View Full Version : Intro to Anatomy 6: Tissues, Membranes, Organs

The Lone Ranger
01-07-2007, 05:18 AM
An Introduction to Human Anatomy and Physiology
Chapter Six: Tissues, Membranes, Organs, and Organ Systems

Introduction: So, the fundamental unit of life is the cell. For many organisms, that’s as complex as it gets. In fact, the great majority of living organisms are single-celled. However, most of the organisms we’re familiar with are comprised of millions, billions, or even trillions of cells. In multicellular animals such as ourselves, the cells are organized into tissues, membranes, organs, and organ systems. All of these tissues, etc. work together to make up a living organism. It’s to these structures that we’ll now turn our attention. You were probably wondering when we’d start talking about some actual anatomy, weren’t you?

A tissue is a structure made up of cells of the same general type, in which the cells perform a common function. In animals such as ourselves, there are four basic types of tissues. Epithelial tissue covers body surfaces and lines body cavities. Connective tissue binds body structures together, provides structural support and protection, stores substances, and fills body spaces. Muscle tissue is specialized to contract and to cause movement. Nervous tissue is specialized for transmitting information throughout the body and for coordinating body movements.

Epithelial Tissue (Epithelium): Epithelial tissue (also called epithelium) covers body surfaces. It also lines body cavities and covers many of the internal organs. It forms a protective layer around body organs (and the epidermis of the skin protects the entire body against water loss and against invasion by pathogenic organisms), and is often specialized for secreting substances.

Epithelium lines various body tubes, including blood vessels, the passageways in the lungs, the kidney tubules, and the digestive tract. The epithelium lining the inside of body tubes such as blood vessels is sometimes referred to as endothelium. (The hollow space within a body tube is known as a lumen.)

Epithelium almost always forms relatively thin sheets of tissue, and the underside of epithelium is typically attached to an underlying layer of connective tissue by a non-cellular structure (largely made up of protein fibers) called the basal lamina or basement membrane. (The term “basement membrane” isn’t used too often nowadays, since the basal lamina isn’t a true membrane.) You can think of the basal lamina as consisting of protein fibers (especially collagen) that penetrate into both the epithelium above and the connective tissue below and so knit the tissues together.

The cells of epithelium are typically packed together very tightly, and blood vessels don’t normally penetrate into it. On the other hand, since epithelium tends to be rather thin and because the connective tissue that underlies it is well-supplied with blood vessels, the cells that make up epithelium are never far from blood vessels and so are well-supplied with oxygen and nutrients. This is important because epithelial tissue is often damaged. Fortunately, the cells that make up epithelium generally grow and reproduce quite rapidly, and so epithelial tissues are quick to heal when injured.

Epithelial tissues are generally classified according to two factors: the arrangement of the cells that make them up, and the shape of the cells. Simple epithelium consists of a single layer of cells. Stratified epithelium consists of layers of cells. Then there’s pseudostratified epithelium; pseudostratified epithelium looks like it’s layered at first glance, but if you look carefully, you can see that it consists of only a single layer of cells.

Squamous epithelium is made up of cells that are flattened in shape. Flattened cells allow for the quick movement of dissolved substances across the tissue. Cuboidal epithelium, as you might imagine, is made up of cube-shaped cells. Finally, there’s columnar epithelium, in which the cells are elongated and much “taller” than they are wide.

Types of Epithelial Tissues:
Simple Squamous Epithelium: Simple squamous epithelium, of course, consists of a single layer of flattened cells. Because the cells are flattened (and therefore very thin) and because there’s only a single layer of them, dissolved substances can quickly and easily cross simple squamous epithelium. So, you probably wouldn’t be surprised to learn that this type of epithelium is found lining the insides of your lungs. Oxygen can rapidly pass from the air in the lungs into your blood through the epithelium, and CO2 can rapidly pass from the blood and into your lungs to be exhaled. Simple squamous epithelium also lines the smaller blood vessels, allowing rapid exchange of gases and nutrients between the blood and surrounding tissues.

http://www.freethought-forum.com/images/anatomy6/ss_epithelium.gifSimple Squamous Epithelium in a Chicken Embryo
The arrows indicate a layer of simple squamous epithelium.
Simple squamous epithelium surrounds most of the internal organs, such as the intestines, for instance. These tissues don’t have much of a protective function, of course, but they may help to hold the organs in position, and they sometimes secrete fluids that lubricate and cushion the organs.

The skin of most frogs and other amphibians consists of simple squamous epithelium. This is important to them, because most amphibians get much (in some species, all) of their oxygen not from their lungs, but by absorbing it across the skin surface. The downside to this, of course, is that they rapidly lose water across their very thin skins, and so must live in moist environments.

Simple Cuboidal Epithelium:
http://www.freethought-forum.com/images/anatomy6/sc_epithelium.jpgSimple Cuboidal Epithelium Lining the Kidney Tubules

Simple cuboidal epithelium consists of a single layer of cube-shaped cells – that is, cells that are as wide as they are tall. Since it is much thicker than simple squamous epithelium, substances don’t cross simple cuboidal epithelium so easily. Instead, simple cuboidal epithelium often consists of cells that are specialized for actively transporting substances, and so they either secrete substances into body cavities or absorb substances from body cavities. For example, the tubules of the kidney are lined with simple cuboidal epithelium. The cells of this tissue actively transport urea and other substances out of the blood and then secrete it into the lumen of the kidney tubules, where it can be excreted out of the body in the form of urine.

Because of their shape, cuboidal cells can be packed together very tightly, and that’s just what happens in the simple cuboidal epithelium lining the kidney tubules. The cells are bound together by tight junctions (remember those?), so that even water molecules cannot easily pass between them and escape from the lumen of the kidney tubules.

