of theground substanceCOMMON LOCATIONS: In skin and in capsules around some organsFUNCTION: Structural support
TYPE: Fibrous, regularconnective tissueDESCRIPTION: Collagen fibersbundled in parallel, long rows offibroblasts, little ground substanceCOMMON LOCATIONS: Tendons,ligamentsFUNCTION: Strength, elasticity
TYPE: CartilageDESCRIPTION: Chondrocytes insidepliable, solid ground substanceCOMMON LOCATIONS: Nose, ends of long bones, airways, skeleton ofcartilaginous fish, vertebrate embryoFUNCTION: Support, flexion, low-friction surface for joint movements
Bone tissue is a hardened connective tissue withliving cells imprisoned in their mineralized secretions(Figure 33.6e). It is the main tissue of bones, the organsthat interact with muscles to move the body and thatsupport and protect soft internal organs. Figure 33.7shows a limb bone, which has load-bearing functions.As explained in Sections 37.3 and 37.4, some bones aresites of blood cell formation.
Adipose tissue is an energy reservoir. Fat dropletsform in many cells as excess carbohydrates and lipidsare converted to fats (Section 3.4). However, cells ofadipose tissue get so swollen with stored fat that theirnucleus and a few fibroblast nuclei are flattened andpushed to one side (Figure 33.6f ). This tissue has littleextracellular matrix but many fine blood vessels thatrapidly move fats to and from individual cells. Fatdeposits under the skin form an insulating layer andcushion certain body parts. They accumulate aroundsome organs, such as the kidneys and heart.
Blood is considered a connective tissue because itscellular components arise from stem cells in bone, aconnective tissue. Blood cells are suspended in plasma,a fluid extracellular matrix that functions in transportand heat transfer. Plasma is mostly water with diverseproteins, gases, ions, sugars, and other substancesdissolved in it. Red blood cells, white blood cells, andplatelets tumble through it (Figure 33.8). Red blood
Connective tissues support, protect, organize, or insulateother tissues. They consist of cells within an extracellularmatrix. Except for blood, each contains fibroblasts.
The matrix of soft connective tissues contains characteristicproportions and arrangements of fibroblasts and fibers.
Cartilage, bone, adipose tissue, and blood are specializedconnective tissues. Cartilage and bone are both structuralmaterials. Adipose tissue is a reservoir of stored energy.Blood, a fluid connective tissue, has transport functions.
TYPE: Bone tissueDESCRIPTION: Collagen fibers,osteocytes occupyingextensive calcium-hardenedground substanceLOCATION: Bones of allvertebrate skeletonsFUNCTION: Movement,support, protection
TYPE: Adipose tissueDESCRIPTION: Large, tightlypacked fat cells occupyingmost of ground substanceCOMMON LOCATIONS: Underskin, around the heart andkidneysFUNCTION: Energy storage,insulation, padding
cartilageon knobbyend of a longbone
compactbonetissue
spacesin spongybone tissue
Figure 33.7 Cartilage and bone tissue.Spongy bone tissue has needlelike hard partswith spaces between. Compact bone tissue is more dense. Bone, a load-bearing tissue,resists being compressed and gives giraffesand other big animals selective advantages.Big animals can ignore most predators androam farther for food and water. They gain orlose heat more slowly than smaller animalsdo; they have a lower surface-to-volume ratio.
Figure 33.8 Cellularcomponents of humanblood. Many diverseproteins, nutrients,oxygen and carbondioxide, and othersubstances also aredissolved in plasma,blood’s straw-coloredfluid portion.
whitebloodcell
redbloodcell
nucleus
cell bulging with fatdroplet
compact bone tissue
blood vessel
bone cell(osteocyte) fe
platelet
cells get oxygen to metabolically active tissues and getrid of carbon dioxide wastes. White blood cells defendand repair tissues. Platelets function in blood clotting.
