Chapter 3

Scoping Out the Body’s Structural Layers

IN THIS CHAPTER

Discovering the functions of your skeleton

Understanding your muscular system

Appreciating your skin, hair, and nails

You’re not an amorphous blob of cells. The things that give your body its form, allow it to move, and protect it are the structural layers. Following are the three main structural components in the body:

• The skeletal system: The skeleton determines the general shape and size of humans as a species. In humans, as in all vertebrates, the skeleton is part of the musculoskeletal system.
• The muscular system: Muscle tissues work together with all the other systems in your body, but more than any other organ system, the muscular systems specialize in movement.
• The integumentary system: The skin and its appendages (hair and nails), together called the integument, make up the body’s largest organ system.

This chapter shows you what your skeleton, muscle, and skin are made of and explains how they’re organized.

The Skinny on Your Skeleton

The structural functions of the skeletal system are

• Protection: Bones and joints are strong and resilient, so they shelter and protect your organs. For example, the rib cage provides a protected inner space for your more delicate internal organs. The vertebral column partially encases and protects the spinal cord, and the skull completely encases the brain.
• Movement: The musculoskeletal system is a motion machine: The bones anchor the skeletal muscles and act as levers, the joints act as fulcrums, and muscle contraction provides the force for movement.
• Support: The curved vertebral column supports most of your body’s weight. The arches of your feet support the weight of your body in a different way.

Examining the skeleton’s makeup

This section explains how your body builds the tissues of the bones and the joints and describes how they all fit together to protect, move, and support the entire body.

Connective tissue

The skeleton is made up of three types of connective tissue:

• Bone tissue is physiologically active, is constantly generating and repairing itself, and has a generous blood supply running through it. It also makes a huge amount of cells that are exported to the rest of the body, notably the very cells of the blood. Bone tissue contains dozens of specialized cell types, and all the skeletal system’s functions depend on the functioning of specialized cells in the tissue.

• Cartilage is a firm but flexible tissue that’s made up of mostly protein fibers and serves as the main component of joints. Among the functions of cartilage tissue is the building of new bone. The two types of cartilage in the skeletal system are hyaline cartilage and fibrocartilage.

  • Hyaline cartilage is the type that forms the septum of your nose. It also forms a portion of the very first version of the fetal skeleton. It’s the most abundant type of cartilage in several kinds of joints. For example, it’s a major component of the freely movable joints called synovial joints.
  • Fibrocartilage is a fibrous, spongy tissue that acts as a shock absorber in the vertebral column (spine) and the pelvis.

  Cartilage isn’t generated and replaced as actively as bone, so cartilage gets by with fewer cells. Mature cartilage has no blood supply.

• Fibrous connective tissue contains very few living cells and is composed mainly of protein fibers, complex sugars, and water. It forms a structure called the periosteum, which is a protective sheet that covers bones. The sheet morphs into cordlike structures, called ligaments and tendons, wherever necessary. Ligaments connect a bone to another bone, and tendons connect a muscle to a bone.

  The periosteum is said to be continuous with the ligaments and tendons, because no real separation exists between the “sheet” and the “cords.”

Bone structure

The structures called bones (such as the femur, the vertebrae, and the finger bones) are made of bone tissue. (No surprise there. However, note that the structures called joints are made of tissue called cartilage.) Remember that individual types of bones have different forms of bone tissue.

Long bones, such as your thighbone (femur) or forearm bone (radius), are the type of bones people usually think of first. And in fact, they make good illustrations of the general anatomy and physiology of bone tissue (see Figure 3-1). You can read about long bones and the other types of bones in the later section “Bone names and characteristics.”

FIGURE 3-1: Long bone structure.

In cross section, bone is structured in concentric layers. In other words, an outer layer surrounds a middle layer, which in turn surrounds an inner layer. In longitudinal section, a bone has two (mostly similar) ends and a long middle area, which has cells and tissues that are mostly different from the ends. The following list names and briefly describes cellular and material composition of the areas of a long bone:

• Compact bone (the outer layer) is a dense layer of cells in a hard matrix of protein fibers and compounds made of calcium and other minerals. This layer gives bones their amazing strength.
• Spongy bone (the middle layer) is, like compact bone, a variety of cell types within a matrix of mineralized protein fibers. But spongy bone is more open in structure than compact bone, creating a physiological trade-off between strength and lightness. Structures called trabeculae, which follow stress lines in the bone, act like braces, providing support.
• The medullary cavity and bone marrow is the inner layer in the shaft of a long bone (the diaphysis) and the inner layer of other bones. Bones have both yellow marrow, which is mostly fat (think butter), and red marrow, the site of hematopoiesis, which is the production of blood cells.
• Epiphysis is the enlarged, knobby end of a long bone. It consists of a layer of spongy bone covered by an outer layer of compact bone. The epiphysis is the site of bone elongation. Within the epiphysis, bone and cartilage tissue are intimately connected: As cartilage cells divide, cartilage morphs into bone tissue. This process continues from before birth until the bones reach their full adult size.

Bone names and characteristics

Bones come in different shapes and sizes. Appropriately, many bone type names match what they look like, such as flat bones, long bones, short bones, and irregular bones. Check out Table 3-1 for the differences among the four types of bones.

TABLE 3-1 Characteristics of Bone Types

Bone Type	Example Location in the Body	Characteristics
Flat	Skull, shoulder blades, ribs, sternum, pelvic bones	Like plates of armor, flat bones protect soft tissues of the brain and organs in the thorax and pelvis.
Long	Arms and legs	Like steel beams, these weight-bearing bones provide structural support.
Short	Wrists (carpal bones) and ankles (tarsal bones)	Short bones look like blocks and allow a wider range of movement than larger bones.
Irregular	Vertebral column, kneecaps	Irregular bones have a variety of shapes and usually have projections that muscles, tendons, and ligaments can attach to.

Surveying joints and their movements

A joint is a connection between two bones. Some joints move freely, some move little, and some never move. Joints, which vary greatly in their size and shape, are classified by the amount of movement they permit. The following sections tell you about the different joint structures and what they allow you to do.

Categorizing the types of joints

The human skeleton features the following three types of joints, and they all provide a different range of motion:

• Immovable joints: Joints that don’t move, such as those between the bones of the skull, are called synarthroses. A thin layer of fibrous connective tissue, called a suture, joins the bones of the joint together.
• Slightly movable joints: Joints that are slightly movable and connected by fibrocartilage or hyaline cartilage are called amphiarthroses. An example of this type of joint includes the intervertebral disks, which join each vertebra and allow slight movement of the vertebral column.
• Freely movable joints: Joints that are freely movable are called diarthroses. They’re also called synovial joints because a cavity between the two connecting bones is lined with a synovial membrane and filled with synovial fluid, which helps to lubricate and cushion the joint. The ends of the bones are cushioned by hyaline cartilage. Diarthroses are joined together by ligaments. The following list shows the many types of diarthroses:
  • Ball-and-socket joints: With this joint, the ball-shaped head of one bone fits into a depression (socket) in another bone. This joint, which can move in all planes, allows circular movements and rotation. Examples of this type of joint include the shoulder and the hip.
  • Condyloid joints: With this joint, the oval-shaped condyle of one bone fits into the oval-shaped cavity of another bone. This joint can move in different planes but can’t rotate. An example of this type of joint is the knuckles (the joints between the metacarpals and the phalanges).
  • Gliding joints: This joint joins flat or slightly curved surfaces and allows sliding or twisting in different planes. Examples include the joints between the carpal bones (wrist) and between the tarsal bones (ankle).
  • Hinge joints: With this joint, a convex surface joins with a concave surface. This joint provides up and down motion in one plane, so it either bends (flexes) or straightens (extends). Examples of this joint include elbows and knees.
  • Pivot joint: With this joint, a cylinder-shaped projection on one bone is surrounded by a ring of another bone and ligament. Rotation is the only possible movement with this joint. Examples include the joint between the radius and ulna at the elbow and the joint atlas and axis at the top of the vertebral column.
  • Saddle joint: With this joint, each bone is saddle shaped and fits into the saddle-shaped region of the opposite bone. Many movements are possible with this joint; it can move in different planes but it can’t rotate. An example is the joint between the carpal and metacarpal bones of the thumb.

Discovering what your joints can do

Your joints perform either angular or circular movements. Angular movements make the angle formed by two bones larger or smaller. Examples of these movements include the following:

• Abduction moves a body part to the side, away from the body’s middle. When you make a snow angel and move your arms and legs out and up, that’s abduction.
• Adduction moves a body part from the side toward the body’s middle. When you’re in the snow angel position and you move your arms and legs back down, that’s adduction.
• Extension makes the angle larger. Hyperextension occurs when the body part moves beyond a straight line (180 degrees).
• Flexion decreases the joint angle. When you flex your arm, for example, you move your forearm to your arm.