Simple Columnar Epithelium: Simple columnar epithelium consists of a single layer of cells that are much taller than they are wide, and so appear column-shaped under a microscope. These cells, though they form a relatively thick layer, tend to be fairly fragile, and so they typically line body cavities where there’s a minimum of wear and tear. They are found lining the uterus and most of the digestive tract, for instance.

Simple Columnar Epithelium
The scale bar represents 20 micrometers, giving you an idea of the size of a typical cell.
(A micrometer is one-millionth of a meter.)

Simple columnar epithelium is often specialized for secretion. For example, the epithelium lining both the uterus and the digestive tract secretes large amounts of mucus. The mucous layer lining the digestive tract helps protect the tissues of the digestive tract from digestive enzymes and acids, and the mucous layer lining the uterus helps protect against invasion by bacteria and viruses.

The cells of the simple columnar epithelium that lines the lumen of the intestine often have finger-like projections that extend into the lumen. These microvilli greatly increase the cells’ surface areas, making them very efficient at absorbing amino acids, simple sugars, and other small molecules produced by digestion of larger molecules. The molecules can then by secreted by the intestinal epithelium into the blood, which carries them to wherever they’re needed in the body.

Pseudostratified Columnar Epithelium: Pseudostratified columnar epithelium is confusing at first, because under a microscope it looks as if it’s layered. That’s because adjacent cells often have their nuclei at different levels, and this creates the illusion that they are layered. If you look carefully though, you can see that each cell touches the basal lamina, and so the cells are not layered.

http://www.freethought-forum.com/images/anatomy6/pseudostrat.gifPseudostratified Epithelium in the Trachea (Windpipe)
Note the presence of cilia and of mucus-secreting goblet cells.

Pseudostratified columnar epithelium, like simple columnar epithelium, is typically found lining body tubes. It lines the respiratory passages, for instance, and much of the reproductive tract.

Pseudostratified columnar epithelium often has hairlike projections called cilia that extend into the lumen of the tubes they line. These cilia beat back-and-forth and can move substances through the tubes. For instance, the ciliated cells lining the female reproductive tract help create currents in the fluid filling the tract; these currents move an ovum from the ovary through the oviduct and into the uterus. Ciliated cells lining the respiratory passages help to prevent mucus from clogging the tubes and also help to sweep dust particles, microbes, and other potentially harmful substances out of the air as it passes through the tubules.

Pseudostratified columnar epithelium that is specialized for mucus production typically contains specialized cells known as goblet cells. Goblet cells are specialized for secreting mucus, and are named for their very distinctive shape.

Stratified Squamous Epithelium: Stratified squamous epithelium, as you have probably guessed, consists of several layers of flattened cells. It forms a thick, protective layer, and makes up the outer portion of the skin. The tongue and the esophagus, like the skin, are subject to a great deal of abrasion, and so the outer surface of the tongue and the inner surface of the esophagus consist of stratified squamous epithelium. The lining of the vagina also consists largely of stratified squamous epithelium. The cornea of the eye is another place where you can find stratified squamous epithelium.

Stratified Squamous Epithelium in the Tongue

It’s an imperfect world, and just because a tissue is classified as “stratified squamous epithelium” doesn’t mean that every cell in it is flat. Typically in stratified squamous epithelium, the uppermost cells are squamous in shape, while those closer to the basal lamina are more cuboidal.

Stratified Cuboidal Epithelium: Stratified cuboidal epithelium lines some of the larger ducts in the body, where it’s important to have a thick and (more or less) waterproof layer of cells. Stratified cuboidal epithelium lines the larger ducts of the mammary glands, some of the sweat glands, and the salivary ducts.

Stratified Cuboidal Epithelium Lining the Lumen of a Dog’s Mammary Gland

Stratified Columnar Epithelium: Stratified columnar epithelium is quite rare, but it does form a thick layer of cells lining the largest tubules in the mammary glands and the salivary glands.

Stratified Columnar Epithelium Lining a Body Tube

Transitional Epithelium: Transitional epithelium is found lining the urinary bladder, and the cells of this tissue are specialized to change shape in response to pressure. When the bladder is empty, these cells are more or less cuboidal in shape, but as the bladder fills the cells become compressed and flattened.

Transitional Epithelium in the Urinary Bladder

[B]Glands and Glandular Epithelium:
Glandular epithelium is named not for the cells’ shapes or arrangements, but for what the cells do. Glands are organs that secrete substances into the blood, or into body cavities or ducts – and glandular epithelium lines the glands. More precisely, glandular cells are those which produce chemical substances (proteins, lipids, etc.) that are not used by the cells themselves, but are instead stored and eventually secreted to be used by other parts of the organism.

Broadly speaking, there are two types of glands, endocrine glands and exocrine glands. Endocrine glands have no ducts, and secrete the substances they produce (called hormones) directly into the blood, which transports the hormones to where they’re needed. We’ll consider endocrine glands in some detail when we discuss the endocrine system.

Exocrine glands, by contrast, secrete their products into ducts that transport them onto body surfaces or into body cavities. The exocrine glands are classified according to how they produce their products.

Merocrine Glands: The cells of merocrine glands secrete watery, protein-rich fluids through exocytosis. Most glandular cells are merocrine in nature, and merocrine glands include some (but not all) of the sweat glands. The salivary glands are also merocrine glands.

If the substance produced by merocrine cells is thin and watery, it is known as serous fluid. For example, serous fluid is secreted by the serous membranes (serosa) lining the body cavities; the serous fluid produced by these membranes helps to lubricate the internal organs so that they don’t damage each other as they rub together.