TYPE: Skeletal muscleDESCRIPTION: Bundles of cylindrical,long, striated muscle fibers and manymitochondria; often reflex-activatedbut can be consciously controlledLOCATIONS: Partner of skeletal bones,against which it exerts great forceFUNCTION: Locomotion, posture;head, limb movements
TYPE: Smooth muscleDESCRIPTION: Contractile cells tapered at both ends; not striatedLOCATIONS: Wall of arteries, sphincters,stomach, intestines, urinary bladder,many other soft internal organsFUNCTION: Controlled constriction;motility (as in gut); arterial blood flow
TYPE: Cardiac muscleDESCRIPTION: Cylindrical, unevenlystriated muscle fibers that abut at theirends; signal flow through gap junctionsmake them contract rapidly as a unitLOCATIONS: Heart wallFUNCTION: Pump blood forcefullythrough circulatory system
564 Unit VI How Animals Work
Vertebrates have three types of muscle tissue: skeletal,cardiac, and smooth muscle tissues. Each type has uniqueproperties that reflect its functions.
Muscle Tissues
In muscle tissues, cells contract, or forcefully shorten inresponse to stimulation, then they relax and passivelylengthen. These tissues consist of many cells arrangedin parallel with one another, in tight or loose arrays.Coordinated contractions of layers or rings of musclesmove the whole body or its component parts. We findsmooth and striated muscle cells among invertebrates,but focus here on the kinds found in vertebrates.
SKELETAL MUSCLE TISSUE
Skeletal muscle tissue, the functional partner of bone(or cartilage), helps move and maintain the positions
Figure 33.9 (a) Skeletal muscles, the functional partners of bones, move the vertebrate body.Row after row of contractile units give skeletal muscle cells a striated appearance. (b) Striatedcells of cardiac muscle tissue. Adhering junctions are profuse in the horizontal bands; they holdabutting cells together. (c) Smooth muscle tissue, showing tapered cells and no striations.
a
33.3
BASIC TYPES OF ANIMAL TISSUES
LINKS TOSECTIONS
3.4, 4.10, 8.1
of the body and its parts. Skeletal muscle tissue hasparallel arrays of long, cylindrical muscle fibers (Figure33.9a). The fibers are not single cells. While embyrosare developing, groups of cells fuse together and formeach fiber, which ends up with multiple nuclei. Insidethe fiber are myofibrils—long strands with row afterrow of contractile units. These rows are so regular thatskeletal muscle has a striated, or striped, appearance.
Each unit, a sarcomere, is contractile. It has parallel,interacting arrays of the contractile proteins actin andmyosin (Section 4.10). The structure and function ofskeletal muscle is the focus of Section 37.6.
Skeletal muscle tissue makes up 40 percent or so ofthe weight of an average human. Reflexes activate it,but we also make it contract simply by thinking aboutit, as is happening in Figure 33.9a. That is why skeletalmuscles are commonly called “voluntary” muscles.”
CARDIAC MUSCLE TISSUE
Cardiac muscle tissue occurs only in the heart wall(Figure 33.9b). Like skeletal muscle tissue, it containssarcomeres and looks striated. Unlike skeletal muscletissue, it consists of single, branching cells that have anucleus. Cardiac muscle cells abut at their ends, whereadhering junctions help keep them from being rippedapart during forceful contractions. Signals to contract
Of all animal tissues, the one with the communication lines made of neurons exerts the most control over how the body senses and responds to changing conditions.
Muscle tissue, which functions in movement, contracts inresponse to stimulation.
Skeletal muscle is the functional partner of bones. Cardiacmuscle is present only in the heart wall. Smooth muscle tissue is present in many soft internal organs.
Neurons are the basic units of communication in nervoustissue. Different kinds detect specific stimuli, integrate information, and issue or relay commands to other tissues.
Nervous tissue also contains neuroglia. Diverse cells in thiscategory structurally and functionally support the neurons.
Figure 33.11 One good example of thecoordinated interaction between muscletissue and nervous tissue. Interneurons in the brain of this lizard, a chameleon,calculate the distance andthe direction of a tasty fly. In response to this stimulus, signals from the interneuronsflow along certain motor neurons and reachmuscle fibers inside the lizard’s long, coiled-up tongue. The tongue uncoils swiftly in theprecise direction of the fly.
Nervous tissue is composed of neurons and a varietyof cells, collectively called neuroglia, that structurallyand functionally support them. Neurons are a kind ofexcitable cell that makes up the communication linesin most nervous systems. Figure 33.10 offers a look atone type, a motor neuron.