Circular movements occur only at ball-and-socket joints like in the hip or shoulder. Examples of circular movements include the following:

• Circumduction is the movement of a body part in circles.
• Depression is the downward movement of a body part.
• Elevation is the upward movement of a body part, such as shrugging your shoulders.
• Eversion only happens when the foot is turned so the sole is facing outward.
• Inversion only happens when the foot is turned so the sole is facing inward.
• Rotation is the movement of a body part around its own axis, such as shaking your head to answer “no.”
• Supination and pronation refer to the arm, and they stem from the terms supine and prone. Supination is the rotation of the forearm to make the palm face upward or forward. Pronation is the rotation of the forearm to make the palm face downward or backward.

The 411 on the Muscular System

The human muscular system is always busy pulling things up, pushing things down and around, and moving things inside you and outside you. This section clues you in to the different jobs of your skeletal muscles, the various muscle tissue types, and the basics of muscle contraction.

Seeing what your skeletal muscles do

The skeletal muscles, the muscles that move your bones, comprise a substantial portion of your body mass, and most of what you eat goes to fuel their metabolism. In the following sections, you find out what they do with all that energy.

Support your structure

Muscles are attached to bones on the inside of your body and to skin on the outside, with various types of connective tissue between the layers. Thus, they hold your body together. Along with your skin and your skeleton, your muscles shield your internal organs from injury from impact or penetration.

As it does with everything else, gravity pulls your weight downward (toward the planet’s center). But gravity doesn’t only pull on the soles of your feet — it pulls on all your weight. If gravity had its way, you’d be lying on the floor right now. Thankfully, your muscles pull your weight up and hold you upright.

Move you

Contracting and releasing a muscle moves the bone it’s attached to relative to the rest of the body. The movement of the bone, in turn, moves all the tissue attached to it, such as when you raise your arm. Certain combinations of these types of movements move the entire body, as when you walk, run, swim, skate, or dance.

Muscle contraction is responsible for little movements, too, like blinking your eyes, dilating your pupils, and smiling.

Position and balance you

A very close interaction outside of your conscious control between some muscle cells and the nervous system keeps you not just upright, but in balance. Nerve impulses throughout the muscular system cause muscles to contract or relax to oppose gravity in a more subtle way when, say, you’re shifting your weight from one side to the other as you step. This interaction is called muscle tone.

When you step down a steep incline in rough terrain, your muscle tone brings your abs and your back muscles into action in a different way than when you step across your living room rug. The mechanisms of muscle tone may move your arms up and away from your body to counterbalance the pull of gravity with an accuracy and precision you could never calculate cognitively. Below your conscious level, the mechanisms of muscle tone are active every minute of every day, even when you’re asleep.

Muscle tone relies on muscle spindles, which are specialized muscle cells that are wrapped with nerve fibers. The central nervous system stays in contact with the muscles through the muscle spindles. Spindles send messages about your body position through the spinal cord to the brain; to initiate the fine adjustments, the brain sends signals about which muscles to contract and which to release through the spinal cord and nerves to the muscle spindles.

Maintain your body temperature

Muscle contributes to homeostasis by generating heat to balance the loss of heat from the body surface. Muscle contraction uses energy from the breakdown of ATP and generates heat as a byproduct. The heat generated by the muscles interacts with other physiological processes that release heat from the body — sweating, for example — to maintain thermoregulation. Similarly, shivering is a series of muscle contractions that generate extra heat to increase your temperature in cold situations.

Push things around inside you

Following are some of the muscles that keep things moving within your body, all without any thought from you:

• Cardiac muscle: The heart is a muscle that contracts rhythmically, pumping blood into the arteries. A muscular lining in the arteries rhythmically dilates and contracts, pushing blood along with enough force (blood pressure) to drive this relatively viscous fluid into the capillary beds. The rhythmic movements of the heart and arteries are detectable as your pulse.

  The ability of the arterial wall muscles to dilate and contract in response to physiological stimuli enables the subtle control of blood pressure. Damage to this muscular lining causes arteriosclerosis (hardening of the arteries), which inhibits the muscle lining’s ability to move (dilate and contract) the vessel, an underlying factor in cardiovascular disease and dysfunction.