If the substance produced by merocrine cells is thicker and contains relatively large amounts of the protein mucin, it is known as mucus. Mucus in the respiratory passages helps to trap foreign particles and pathogens in the air that would otherwise damage or infect lung tissues, and mucus in the alimentary canal helps protect the linings of the stomach and intestines against digestive enzymes and acids.

Apocrine Glands: The cells of apocrine glands lose small portions of their cytoplasm during secretion. Basically, the portion of a cell that is closest to the lumen of the duct ruptures, releasing its contents into the duct – but the remainder of the cell survives. (In apocrine glands, only the apex of the cell ruptures.) Some of the sweat glands are apocrine glands, as are the mammary glands. (So milk is made of ruptured mammary gland cells.)

Holocrine Glands: The cells of holocrine glands rupture completely during secretion, and so the substances produced by holocrine glands consist of the remains of ruptured cells. (In holocrine glands, the whole cell ruptures.) The sebaceous glands of the skin that secrete an oily, waterproofing substance are holocrine glands.

[B]Epithelial Tissues and Cancer:
Epithelial tissues form the boundary between your body and the external environment – whether it’s the skin that covers your body or the epithelial tissues that line your respiratory, reproductive, and digestive passages. As such, epithelial tissues are your first line of defense against pathogenic organisms, noxious chemicals, ultraviolet radiation, and any other substances that might cause damage to the body.

Perhaps it’s not too surprising, therefore, that something like 90% of the cancers that afflict us occur in epithelial tissues – skin cancers, cancers of the tissues lining the lungs, cancers of the stomach and intestines, and so forth. (Cancer of epithelial tissues is known as carcinoma.) Not only is epithelium directly exposed to noxious agents to a far greater extent than are other body tissues, but the cells of epithelial tissues are fast-growing and rapidly-dividing, which may make them more prone to become cancerous than are most body cells.

Connective Tissue: Connective tissue performs a wide variety of functions in the body. It binds together (connects) body structures; it forms the structural framework of the body; it provides protection for vulnerable areas of the body; it stores fat and some minerals; it fills body spaces; it transports substances throughout the body; and it helps fight infection.

There are lots of different kinds of connective tissues, but the common factor is that the cells, instead of being tightly packed together, are separated by a non-living substance known as matrix. The matrix consists of protein fibers embedded within a ground substance. The two most common protein fibers in the matrix are collagen and elastin. Collagen fibers are strong and relatively inflexible, and so provide strength. (Collagen fibers are white in color, and so connective tissues with lots of collagen are typically white.) Elastin fibers (as you might guess from the name) aren’t as strong as collagen, but they’re quite elastic. When stretched, they act like a rubber band and resume their original shape when the tension is released. Connective tissues in which flexibility is more important than is strength tend to have lots of elasin fibers. (Elastin is yellowish in color, so these tissues generally look yellow.)

Some connective tissues contain very thin collagenous fibers that form a highly branched network. These fibers are known as reticular fibers, and they form a supporting network in some tissues.

Generally speaking, the cells found in connective tissues are either resident cells or wandering cells. Resident cells remain within the tissue, while wandering cells can move into connective tissues from the blood, usually in response to an injury or infection.

Fibroblasts are the most common resident cells in connective tissue. These are relatively large, star-shaped cells that secrete protein fibers (especially collagen) into the matrix. When tissues are damaged, fibroblasts in the area multiply rapidly and secrete large amounts of collagen and other macromolecules that help to seal the wound. As you might expect, given that it’s specialized for secreting protein, a fibroblast has relatively large amounts of rough endoplasmic reticulum extending throughout its cytoplasm.

Mast cells are resident cells that are found scattered throughout connective tissues, but they are especially common near blood vessels. These relatively large cells release heparin, which prevents blood from clotting – needless to say, if your blood were to clot inside the veins and arteries, this would be a very bad thing. In response to invasion of the body by a foreign substance, mast cells release histamine, which causes inflammation, part of the body’s immune response. (Overactivity of mast cells can cause allergies.)

Macrophages are large wandering cells that can move into and out of body tissues under their own power. A macrophage moves by extending pseudopods, much the same way that an amoeba does. Macrophages are specialized to perform phagocytosis, and they play an important role in the body’s defense against disease. They engulf and destroy bacteria, viruses, and other foreign particles; they also scavenge and destroy dead or damaged body cells. Injured or infected cells release chemicals that attract macrophages to the site of injury or infection.

Loose Fibrous Connective Tissue:
Loose fibrous connective tissue forms thin, delicate membranes throughout the body. The resident cells of this tissue are mostly fibroblasts and are widely separated by a gel-like matrix that contains lots of collagen and elastin fibers.

Loose fibrous connective tissue binds the skin to underlying organs and fills the spaces between muscles. It also lies beneath most layers of epithelium, where its many blood vessels provide oxygen and nutrients to the epithelial cells.

Loose Fibrous Connective Tissue

Adipose Tissue: Adipose tissue is a specialized form of loose fibrous CT that stores fat. The cells of adipose tissue store fat as droplets in their cytoplasm, and release it into circulation when food stores are low.

Adipose tissue, in addition to storing fat, provides insulation and protection. Adipose tissue lies beneath the skin and helps insulate against heat loss. (Many aquatic mammals such as whales and sea lions have thick layers of adipose tissue beneath the skin – called blubber – that greatly slow the loss of body heat to the surrounding water.)

Adipose tissue fills some of the spaces between muscles, cushions some of the joints, surrounds and cushions the kidneys, surrounds and cushions the heart, and lies behind the eyeballs. Basically, wherever a little extra cushioning is needed to protect an internal organ, you’ll find adipose tissue.