All cells respond to stimulation, but the neuron ishighly excitable in a specific way. When it is suitablystimulated, it propagates a message along its plasmamembrane, all the way to some output zone, withoutaltering it. There, the message triggers the release ofsignaling molecules called neurotransmitters. Thesesignals diffuse to another cell that is almost but notquite touching the neuron that sent them.
Your nervous system contains more than 100 billionneurons, and half of its volume consists of neuroglialcells that keep neurons positioned and functioning asthey should. Sensory neurons detect specific stimuli,such as light and pressure. Neurons in your brain andspinal cord are called interneurons. They receive andintegrate sensory information, store the bits that holdmeaning, and coordinate the body’s short-term andlong-term responses to stimuli. Motor neurons relaycommands from the brain and spinal cord to musclecells, as in Figure 33.11, and to glands. Such signalsstimulate or inhibit activity in target cells, which is atopic of later chapters.
33.4
Figure 33.10Motor neuron,which relayssignals from thenervous systemto muscle cells.
pass so swiftly from cell to cell at gap junctions thatall of the cells in cardiac muscle tissue contract as aunit. This tissue has more mitochondria than we findin skeletal or smooth muscle tissue, because it takes acontinuous supply of ATP from aerobic respiration tokeep the heart beating nonstop. Cardiac muscle tissuedoes not store as much glycogen, so glycolysis cannotdo much when oxygen is scarce. If something ends theflow of oxygen to them, cardiac muscle cells will falteror die. That is what happens during a heart attack.
Like smooth muscle tissue, cardiac muscle tissue issaid to be “involuntary” muscle; we usually cannotmake its cells contract just by thinking about it.
SMOOTH MUSCLE TISSUE
We find layers of smooth muscle tissue in the wall ofmany soft internal organs, including the stomach,bladder, and uterus. It has single, unbranching cells,tapered at both ends, with one centrally positionednucleus. Contractile units are not arranged in orderlyrepeating fashion, as they are in skeletal and cardiacmuscle tissue, so smooth muscle tissue is not striated(Figure 33.9c). Even so, cells of this tissue do containactin and myosin filaments, which are anchored to theplasma membrane by intermediate filaments.
Smooth muscle tissue contracts more slowly thanskeletal muscle, but its contractions can be sustainedmuch longer. Contractions drive many internal events,as when they propel material through the gut, shrinkthe diameter of arteries, and close sphincters.
proximal (closestto trunk or topoint of origin ofa body part)
distal (farthestfrom trunk or frompoint of origin ofa body part)
transverseplane(yellow)
ANTERIOR
(of two body parts,the one farthest from head)
INFERIOR
(at or nearback of body)
POSTERIOR
(of two body parts,the one closer to head)
SUPERIOR
midsagittalplane(green)
566 Unit VI How Animals Work
TISSUE AND ORGAN FORMATION
Figure 33.12 has an overview of eleven organ systemsof a typical vertebrate, an adult human. It lists someterms used when describing the positions of organs.It also shows major body cavities in which a numberof important organs are located.
The amazing thing is that the internal environmentstays within tolerable limits even though all of theseorgan systems are putting various amounts and kindsof substances into it and withdrawing substances fromit while they grow, develop, and maintain themselves.Talk about clockwork!
The clock starts ticking when germ cells, a type ofimmature reproductive cell, give rise to a sperm andan egg. (All other body cells, remember, are somatic.)At fertilization, a zygote forms. Mitotic cell divisionsturn it into a ball of cells that arrange themselves intothree primary tissue layers, which are forerunners of alladult tissues and organs. Ectoderm, the outer layer, isthe first to form. It will give rise to epidermis and thenervous system. Mesoderm, the middle layer, is thestart of muscles, bones, and most of the circulatory,urinary, and reproductive systems. Endoderm is theinner primary tissue layer. It is the start of the liningof the digestive tract and organs derived from it.
REGARDING THE DIVISION OF LABOR
Remember, new properties emerge at higher levels ofbiological organization (Section 1.1). Collectively, theorgan systems of a multicelled body show a divisionof labor—a compartmentalization of functions—thathelp the body survive in ways that no one tissue canoffer. Organ systems divide up the tasks of securing,processing, and distributing materials, and expellingwastes, protecting the body, integrating its activities,and reproducing. In Chapter 25, you glimpsed howthis emergent property—the division of labor—turnedout to be a key innovation when large-bodied animalsevolved. In the rest of this unit, you will come acrossorgan systems that reflect its extraordinary potential.