• Diaphragm: The diaphragm is a skeletal muscle whose contraction and release forces air in and out of the lungs.
• Digestive smooth muscle: Your digestive system is lined with a kind of muscle tissue called smooth muscle that contracts in pulsating waves, pushing ingested material along the digestive tract. Think of this muscular lining as a conveyor belt on a disassembly line. Refer to Chapter 6 for details.
• Sphincter muscles: These muscles are essentially valves: rings of smooth muscle that are fully in contraction in their resting state, holding some material in one place, and then relaxing only briefly to allow the material to move through. You find sphincters at various places in the digestive system, from the very beginning to the very end, and in other parts of the body as well.

Taking a look at tissue types

A muscle tissue type isn’t the same as a muscle. Your left bicep is a muscle; in all, you have hundreds of named muscles. However, you have only three muscle tissue types: skeletal muscle tissue, cardiac muscle tissue, and smooth muscle tissue. The following sections fill you in on each type as well as the cellular characteristics of muscle tissue.

Browsing the unique features of muscle cells

Your muscle tissue is made up of cells that are different from the other cells of your body. These cells are so unique that they’re even different from each other based on the type of muscle tissue they belong to. The three muscle types are distinguishable anatomically by their characteristic cells and structures. They’re also distinguishable physiologically as voluntary or involuntary.

Muscle cells feature these characteristics, which you can see in Figure 3-2:

• Single or multiple nuclei: Cardiac muscle cells and smooth muscle cells have one nucleus apiece, like most other cells. Skeletal muscle cells (fibers), however, are multinucleate, meaning they have numerous nuclei within one cell membrane.

  Skeletal muscle cells don’t grow these extra nuclei; during the development of skeletal muscle tissue, numerous skeletal muscle cells merge into one large cell, and most of the nuclei are retained within one continuous cell membrane, along with most of the mitochondria. (For the scoop on mitochondria and other components of cells, see Chapter 2.)

• Striation: Skeletal muscle is striated, meaning that, under a microscope, alternating light and dark bands are visible in the muscle cell (fiber). Striation is the result of the subcellular structure of skeletal muscle cells (as explained in the later section “Reviewing skeletal muscle”) and is integral to the mechanism of contraction called the sliding filament model (which we explain in the later section “Making muscles contract: The sliding filament model”).

  Cardiac muscle cells are striated as well, and they also contract by a variation of the sliding filament model. Smooth muscle cells aren’t striated in appearance but do follow a version of the sliding filament model.

FIGURE 3-2: Muscle cell and tissue types.

Muscle cells can also be categorized by the type of contraction they perform. They can be categorized as one of the following:

• Involuntary: Smooth and cardiac muscle cells are involuntary, meaning their contraction is initiated and controlled by parts of the nervous system that are far from the conscious level of the brain. You have no practical way to consciously control, or even become aware of, the smooth muscle contractions in your stomach that are grinding up this morning’s muffin.
• Voluntary: Skeletal muscle is classified as voluntary because you make a decision at the conscious level to move the muscle. For example, when you decide to reach for a doorknob and turn it, your muscles carry out the command from your brain to do so.

Table 3-2 sums up the characteristics and classifications of muscle cells.

TABLE 3-2 Cell Characteristics of Muscle Cells

	Skeletal	Cardiac	Smooth
	Skeletal	Cardiac	Smooth
Multinucleate	Yes	No	No
Striated	Yes	Yes	No
Voluntary	Yes	No	No

Reviewing skeletal muscle

Skeletal muscle tissue is made of bundles of fibers. Like fibrous material of every kind, skeletal muscle tissue gets its strength from assembling individual fibers together into strands, and then bundling and rebundling the strands. Two properties make this particular fibrous material special: The strands are made of protein, and they renew and repair themselves constantly. See Figure 3-3 for details of skeletal muscle tissue anatomy.

FIGURE 3-3: Anatomy of skeletal muscle tissue.

HOMING IN ON THE CELLULAR LEVEL

Individual muscle cells, which physiologists call fibers, are slender cylinders that sometimes run the entire length of a muscle. Each fiber (cell) has many nuclei located along its length and close to the cell membrane, which is called the sarcolemma. Outside the sarcolemma is a lining called the endomysium, a type of connective tissue.