Adipose Tissue

Dense Fibrous Connective Tissue: Dense fibrous CT contains very few cells, and most of the few that are present are fibroblasts. Dense fibrous CT consists largely of densely-packed collagen fibers with relatively few fibers of elastin; this makes it very strong, but relatively inelastic. Dense fibrous CT often binds body parts together, and it is a major component of the tendons that connect muscles to bones and of the ligaments that connect bones to other bones. Dense fibrous CT also makes up the tough white portion of the eyeball, the sclera.

Because the matrix is so dense but isn’t rigid, blood vessels rarely penetrate into dense fibrous connective tissue. Since it therefore has no direct blood supply, dense fibrous CT heals very slowly or not at all when injured – this is why damage to tendons and ligaments is so slow to heal.

Dense Fibrous Connective Tissue

Cartilage: Cartilage has a very dense, semi-solid matrix with lots of collagen fibers in it. The cartilage cells (chondrocytes) are found within chambers called lacunae that are completely surrounded by the matrix. Cartilage provides support and protection for body tissues, forms the framework of some body structures, and is important in the formation of some bones. Like dense fibrous connective tissue, cartilage has no direct blood supply, and so heals very slowly if damaged.

Cartilaginous tissue is surrounded by a fibrous connective tissue layer called the perichondrium. Blood vessels in the perichondrium provide an indirect blood supply to chondrocytes in the underlying cartilage, but because diffusion of oxygen and nutrients through the dense matrix is so slow, chondrocytes can maintain only very low metabolic rates, compared to most other body cells. So, chondrocytes grow and reproduce very slowly.

Hyaline Cartilage: Hyaline cartilage is the most common of the three types of cartilage. It has very fine collagen fibers in its matrix and looks rather like white plastic. Hyaline cartilage is very strong, though not especially flexible; it forms protective caps at the ends of bones, forms much of the framework of the nose, and forms ring-shaped structures that hold open the respiratory passages and prevent them from collapsing under the weight of surrounding tissues.

The costal cartilage that attaches the ribs to the sternum (breastbone) is hyaline cartilage. Because the ribs are attached to the sternum by cartilage instead of being fused to it directly, the ribcage can flex as we breathe. As you can imagine, breathing would be a lot more difficult if we couldn’t expand the ribcage as we inhaled and compress it as we exhaled.

Elastic Cartilage: Elastic cartilage, as you might imagine, has rather fewer collagen fibers in its matrix than does hyaline cartilage, and many more fibers of elastin. It isn’t as strong as is hyaline cartilage, but it’s much more flexible. When it’s bent or stretched, elastic cartilage quickly returns to its original shape. Elastic cartilage makes up the framework of your outer ear, which is why you can bend and twist your ear without damaging it.

Elastic cartilage also forms the framework of the epiglottis. The epiglottis is a flexible structure in your throat that separates the trachea (windpipe) and the esophagus. When we’re breathing, the epiglottis is positioned such that the windpipe is open, so air passes from the nose (and mouth) into the windpipe. (The esophagus is collapsed when it doesn’t contain food because, unlike the windpipe, it doesn’t contain hyaline cartilage to hold it open against the pressure of surrounding tissues.) When we swallow something, the oncoming food or liquid presses the flexible epiglottis down and over the opening of the windpipe so that we don’t swallow food or water into the lungs. After the food passes, the flexible epiglottis springs back into its original position, the windpipe opens, and we can breathe.

Elastic and Hyaline Cartilage
Note how the chondrocytes are encased within lacunae, and how the hyaline cartilage has
a very dense matrix with relatively few embedded protein fibers.

Fibrocartilage: Fibrocartilage is a very tough tissue that has lots of collagen fibers in its matrix, laid down in more or less parallel rows. This makes fibrocartilage excellent at shock-absorption and at resisting tensile stress. Fibrocartilage makes up the intervertebral disks that separate the vertebrae and absorb shock when we run and jump, thus preventing damage to the vertebrae. Fibrocartilage also forms shock-absorbing pads between bones in the knees.

The symphysis pubis that joins the two pubic bones in the pelvis is made of fibrocartilage as well. The symphysis pubis knits the pubic bones together strongly, yet allows the pelvis to flex during childbirth, so that the baby’s head can pass through the mother’s pelvic girdle.

It’s vitally important that these bones be joined by a strong but flexible joint. If the joint were insufficiently strong, our hips would splay outward, making walking difficult or impossible. But if the joint were fused inflexibly, it would be impossible to expand the pelvic opening during childbirth and humans wouldn’t be able to give birth to their big-headed babies.

Just as an aside, you’ve probably noticed that the hips of men and women are structured slightly differently. The hip bones tend to be flared outward to a greater extent in women than in men. This allows for women to have larger pelvic openings, which is important in childbirth, but it makes for slightly less efficient walking. Typically, when taking a step, a woman must first rotate her hips. For example, to push the right leg forward when stepping, most women have to first rotate their hips counterclockwise, so as to bring the right leg in line with the direction of the stride. Similarly, to push the left leg forward, most women have to first rotate the hips slightly clockwise, to bring the left leg in line with the direction of the step.

Many men seem to enjoy watching this process, I’ve noticed.

Cartilage and Cancer: Chrondrocytes produce a substance known as antiangiogenesis factor, which discourages the growth of blood vessels. For this reason, neither blood vessels nor nerves penetrate into cartilage. Because cartilage lacks a direct blood supply, it’s unsurprising that it grows so slowly and is so slow to repair itself when damaged. Also unsurprisingly, cancer of cartilaginous tissues (chondrosarcoma) is very rare.

Why don’t blood vessels penetrate into cartilage? Well, the matrix of cartilage is very dense, but it isn’t rigid. If blood vessels or nerves penetrated into cartilaginous tissues, they’d surely be crushed by the pressure exerted by the cartilage’s dense matrix.