Organ systems perform compartmentalized functions, such as gas exchange, blood circulation, and locomotion.
Overview of Major Organ Systems
Figure 33.12 Animated! (a) Major cavities in the humanbody. (b,c) Directional terms and planes of symmetry for the body. Most vertebrates move with the main body axis parallel to Earth’s surface. For them, dorsal refers to their back (upper surface) and ventral to the opposite, or lower surface. In humans, who are upright, anterior refers to the front of a standing person; it corresponds to ventral. Posterior, the back, is equivalent to dorsal in the rabbit. (d) Human organ systems and their functions.
Animal organ systems compartmentalize many specializedtasks which, taken together, contribute to the survival andreproduction of the whole body.
Vertebrate organ systems arise from three primary tissuelayers: ectoderm, mesoderm, and endoderm.
Detects external andinternal stimuli; controlsand coordinates theresponses to stimuli;integrates all organsystem activities.
CirculatorySystem
Rapidly transportsmany materials to and from cells;helps stabilizeinternal pH andtemperature.
EndocrineSystem
Hormonally controlsbody functioning;works with nervoussystem to integrateshort-term and long-term activities.
LymphaticSystem
Collects and returnssome tissue fluid tothe bloodstream;defends the bodyagainst infection and tissue damage.
RespiratorySystem
Rapidly delivers oxygento the tissue fluid thatbathes all living cells;removes carbon dioxidewastes of cells; helpsregulate pH.
DigestiveSystem
Ingests food and water;mechanically, chemicallybreaks down food andabsorbs small moleculesinto internal environment;eliminates food residues.
ReproductiveSystem
Female: Produces eggs; after fertilization,affords a protected, nutritive environmentfor the development of new individuals.Male: Produces and transfers sperm tothe female. Hormones of both systemsalso influence other organ systems.
UrinarySystem
Maintains the volumeand composition ofinternal environment;excretes excess fluidand blood-bornewastes.d
Of all vertebrate organ systems, the outer body coveringcalled skin has the largest surface area. It consists of twolayers—an underlying dermis and outer dermis (Figures33.13 and 33.14). Skin stretches, conserves water, and fixessmall cuts or burns. It helps make vitamin D and dissipatesexcess metabolic heat. Its sensory receptors help the brainassess the outside world. White blood cells patrolling ithelp defend the body from external threats.
The Dermis The dermis is primarily a dense connectivetissue with many fibers of stretch-resistant elastin andsupportive collagen. Blood vessels, lymph vessels, and sensory receptors thread through it. The dermis rests on a hypodermis, which is not part of skin. The hypodermis contains loose connective tissue and adipose tissue thatinsulates or cushions some body parts (Figure 33.13).
Human skin has many exocrine glands, including about2.5 million sweat glands, in the dermis. Sweat glands helpdissipate heat. Their fluid secretions are 99 percent waterin which salts, traces of ammonia, vitamin C, and othersubstances are dissolved. Secretions from some sweatglands increase during stress, pain, and sexual foreplay,and prior to menstruation. Except on the palms and soles,the dermis contains oil glands (sebaceous glands). Thesecretions lubricate and soften hair and skin, and they killmany surface bacteria. When bacteria do infect oil glandducts, they can cause acne, an inflammation of skin.
The Epidermis Epidermis is a stratified squamousepithelium with an abundance of adhering junctions andno extracellular matrix. Ongoing mitotic cell divisions inthe deepest epidermal layers push previously formed cellstoward the skin’s surface. Wear and tear from the surface,together with the pressure exerted by the perpetuallygrowing cell mass, flatten and kill epidermal cells beforethey reach the surface. Dead ones are continually rubbedoff or flake away. The main types of epidermal cells arekeratinocytes, melanocytes, and dendritic cells.
When some vertebrates invaded the land, two kinds ofskin cells—keratinocytes and melanocytes—proved useful.The keratinocytes secrete keratin, a tough, water-resistantprotein that makes skin waterproof and more durable. Inmammals, dead, flattened keratinocytes make up most ofthe flexible structures called hairs (Figure 33.15). Figure33.16 shows how hair can be curled or straightened.