Muscle spindles are specialized skeletal muscle fibers that are wrapped with nerve fibers. Figure 3-3 shows how skeletal muscle is connected to the nervous system. Spindles are distributed throughout the muscle tissue and provide sensory information to the central nervous system.

Within the muscle fibers are myofibrils. The myofibrils are composed of sarcomeres, which are distinct units arranged linearly (end to end) along the length of the myofibril. Muscle contraction occurs within the sarcomere. (Refer to the later section “Making muscles contract: The sliding filament model” for more on muscle contraction within sarcomeres.)

EXAMINING THE TISSUE LEVEL

Muscle fibers are bound together into bundles called fascicles. Each fascicle is bound by a connective-tissue lining called a perimysium. Spindle fibers are distributed throughout each fascicle. The fascicles are then bound together to form a muscle, a discrete assembly of skeletal muscle tissue, with a connective-tissue wrapper called an epimysium holding the whole package together.

Tendons — ropy extensions of the connective tissue covering the skeletal bones — grow into the epimysium, holding the muscle firmly to the bone.

How many ways can you say “fiber”? Anatomists need them all when they’re talking about the muscular system. Make sure you’re thinking at the right level of organization (subcellular, cellular, or tissue) when you see these terms: filament, myofibril, fiber, and fascicle.

WORKING TOGETHER: SYNERGISTS AND ANTAGONISTS

Groups of skeletal muscles that contract simultaneously to move a body part are said to be synergistic. The muscle that does most of the moving is the prime mover. The muscles that help the prime mover achieve a certain body movement are synergists. For example, when you move your elbow joint, the bicep is the prime mover and the brachioradialis stabilizes the joint, thus aiding the motion.

Antagonistic muscles also act together to move a body part, but one group contracts while the other releases in a kind of push-pull game. One example is flexing your arm. When you bend your forearm up toward your shoulder, your biceps muscle contracts, but the triceps muscle in the back of your arm relaxes. The actions of the biceps and triceps muscles are opposite, but you need both actions to allow you to flex your arm. Antagonistic actions lower your arm, too: The biceps relaxes, and the triceps contracts.

Checking out cardiac muscle

The heart has its own very special type of muscle tissue, called cardiac muscle. The cells (fibers) in cardiac muscle contain one nucleus (they’re uninucleated) and are cylindrical; they may be branched in shape. Unlike skeletal muscle, where the fibers lie alongside one another, cardiac muscle fibers interlock, which promotes the rapid transmission of the contraction impulse throughout the heart. Cardiac muscle cells are striated, like skeletal muscle cells, and cardiac muscle contraction is involuntary, like smooth muscle contraction. Cardiac muscle fibers contract in a way similar to skeletal muscle fibers, by a sliding filament mechanism.

Cardiac muscle tissue is on the job, day and night, from before birth to the moment of death. Throughout your lifetime, the cardiac muscle cells contract regularly and simultaneously hundreds of millions of times. When cardiac muscle tissue gives up, the game is over.

Unlike skeletal muscle and smooth muscle, contraction of the heart muscle is autonomous, which means it occurs without stimulation by a nerve. In between contractions, the fibers relax completely.

Considering smooth muscle

Smooth muscle tissue lines the organs and structures of many organ systems, including the digestive system, the urinary system, the respiratory system, the circulatory system, and the reproductive system. Smooth muscle tissue is fundamentally different from skeletal muscle tissue and cardiac muscle tissue in terms of cell structure and physiological function. However, smooth muscle sarcomeres are similar, and contraction is affected by a variation of the sliding filament model.

Smooth muscle cells (fibers) are fusiform (thick in the middle and tapered at the ends) and arranged to form sheets of tissue. Smooth muscle cells aren’t striated. However, smooth muscle contractions are affected by a similar sliding filament mechanism as skeletal muscle cells (see the following section for more).

Smooth muscle contraction is typically slow, strong, and enduring. Smooth muscle can hold a contraction longer than skeletal muscle. In fact, some smooth muscles, notably the sphincters, are in a constant or nearly constant state of contraction. Childbirth is among the few occasions in life when humans (some humans, anyway) consciously experience smooth muscle contraction (although they don’t consciously control it).