The antiangiogenesis factor produced by chondrocytes has been thought of as a possible anti-cancer agent. Blood vessels are normally attracted to rapidly-growing body tissues, and tumors need very good blood supplies to support their rapid growth.

The idea, therefore, is that antiangiogenesis factor could be used to discourage blood vessels from growing into tumors, thus “starving” them and preventing their growth. Since sharks’ skeletons are made of cartilage instead of bone, some people have been promoting the notion that sharks never get cancer, and that ingesting ground-up shark cartilage will somehow protect you against cancer.

Nonsense. Sharks, like all vertebrates, do develop cancer, and there’s absolutely no evidence that eating shark cartilage will make you less susceptible to developing cancer. Furthermore, even cartilaginous tissues can become cancerous; chondrosarcoma is rare, it’s not unknown. The fact that even cartilaginous tissues can become cancerous should be all the evidence anyone needs that shark cartilage pills are a waste of money.

Growth and Repair of Cartilage: New matrix is laid down by cells in the perichondrium, but little or no matrix is produced by chondrocytes in the cartilage itself. This means that cartilage grows appositionally, but not interstitially. Appositional growth is when a body structure grows as new material is laid down on the outside of the structure. Interstitial growth is when new material is laid down by cells within the structure.

Because cartilage doesn’t grow interstitially, if it is torn, cells within the cartilage lay down little or no new matrix to repair the tear. This means that though interstital growth can cover over a tear in cartilage, the tear might never be completely repaired on the inside. This is one reason why torn cartilage often fails to completely heal, and never regains its former strength.

Bone: Bone (osseous tissue) is a connective tissue in which the matrix is a rigid solid made up mostly of calcium and phosphorous salts. (The mineral that makes up the matrix is known as calcium hydroxyapatite.) Of course, the matrix of bone also contains collagen fibers, so bones will bend – slightly – when stressed. Even so, bone is far less flexible than is cartilage; on the other hand, it’s much stronger.

Bones make up most of the framework of the body and provide structural support. Bones surround and protect many of the vital organs, including the brain. Blood cells are manufactured inside bone tissue. Bones provide attachment points for muscles, allowing for body movement. Finally, bones can store certain substances until they are needed by the body, including the minerals calcium and phosphorous, and fat.

The living cells within bone are known as osteocytes, and they are located within lacunae, just as are the chondrocytes in cartilage. Unlike chondrocytes, however, osteocytes typically have direct access to blood vessels. This is possible because the matrix of bone is rigid, unlike that of cartilage.

Bone matrix is laid down in concentric layers (called lamellae) around central canals (or Haversian canals). Each central canal contains blood vessels and nerves. From the osteocytes in the lamellae around each central canal extend tiny canals (called canaliculi). These fluid-filled canaliculi penetrate the matrix and connect osteocytes to the central canal, ensuring that the osteocytes are in contact with blood vessels.

Because osteocytes are in contact with blood vessels, they receive oxygen and nutrients from the blood at a much greater rate than do the chondrocytes in cartilaginous tissues. This means that bone cells grow and reproduce much faster than do cartilage cells. That is why bone, unlike cartilage, heals quickly and – ideally – completely after it is broken.

We’ll discuss the internal structure of osseous tissues in considerable detail when we get to the skeletal system.

Compact Bone
The relatively large black structures are the central canals, containing blood vessels and nerves. Note how the osteocytes (small black structures) are arranged in concentric rings around the central canals. You can just make out canaliculi linking the osteocytes to the central canals and to each other.

Blood: Most people don’t think of blood as a tissue, but it is. Blood is a connective tissue with a liquid matrix. Like the matrix of all connective tissues, the matrix of blood contains protein fibers (indeed, they’re vital components of the clotting process, which prevents you from bleeding to death from minor wounds), but they aren’t sufficiently numerous as to prevent the blood from flowing freely.

The primary function of blood is to transport substances through the body. Blood transports oxygen from the lungs to body tissues for aerobic respiration, and transports carbon dioxide from body tissues to the lungs for excretion. Blood transports urea and other metabolic wastes to the kidneys for excretion. Blood transports potentially toxic substances to the liver for detoxification. Blood transports digested molecules (including amino acids, simple sugars, and nucleotides – but not large amounts of water-insoluble lipids) from the intestines to body tissues that need them. Some lipids can be transported in the blood if they’re first bound to proteins; this makes them (more or less) water-soluble, so that they can be transported in the blood.

Blood also contains specialized cells known as leukocytes (white blood cells) that are important in defending the body against invasion by pathogens. Your blood is therefore your second line of defense (after the skin) against infection.

Blood consists of a liquid matrix (known as blood plasma) and various solids that are collectively known as the formed elements. Blood plasma consists mostly of water, with various proteins and electrolytes dissolved in it. Glucose, some lipids, hormones, amino acids, carbon dioxide, (small amounts of) oxygen, metabolic wastes such as urea, and other substances can be found dissolved into the blood plasma as well.

Blood plasma is transparent and slightly yellowish in color. When the clotting factors are separated out, blood plasma is known as serum. Removal of the clotting factors allows blood plasma to be stored without concern that it will clot and thus be rendered useless.

The formed elements in blood consist of three different types of cells: erythrocytes, leukocytes, and throbocytes. By far the most common are the erythrocytes, or red blood cells. The erythrocytes, like all blood cells, are manufactured inside of bones. In mammals such as ourselves, erythrocytes have no nuclei or other organelles, which means that they don’t live for very long – about 120 days at most. (Most other vertebrates have nucleated erythrocytes.)