An average human scalp has about 100,000 hairs, butgenes, nutrition, and hormones affect hair growth. Proteindeficiency thins hair; amino acids are required for keratin
Vertebrate Skin—Example of an Organ System
Go back in time to the Cambrian seas, when jawed fisheswere first evolving. The body covering—integument—of some species had protective, heavy armor plates thathindered speed and precision movements. Among somebony fishes, a more flexible integument evolved intovertebrate skin.
Figure 33.15 Animated! Hair fine structure. Dead, flattened hair cellsaccumulate and form a tubelike cuticle around a hair’s shaft. They arederived from modified skin cells that synthesize polypeptide chains of theprotein keratin. Disulfide bridges link three chains into thin fibers, whichbecome bundled into larger fibers. The fibers almost fill the cells, which intime die off. Figure 33.16 shows how people play with the disulfide bridges.
hair
smooth muscleblood vessels
sweat glandhair follicle
sensory neuronoil gland
hypodermis (below skin, not part of it)
epidermis
dermis
Figure 33.13 Animated! Structure of skin that includes hair, oil glands,and sweat glands. Skin components differ in different body regions.
synthesis. High fever, emotional stress, and too much vitamin A in the diet also cause hair to thin.
Melanocytes in the epidermis produce the brownish-black pigment melanin, then give up these pigments to the keratinocytes. Melanin is one of the body’s barriers to harmful ultraviolet (UV) radiation in the sun’s rays.
Variations in skin color arise from differences in the distribution and activity of melanocytes. In pale skin, littlemelanin forms. Such skin often appears pink because thered color of the iron in hemoglobin shows through thin-walled blood vessels and epidermis. An orange pigment,carotene, also contributes to skin color.
Langerhans cells migrate through the epidermis. Thesephagocytic cells engulf bacteria or viruses and notify theimmune system of the threat (Section 39.4). Ultraviolet(UV) radiation damages Langerhans cells. When that happens, skin becomes much more vulnerable to viral outbreaks, such as the cold sores caused by Herpes virus.
Lab-grown epidermis is used to protect tissues and aidwound healing in some patients. One company makes it bygrowing cells of foreskins that were discarded earlier fromcircumcised male infants. The resulting product is missingsome cell types, such as melanoctyes, and it has no glands.
On Suntans and Shoe-Leather Skin Sunlight canburn unprotected light skin, sometimes severely. UV lightstimulates the melanocytes in skin to make melanin, whichgives skin the “tan” that many light-skinned people covet(Figure 33.17). Melanin production accelerates slowly, thenpeaks about ten days after tanning starts.
At first, dark-skinned people are better protected. Butprolonged or repeated UV exposure damages collagen and causes elastin fibers to clump. Chronically tanned skingets less resilient and starts to look like shoe leather. UValso attacks DNA, which invites skin cancer (Section 9.5).
As we age, epidermal cells divide less often. Skin thinsand becomes less elastic as collagen and elastin fibersbecome sparse. Glandular secretions that kept it soft andmoist dwindle. Wrinkles deepen. Many people needlesslyaccelerate the aging process by indulging in tanning orsmoking, which shrinks the skin’s blood supply.
Figure 33.16 (a) Curly or straight hair? Hair’s long keratin chains are bonded together by disulfide bridges, which break when exposed to chemicals. Ironing (flattening out) hair or rolling it around curlers holds the unbonded chains in new positions. Exposure to a different chemical makes new disulfide bridges form between different sulfur-bearingamino acids. The displaced bonding locks chains in new positions. That is how manywomen, including actress Nicole Kidman (b,c), straighten their naturally curly hair. cb
a
FOCUS ONHEALTH
The Vitamin Connection UV light also stimulatesmelanocytes to synthesize a precursor of vitamin D called cholecalciferol. “Vitamin D” is a generic name for steroid-like compounds that help the body absorb calcium ionsfrom food. At the same time, the UV exposure causes thebreakdown of folate, a B vitamin. Folate deficiency can leadto many problems. For one thing, the nervous system of an embryo cannot develop normally without it.