Making muscles contract: The sliding filament model

A muscle contracts when all the sarcomeres in all the myofibrils in all the fibers (cells) contract all together. The sarcomere is the functional unit within the myofibril. (Sarcomeres line up end to end along the myofibril.) The sliding filament model describes the fine points of how this contraction happens.

The key to the sliding filament model is the distinctive shapes of the protein molecules myosin and actin and their partial overlap in the sarcomere. The special chemistry of ATP supplies the energy for the filaments’ movement. The following sections explain how sarcomeres create muscle contraction.

Assembling a sarcomere

The sarcomere is composed of thick filaments and thin filaments. The thick filaments are molecules of the protein myosin, which is dense and rubbery. The thin filaments are primarily made up of two strands of the lighter (less dense) protein actin, wrapped in a double helix, which, as in DNA, is springy.

The thin and thick filaments line up together in an orderly way to form a sarcomere. One end of a thin filament touches and adjoins the end of another thin filament. Adjoined thin filaments adjust themselves so the joining points form a structure called a Z line — a straight line that runs perpendicular to the filament axis. The sarcomere begins at one Z line and ends at the next Z line. The thick filaments line up precisely between the thin filaments. Sarcomeres and Z lines are shown in Figure 3-3.

The two types of filaments overlap only partially when the sarcomere is at rest. The partial overlap gives skeletal muscle cells their striations. Where thick and thin filaments overlap, the tissue appears dark (dark band); where only thin filaments are present, the tissue appears lighter (light band).

Contracting and releasing the sarcomere

Myosin molecules have binding sites with a high affinity for ATP (refer to Chapter 1 for details on ATP). Actin molecules have binding sites with a high affinity for myosin. When a nerve impulse sends calcium ions into the cytoplasm of the muscle fiber, things start to happen.

The binding of calcium ions on actin molecules exposes the myosin binding sites of actin. Myosin, with its ATP cargo, binds to actin, forming cross-bridges between the thick and thin (actin) filaments of the sarcomere. The bond with actin distorts the myosin-ATP binding site, leading to the hydrolysis of the ATP and the release of its energy. This energy fuels the motion of the cross-bridges that pulls (slides) the thin filaments past the thick filaments toward the middle of the sarcomere. The distance between the Z lines becomes shorter because the length of the nonoverlapping portion of the two types of filaments becomes shorter. All sarcomeres in a fiber contract simultaneously, transmitting the force to the fiber ends.

The myosin then drops the products of the hydrolysis (ADP and P_i) from the binding site. Another molecule of ATP takes its place, reshaping the myosin molecule once again and pulling the actin-myosin bond apart. At this point, the cycle begins again.

Both the binding action and the release action require energy in the form of ATP. One molecule of ATP is needed for each binding and each release of each filament pair within each sarcomere. Thousands of molecules of ATP are required for every second of muscle contraction.

An Introduction to the Integument

The entity known as you is bounded by your integument. Everything inside the outermost layer of skin is you. Everything outside the outermost layer of skin is not you.

Your skin mediates much of the interaction between you and not you, which we henceforth call the environment. Your integument identifies you to other humans, a very important function for members of the hypersocial human species. Here’s a look at the integument’s other important functions:

• Incoming messages: Many types of sensory organs are embedded in your skin, including receptors for heat and cold, pressure, vibration, and pain.
• Outgoing messages: The skin and hair are messengers to the outside environment, mainly to other humans. People get information about your state of health (physical and emotional) by looking at your skin and hair. Your emotional state is signaled by pallor, flushing, blushing, goose bumps, sweating, and more. The odors of sweat from certain sweat glands signal sexual arousal.
• Protection: Skin protects the rest of the body by keeping out many threats from the environment, such as infection and predation by other organisms, damaging solar radiation, and nasty substances everywhere.
• Substance production: Sebaceous glands in the skin, usually associated with a hair follicle, produce a waxy substance called sebum. Similarly, sweat glands in the skin make sweat. In fact, your skin has several different types of glands, and each makes a specific type of sweat. Also, skin cells produce keratin, a fibrous protein that’s an important structural and functional component of skin and is, essentially, the only component of hair and nails.
• Thermoregulation: The skin supports thermoregulation (the maintenance of optimum body temperature) in several ways — by producing sweat, for example.
• Water balance: The skin’s outer layers are more or less impermeable to water, keeping water and salts at an optimum level inside the body and preventing excess fluid loss. A small amount of excess water and some bodily waste (urea) are eliminated through the skin.