The primary function of eythrocytes is to transport oxygen. Erythrocytes contain large amounts of the protein hemoglobin, which binds to oxygen in the lungs and then releases it to body tissues. The hemoglobin in erythrocytes allows blood to transport far more oxygen than could be dissolved into the blood plasma. It's the presence of large numbers of red blood cells that makes blood look red.

A person has the condition known as anemia if (s)he has reduced amounts of erythrocytes and/or hemoglobin for some reason. (“Anemia” literally means “without blood.”) Anemia can be life-threatening, since severe anemia means that the blood cannot deliver sufficient oxygen to body tissues.

Anemia can be caused by severe blood loss, or by insufficient amounts of iron in the diet. (Iron, you recall, is a key component of hemoglobin; iron deficiency is particularly common in menstruating women.) Some diseases (particularly those that affect the bone marrow, where all blood cells originate) can cause anemia, as can certain inherited conditions, such as sickle-cell disease. (Sufferers of sickle-cell disease don't lack erythrocytes; it's that their erythrocytes don't function normally.)

Leukocytes are also known as white blood cells, because they’re more or less white in color, when seen under a microscope. They are primarily important in defending you against infectious agents. There are several different kinds of leukocytes, and we’ll discuss their specific functions when we cover the immune system.

Leukemia is a cancer of the blood or of the bone marrow that causes abnormal production of leukocytes. Because the leukocytes produced by leukemia sufferers typically function poorly or not at all, victims of this condition are often much more vulnerable to infections than are most of us.

Thrombocytes (platelets) are responsible for coagulation (clotting) of the blood. Most forms of hemophilia are the result of a genetically-caused inability to manufacture one or more proteins that are important in clotting, but if your body doesn’t manufacture enough platelets for one reason or another, even small wounds may bleed so much that they can be life-threatening.

The condition known as thrombocytopenia occurs when a person has an abnormally low thrombocyte concentration. TCP sufferers tend to bruise very easily, and often suffer from nosebleeds or bleeding of the gums. Even small cuts bleed for a long time.

TCP can be caused by some forms of leukemia as well as infection of bone marrow. Since Vitamin B12 is important in platelet formation, too little of it in the diet can cause TCP. Ironically, some anti-cancer medications can induce TCP. TCP can be a life-threatening condition, because if bleeding occurs inside the skull, increased pressure on the brain can cause severe brain damage or even death.

Most of the cells you see are erythrocytes. Two leukocytes (neutrophils, to be precise) can also be seen.
The small purple-stained structures are thrombocytes.

[BREAK=Muscle Tissue]
[B]Muscle Tissue: Muscle tissue is specialized for contraction. Contraction of muscle tissues moves parts of the body and also moves substances within the body. Contraction of muscles also helps to generate body heat.

The cells of muscle tissues are usually elongated in shape, and contain large amounts of the proteins actin and myosin. We’ll discuss how these proteins interact to cause muscle contraction when we consider the muscular system.

There are three different kinds of muscle tissue found in humans and other vertebrates. These are skeletal (voluntary) muscle, smooth muscle, and cardiac muscle.

Skeletal Muscle Tissue: Skeletal muscles, as the name implies, are typically attached to bones. When they contract, they move the bones to which they’re attached. Since the bones form the framework of the body, when a bone moves, the body part for which it forms the framework moves as well. Skeletal muscles are the only muscles that are under our voluntary control.

Skeletal Muscle Fibers
Note that each cell has many nuclei.
The striations are caused by alternating bands
of actin and myosin. The dark bands
(A bands) are thick filaments of
myosin with thinner fibers of actin between
them, and the light bands (I bands) are
actin fibers extending beyond the A bands.

Under a microscope, the cells of skeletal muscles are elongated and cylinder-shaped. Each cell has several nuclei (not just one nucleus per cell, as in most other tissues), and the arrangement of actin and myosin fibers gives the cell a distinctly striated appearance. (This is why skeletal muscles are sometimes known as striated muscles.)

Skeletal muscle cells can contract quickly and with a great deal of force, but they don’t relax completely between contractions. (The fact that your skeletal muscles are always partially contracted contributes to muscle tone.) Because they don’t relax completely between contractions, skeletal muscles can experience fatigue if forced to contract to often or for too long. This happens because the cells can use up their available oxygen and so lactic acid levels begin to build up. If lactic acid levels become too high, the muscle fatigues and stops functioning.

Skeletal muscles actually contain two different types of fibers. The proportion of the two fiber types depends upon the kinds of exercise you do.

Red fibers (“slow-twitch fibers”) contain more mitochondria, store oxygen in the protein myoglobin (myoglobin is very similar to hemoglobin as you might imagine, and, like hemoglobin, is red in color), and rely on aerobic respiration to produce energy. They metabolize ATP relatively slowly and contract relatively slowly, but have great endurance. Marathon runners tend to have lots of red fibers in their skeletal muscles.

White fibers (“fast-twitch fibers”) have fewer mitochondria, metabolize ATP more quickly, and contract more quickly and more forcefully. On the other hand, they're more likely to exhaust their oxygen supplies and so produce lactic acid, leading to muscle fatigue. Weight-lifters and sprinters tend to have lots of white fibers in their skeletal muscles.

The cylindrical cells of skeletal muscles form muscle fibers that are bound together by connective tissues to form bundles of muscle fibers. In turn, the muscle fiber bundles are bound together to form muscles that are attached to bones by tendons.

Skeletal Muscle Structure

Smooth Muscle Tissue: Smooth muscles don’t have obvious striations when observed under a microscope. These muscles are involuntary, meaning that they aren’t under your conscious control. They line the digestive tract and other hollow body tubes, and their contractions help to move substances through these tubes.

Smooth muscle tends to be laid down in sheets, and the cells are spindle-shaped instead of elongated and cylindrical. Each cell has only a single nucleus.