Variations in skin color among human populations maybe adaptations to differences in sunlight exposure. Ourearly ancestors evolved in Africa, where the sun’s rays areintense (Sections 26.14 and 26.15). Their melanin-rich skinprotected folate but still produced enough vitamin D. Aftersome human populations migrated to colder regions, theirdescendants spent much of the time indoors or bundled upoutside. We can hypothesize that embryo-protecting darkskin became disadvantageous and that paler skin, whichdoes not block as much UV light, proved advantageous.
Researchers tested the hypothesis by comparing annualUV levels with the skin color of people who were native tomore than fifty countries. The data support the hypothesisthat latitudinal decreases in the annual levels of UV lightcorrelate with latitudinal increases in lighter skin.
Figure 33.17 Demonstration of how to encouragethe formation of shoe-leather skin.
Section 33.1 A tissue is an aggregation of cells andintercellular substances that interact in performing oneor more common tasks. Epithelial tissues cover the bodysurface and line internal cavities and tubes. Epitheliumhas one free surface exposed to extracellular fluid or theenvironment. Gland cells and glands are derived fromepithelium. Endocrine glands are ductless and secretehormones. Exocrine glands secrete other products, suchas sweat or milk, through ducts to body surfaces.
Animal tissues have a variety of cell-to-cell junctions.Adhering junctions cement neighboring cells together.Tight junctions prevent substances from leaking across a tissue. Gap junctions are open channels that connectthe cytoplasm of abutting cells. They permit the rapidtransfer of ions and small molecules between cells.
Compare the structure and function of the maintypes of animal cell junctions with the animationon BiologyNow.
Section 33.2 Connective tissues structurally andfunctionally “connect” other animal tissues. Differenttypes bind, organize, support, strengthen, protect, andinsulate other tissues. All contain cells scattered withinan extracellular matrix of their own secretions. Exceptin blood, the main cell type is the fibroblast. It makesand secretes structural fibers of collagen and elastin intothe extracellular matrix.
Loose connective tissue and dense connective tissuehave the same components but differ in the proportions.They are classified as soft connective tissues. Cartilage,bone tissue, adipose tissue, and blood are classified asspecialized connective tissues.
Section 33.3 Muscle tissues contract (shorten), thenpassively lengthen. They help move the body and itscomponent parts. The three types are skeletal muscle,cardiac muscle, and smooth muscle tissue. Only skeletalmuscle and cardiac muscle tissues are striated. Onlyskeletal muscle is under voluntary control.
Section 33.4 Neurons in nervous tissue make upcommunication lines through the body. Different kindsdetect, integrate, and assess stimuli about internal and
external conditions, and deliver commands to musclesand glands that carry out responses. Nervous tissuealso contains diverse cells collectively called neuroglia,which protect and support the neurons.
Section 33.5 An organ system consists of two ormore organs that interact chemically, physically, or bothin tasks that help keep individual cells as well as thewhole body functioning. Most vertebrate organ systemscontribute to homeostasis; they help maintain tolerableconditions in the internal environment that benefitindividual cells and the body as a whole.
All tissues and organs of an adult animal arise fromthree primary tissue layers that form in early embryos:ectoderm, mesoderm, and endoderm. Ectoderm, theouter primary tissue layer, forms first and gives rise toepidermis and part of the nervous system. The other twogive rise to the rest of the internal tissues and organs.
Collectively, the organ systems of a multicelled bodyshow a division of labor—a compartmentalization offunctions—that help the body survive in ways that noone tissue can offer.
Investigate the function of vertebrate organ systemsand learn about terms used to describe theirlocations with the animation on BiologyNow.
Section 33.6 An organ system called skin functionsin protection, temperature control, detection of shifts inexternal conditions, vitamin D production, and defense.
Explore the structure of skin and hair withthe animation on BiologyNow.
Self-Quiz Answers in Appendix II
1. The four light micrographs at lower left show fourtypes of animal tissues. Identify each type and write out a brief description of its defining features.