Studying up on the structure of the integument

Your skin, itself a thin layer, is made up of many layers. This layering is visible to the unaided eye because each layer is different from the others and the transitions between layers appear to be relatively abrupt. We look at the three layers of the integument — the epidermis, the dermis, and the subcutaneous layer (hypodermis) — in the following sections.

When describing the integument, we use up and above to mean “toward the surface of the body,” and down and below to mean “toward the center of the body.” Sometimes, anatomists use the terms superficial and deep to mean the same things, respectively.

Touching the epidermis

The most familiar aspect of the integument is the epidermis, the outermost surface that you see on yourself and other people. The epidermis feels soft, slightly oily, elastic, resilient, and strong. In some places, the surface has a dense cluster of coarse hairs; in other spots, it has a lighter covering of light hairs; and in a few places, it has no hairs at all. The nails cover the tips of the fingers and toes.

The epidermis itself is made up of four to five different layers not visible to the unaided eye, from the stratum corneum at the top to the stratum germinativum (or stratum basale) at the bottom. All layers of the epidermis are composed of stratified squamous epithelial tissue, but the layers perform different functions. The epidermis has no blood supply; it’s nourished by diffusion from the dermis layer below.

STRATUM CORNEUM: THE THIN, IMPERVIOUS COVER

Think of the stratum corneum, the top layer, as a sheet of self-repairing fiberglass over the other layers of the epidermis. It’s only 25 to 30 cells thick, but it’s dense and relatively hard. All the cells are keratinocytes, which produce the fibrous protein keratin.

The keratinocytes of the stratus corneum originate as squamous epithelial cells in the stratum germinativum (the bottom layer of the skin).

The uppermost surface of the stratum corneum is covered with a waxy, waterproof coating of sebum. This layer of the skin protects the entire body by making sure some things stay in and everything else stays out.

The stratum corneum doesn’t seal out ultraviolet radiation. This form of energy goes directly through the skin’s surface and down to the layers below, where it stimulates the production of vitamin D. In high doses, it burns the skin and damages DNA, which can cause cells to become cancerous. Evolution’s response to the threat of UV-induced cell damage is the pigment melanin, which absorbs harmful UV-radiation and transforms the energy into harmless heat.

STRATUM LUCIDUM: THE LAYER ON THE HANDS AND FEET

The stratum lucidum, found only on the palms of the hands and soles of the feet (thick skin), the stratum granulosum, and the stratum spinosum lie in distinct layers below the stratum corneum. Old cells slough off above and new cells push up from below, finally getting up into the stratum corneum. The process takes about 14 to 30 days. Like other epidermal cells, these cells live in the stratum corneum for about a month.

The keratinocytes produce lipids (fatty substances) and undergo successive stages of keratinization and other kinds of differentiation in these layers. These layers also contain Langerhans cells, immune system cells that arrest micro-bial invaders and transport them to the lymph nodes for destruction.

STRATUM GERMINATIVUM: THE CONSTANTLY RENEWING LAYER

The stratum germinativum, also called the stratum basale or basal layer, is like a cell farm, constantly producing new cells and pushing them up into the layer above. This stratum contains melanocytes, which produce the melanin pigment that gives color to your skin, hair, and eyes and protects the skin from the damaging effects of UV radiation in sunlight. Melanin absorbs UV radiation and dissipates more than 99.9 percent of it as heat.

Exploring the dermis

Below the layers of the epidermis (and several times thicker) is the dermis. The dermis itself is made up of two layers:

• The papillary region: This region consists of the basement membrane, which sits just below the epidermis, and the papillae (finger-like projections) that push into the basement membrane, increasing the area of contact between the dermis and the epidermis.

  In your palms, fingers, soles, and toes, the papillae projecting into the epidermis form friction ridges. (They help your hand or foot to grasp by increasing friction.) The pattern of the friction ridges on a finger is called a fingerprint.

• The reticular region: This region is chock-full of protein fibers and is a complex and metabolically active layer. Cells (which migrate down from the epidermis during development) and structures of the reticular region manufacture many of the skin’s characteristic products: hair and nails, sebum, and sweat. The region also contains structures that connect the integument to other organ systems: sensors of pressure (touch) and heat, lymph vessels, and a rich blood supply.