Smooth muscles contract slowly and with relatively little force, but unlike skeletal muscles, they don’t fatigue. The cells within a sheet of smooth muscle tend to coordinate their contractions, so “waves” of contraction move through a sheet of smooth muscle, which helps to push substances through body tubes efficiently, if not very quickly.

Smooth Muscle of the Intestinal Wall
The outermost layer of smooth muscle is oriented longitudinally, meaning that the cells run down the length of the intestine. When these cells contract, they tend to shorten the intestine. The inner layer of smooth muscle is oriented circularly -- that is, around the intestine. When these circular muscles contract, they cause the intestine to become narrower. Coordinated contraction of the longitudinal and circular muscles squeezes material through the intestine in much the same way you squeeze toothpaste out of the tube.

Cardiac Muscle Tissue: Cardiac muscle makes up the bulk of the heart, and its properties are rather like a blending of the properties of skeletal muscles and smooth muscles. Like skeletal muscle, it is striated and contracts quickly and forcefully; like smooth muscle, it relaxes completely between contractions and is involuntary. (It’s fortunate for us that it is! Imagine what would happen if you had to lie awake all night remembering to make your heart beat!)

The cells of cardiac muscle are cylindrical in shape and striated, like those of skeletal muscles. Like the cells of smooth muscle, each cardiac muscle cell has only a single nucleus. Unlike the cells of either skeletal muscle or smooth muscle, cardiac muscle cells are branched – this makes them look “Y” shaped under a microscope.

Because cardiac muscle relaxes completely between contractions, it does not fatigue as skeletal muscle does. (Imagine what would happen if your heart simply stopped working if you demanded too much of it!) Individual cells in cardiac muscle are connected by thin structures called intercalated disks, which appear to transmit impulses between the cells so that the heart muscle contracts as a unit. [This is sometimes misinterpreted. According to the very bad movie The Amazing Colossal Man (I saw it on Mystery Science Theater 3000), the entire heart muscle is made up of just one cell. That’s complete nonsense! A man growing to 50 feet in height is physiologically impossible for many reasons, but not because the heart contains only a single cell!]

Like all muscle cells, cardiac muscle cells are surrounded by a network of collagen fibers that knit the cells together. Cardiac muscle cells aren’t so tightly bound together as are skeletal muscle cells, however.

Cardiac Muscle Cells
Note the striations, similar to those in skeletal muscle cells. A cardiac muscle cell has only a single nucleus, however. You can see that the cardiac muscle cells are branched and that cells are connected to those above and below by thin intercalated disks. Collagen fibers between adjacent cells bind them together.

[B]Nervous Tissue: Nervous tissue is specialized for the rapid transmission of messages throughout the body. It makes up the brain, the spinal cord, and the various peripheral nerves that extend throughout the body and relay information to and from the brain, spinal cord, and sense organs.

Broadly speaking, neural tissue contains two major types of cells: neurons and neuroglial cells. Neurons transmit electrochemical impulses along their length, and so allow rapid communication between different parts of the body. Neuroglial cells, by contrast, typically provide support for neurons.

Neurons: The great bulk of a neuron, containing the cell’s nucleus and its various organelles makes up the cell body. Numerous thin, branching structures (up to 10,000 or so) called dendrites extend from one end of the cell body. The dendrites transmit electrochemical impulses from sensory cells or other neurons toward the cell body of the neuron. On the other side, a single axon (it may be branched, however) extends. The axon transmits electrochemical impulses away from the cell body of the neuron, to the next neuron or to a muscle cell or a gland. Because electrochemical impulses always travel from dendrites through the cell body and out through an axon, any given neuron can transmit information in one direction only.

Dendrites and axons are very long and thin compared to the cell body, and they’re collectively called nerve fibers. Outside of the central nervous system (the brain and the spinal cord), the nerve fibers are covered by a white substance called myelin, which both insulates the nerve fibers and greatly increases the speed with which they can transmit impulses.

A single nerve fiber can be three feet long or so (!). The fibers of the sciatic nerve, for instance, extend from the spinal cord down through the leg and all the way to the toes. Nerve fibers carrying impulses to or from a particular part of the body are typically bound together by connective tissue to form nerves.

Afferent (sensory or receptor) neurons have long dendrites and short axons. The elongated dendrites of afferent neurons carry impulses from sensory organs toward the central nervous system. If numerous afferent fibers are bound together to form a nerve, it is known as an afferent or sensory nerve, since it contains only afferent fibers. The optic nerve, which transmits information from the eye to the brain, is an example of an afferent nerve.

Efferent (motor or effector) neurons have short dendrites and long axons. The elongated axons carry impulses away from the central nervous system, to muscles or glands. Of course, if numerous efferent fibers are bound together to form a nerve, it is known as an efferent or motor nerve. The oculomotor nerve is an example of an efferent nerve; it transmits impulses from the brain to the muscles that move the eye.

Interneurons (relay neurons or association neurons) are found only in the central nervous system, and they connect only to other neurons. Interneurons bridge efferent and afferent neurons, allowing for reflex arcs, and they allow communication within the central nervous system. Because they’re typically unmyelinated, interneurons are generally gray in color. This is the distinction between the “gray matter” and the myelinated “white matter” in the nervous sytem.

Some nerves contain both afferent and efferent fibers, and so can transmit messages both to and from the central nervous system. (No individual fiber can do this, of course, since a neuron will transmit impulses in one direction only.) These nerves are known as mixed nerves. The facial nerve is an example of a mixed nerve; it contains afferent fibers that allow you to taste food and it contains efferent fibers that control your facial muscles.

A Typical Neuron (This is a Motor Neuron)
Note that it has many dendrites but only a single axon.