2. tissues are sheetlike with one free surface.a. Epithelial c. Nervousb. Connective d. Muscle
3. function in cell-to-cell communication.a. Tight junctions c. Gap junctionsb. Adhering junctions d. all of the above
4. In most animals, glands are located in tissue.a. epithelial c. muscleb. connective d. nervous
5. Most have many collagen and elastin fibers.a. epithelial tissues c. muscle tissuesb. connective tissues d. nervous tissues
6. is mostly plasma.a. Adipose tissue c. Cartilageb. Blood d. Bone
7. Your body converts excess carbohydrates and proteinsto fats. specializes in storing the fats.
a. Epithelial tissue c. Adipose tissueb. Dense connective tissue d. both b and c
advantages and disadvantages of tests that use specificlab-grown tissues as opposed to living animals.
4. After a cold night in Africa’s Kalahari Desert, animalssmall enough to fit inside a coat pocket emerge stiffly from burrows. These “meerkats” are a type of mongoose.They stand on tiny hind legs and face east, exposing theirchilled bodies to the warm rays of the morning sun (Figure33.19). Once meerkats warm up, they fan out and search forfood. Into the meerkat gut go insects and the occasionallizard. These are pummeled, dissolved, and digested intoglucose and other nutritious bits small enough to moveacross the gut wall, into blood, and on to the body’s cells.
Name as many tissues as you can that might have rolesin (1) keeping the meerkat body warm, (2) moving thebody, as during foraging and heart-thumping flights frompredators, and (3) digestion and absorption of nutrients,and elimination of the residues.
5. Porphyria is a name for a set of rare genetic disorders.Affected people lack one of the enzymes in the metabolicpathway that forms heme, the iron-containing group ofhemoglobin. As a result, intermediates of heme synthesis(porphyrins) accumulate in the body. In some forms of thedisorder, porphyrins pile up in the bones, skin, and teeth.When porphyrins in the skin are exposed to sunlight, theyabsorb energy and release energized electrons. Electronscareeening around the cell can break bonds and cause freeradicals to form. The most notable result is the formationof lesions and scars on skin (Figure 33.20). In the mostextreme cases, gums and lips can recede, which makessome front teeth—the canines—look more fanglike.
Affected individuals must avoid sunlight, and garliccan exacerbate their symptoms. By one hypothesis, peoplewho were affected by the most extreme forms of porphyria may have been the source for vampire stories. Would you consider this hypothesis plausible? What other kinds of historical data might support or disprove it?
8. In your body, cells of can shorten (contract).a. epithelial tissue c. muscle tissueb. connective tissue d. nervous tissue
9. Only muscle tissue has a striated appearance.a. skeletal c. cardiacb. smooth d. a and c
10. detects and integrates information aboutchanges and controls responses to those changes.
a. Epithelial tissue c. Muscle tissueb. Connective tissue d. Nervous tissue
11. Match the terms with the most suitable description.exocrine gland a. strong, pliable; like rubberendocrine gland b. secretion through ductcartilage c. outermost primary tissueectoderm d. contracts, not striatedsmooth muscle e. cements cells togetherblood f. fluid connective tissueadhering g. ductless secretionjunction
Additional questions are available on
Critical Thinking1. The nose, lips, tongue, navel, nipples, and genitals areoften targets for body piercing: cutting holes into the bodyso jewelry can be threaded through them (Figure 33.18). Tattooing, or using permanent dyes to make patterns inskin, is another fad. Besides being painful, both skin invasions can invite bacterial infections, chronic viralhepatitis, AIDS, and other diseases if the piercers and tattooers reuse unsterilized needles, dye, razors, gloves,swabs, and trays. Months may pass before any problemsdevelop, so the cause-and-effect connection isn’t alwaysobvious. If despite the risks you think tissue invasions areokay, how can you be sure the equipment used is sterile?
2. Adipose tissue and blood are often said to be atypicalconnective tissues. Compared to other types of connectivetissues, which features are not typical?
3. Many people oppose the use of animals for testing thesafety of cosmetics. They say alternative test methods areavailable, such as the use of lab-grown tissues in some cases.Given what you learned in this chapter, speculate on the
Chapter 33 Animal Tissues and Organ Systems 571
Figure 33.18 Assaults on theintegument: skin piercings.
Figure 33.19 In the Kalahari Desert, gray meerkats (Suricata suricatta). Each morning,they come out of their burrows and face the sun’s warming rays.
Figure 33.20 Skin lesions of an individual affected by a severe form of porphyria.