The blood vessels in the dermis provide nourishment and waste removal from the dermis’s own cells as well as from the stratum germinativum. These blood vessels dilate when the body needs to lose heat and constrict to keep heat in. They also dilate and contract in response to your emotional state, brightening or darkening your skin color, thereby functioning as social signaling.

The sensory receptors in the dermis transmit sensations, such as pressure, vibration, and light touch, to the nervous system. The receptors are sprinkled throughout the dermis and are connected to the nerves that run through the dermis and subcutaneous layer. Not every inch of skin is covered with receptors for every sensation. So at one spot on your skin, you may sense light touch, while a few centimeters away, you may sense pressure.

Getting way under your skin: The subcutaneous layer

The subcutaneous layer (also known as the hypodermis, or superficial fascia) is the layer of tissue directly underneath the dermis. It’s mainly composed of connective and adipose (fatty) tissue. Its physiological functions include insulation, the storage of energy, and help in the anchoring of the skin. The subcutaneous layer contains larger blood vessels, lymph vessels, and nerve fibers than those found in the dermis. Its loosely arranged elastin fibers anchor the hypodermis to the muscle below it.

The thickness of the subcutaneous layer is determined in some places by the amount of fat deposited into the cells of the adipose tissue, which makes up the majority of the subcutaneous layer.

Accessorizing your skin

This section has nothing to do with tattoos or body piercing. It’s all about the accessory structures that work with your skin: hair, nails, and glands.

Locks, beards, and other hair

Your body has millions of hair follicles, about the same number as the chimpanzee, humanity’s closest evolutionary relative. Like chimps, humans have hairless palms, soles, lips, and nipples. Unlike a chimpanzee, however, most of your hair is lightweight, fine, and downy. The hair on your head is coarser and longer to help hold in body heat. Puberty brings about a surge of sex hormones that stimulate hair growth in the axillary (armpit) and pelvic regions and, in males, on the face and neck.

A hair arises in a hair follicle, a small tube made of epidermal cells that extend down into the dermis to take advantage of its rich blood supply. Cells at the bottom of the hair follicle continually divide to produce new cells that are added to the end of the hair and push the older cells up through the layers of the epidermis. On their way up and out, the hair cells become keratinized.

Nails and nail beds

Your fingernails and toenails lie on a nail bed. At the back of the nail bed is the nail root. Just like skin and hair, nails start growing near the blood supply that lies under the nail bed, and the cells move outward at the rate of about 1 millimeter per week. As they move out over the nail bed, they become keratinized. At the bottom of your nails is a white, half-moon-shaped area called the lunula. (Lun- is the Latin root for moon, as in lunar.) The lunula is white because it’s the area of cell growth. In the nail body, the nail appears pink because the blood vessels lie underneath the nail bed. But many more cells fill in the area of growth. This layer is thicker, and you see white instead of pink.

Glands

Glands in the skin make and secrete substances that are transported to your body’s outer surface. The contraction of tiny muscles in the gland accomplishes this secretion. The two main types of skin glands are sudoriferous glands (sweat glands) and sebaceous glands (oil glands). Here’s a rundown of each:

• Sudoriferous glands: Your body contains two types of sudoriferous glands:
  • Eccrine sweat glands, which are distributed all over the skin. These glands open to the skin’s surface, and when you’re hot, they let heat escape in the form of sweat to reduce body temperature by a process of evaporative cooling.
  • Apocrine sweat glands, which start to develop during puberty deep in the hair follicles of the armpits and groin. Apocrine sweat contains a milky white substance and may also contain pheromones, chemicals that communicate information to other individuals by altering their hormonal balance. Apocrine glands become active when you’re anxious and stressed as well as when you’re sexually stimulated. Bacteria on the skin that digest the milky white substance produce unpleasantly odiferous byproducts.

• Sebaceous glands: These glands secrete an oily substance called sebum into hair roots. Sebum helps maintain your hair in a healthy state, which is important in regulating body temperature. It flows out along the hair shaft, coating the hair and the epidermis, forming a protective, waterproof layer. Sebum prevents water loss to the outside. It also helps protect you from infection by making the skin surface an inhospitable place for some bacteria.

  In the watery environment of the amniotic sac, the human fetus produces a thick layer of sebum, called the vernix caseosa. Ear wax (cerumen) is a type of sebum produced by specialized cells in the ear canal.