Neuroglial Cells:
Neuroglial Cells and a Neuron
Neuroglial cells (glial cells or neuroglia) are non-neurons in nervous tissue that provide support for the neurons. (In the human brain, neuroglial cells are estimated to outnumber neurons by 50 to 1 or so.) Some neuroglia are phagocytic and protect neural tissues from infection. Other neuroglial cells form “bridges” between neurons and blood vessels, allowing neurons to rapidly absorb oxygen and nutrients from the blood. Yet other neuroglial cells produce the myelin sheath that covers neural fibers. It has recently been discovered that some neuroglial cells play a role in signal transmission, so it appears that neurons aren’t the only cells in nervous tissue that can transmit information after all.

Think of it: there are something like 100,000,000,000 neurons in the adult human brain, and each of them is connected by its dendrites to perhaps 10,000 others. The number of possible brain states in a human brain is truly staggering. If neuroglial cells play a major role in signal processing, this may raise the possible number of brain states by a factor of 10 or more.

Neural Tissue
Several relatively large neurons can be seen,
as well as many relatively tiny neuroglial cells.

Incidentally, the brain works somewhat like a muscle in that it grows “stronger” with use. Studies with mice have shown that if they’re raised in intellectually challenging environments (where they’re forced to solve puzzles to get food, for instance), they have something like 10 times the density of dendritic connections in their brains as do mice raised in environments where they’re simply provided with food and water and aren’t given any intellectual stimulation in the form of toys.

Autopsies of human brains have provided convincing evidence that people who lead richer intellectual lives (people who read a lot, and otherwise keep mentally active) have more dendritic connections in their brains than do those who don’t read or otherwise keep mentally active. Not only does frequent mental stimulation make it easier to learn new things, people with active mental lives seem to be less susceptible to degenerative brain disorders such as Parkinson’s and Alzheimer’s – possibly because their brains, having more dendritic connections, are generally more “robust.”

A membrane generally consists of two layers of tissue – a layer of epithelium underlain and supported by a layer of connective tissue. (Some membranes consist only of thin layers of connective tissue, with secretory cells scattered through the matrix.) Since membranes contain epithelial tissue, they’re typically found lining body cavities, surrounding organs, and lining tubes that lead out of the body. The skin is also a membrane.

Mucous Membranes: Mucous membranes, as you’ve probably guessed, contain epithelium (usually columnar epithelium) that is specialized to secrete mucus. Mucous membranes line tubes and organs that lead to the outside of the body, such as the digestive, respiratory and reproductive passages.

Mucus secreted by mucous membranes helps protect the digestive, reproductive, and respiratory tracts from infection by viruses and bacteria. In the digestive tract, mucus provides a protective barrier against digestive enzymes and acids that would damage the lining of the tract. Mucus membranes in the mouth and throat provide lubrication that protects against damage during chewing and swallowing. Mucous membranes in the female reproductive tract provide lubrication that protects against damage during sexual intercourse and during childbirth.

Serous Membranes: Serous membranes, of course, secrete serous fluid. These membranes are found lining the internal organs and the body cavities, and they consist of a layer of simple squamous epithelium underlain by a thin layer of connective tissue. These membranes secrete serous fluid that cushions and lubricates the internal organs. In addition, they help support the internal organs and hold them in position; they also compartmentalize body cavities, which helps to slow infections.

Within the thoracic and abdominal cavities, most internal organs are surrounded by two layers of serous membranes. The cavity within which an organ sits is lined by a parietal serous membrane, and the organ itself is surrounded by a visceral serous membrane. For example, the heart sits within a cavity called the pericardial cavity. The pericardial cavity is lined by the parietal pericardial membrane (parietal pericardium), and the heart itself is surrounded by the visceral pericardial membrane (visceral pericardium).

The parietal and visceral membranes secrete serous fluid that fills the space between them, lubricating and cushioning the organ(s) contained within the cavity.

Pleural Membranes: Each lung is contained within a pleural cavity. Naturally, each pleural cavity is lined by a parietal pleural membrane and the lung itself is surrounded by a visceral pleural membrane. Uniquely, the pressure of the serous fluid secreted by these membranes is slightly less than is normal air pressure. This means that the air inside the lungs presses outward with more force than the serous fluid surrounding the lungs presses inward. That pressure difference is what keeps the lungs inflated. If the pleural cavity is punctured and air enters the space between the parietal and visceral membranes, its pressure can force the lung to collapse. This is known as a pneumothorax, and needless to say, is a very bad thing.

Pericardial Membranes: The heart, as mentioned a moment ago, sits within the pericardium, and is surrounded by parietal and visceral pericardial membranes. Were it not for the serous fluid secreted by these membranes, and for the layer of fat surrounding the pericardial cavity, the heart would surely be damaged as it beat because of abrasion against the ribs and sternum.

Peritoneal Membranes: Peritoneal membranes (peritoneum) line the abdominopelvic cavity and most of the organs lying within it. Smooth muscles of the stomach and intestines cause these organs to move somewhat during the digestive process. Were it not for lubrication from the serous fluid secreted by parietal and visceral peritoneal membranes, these organs might be damaged by abrasion.

Synovial Membranes: Synovial membranes line joint cavities where two bones come together to form a movable joint. (Where two bones come together is a joint or articulation. Not all joints are movable – movable joints are known as synovial joints.) Unlike most membranes, synovial membranes contain no epithelium. Synovial membranes secrete synovial fluid into the space between two bones, which lubricates the ends of the bones and prevents them from damaging each other as they rub together. If inadequate amounts of synovial fluid are produced, bones may abrade each other as they move; splinters of bone in the joint cavity may cause irritation and swelling of surrounding tissues. This inflammation of joint tissues is known as arthritis.

So far, no one has developed a synthetic lubricant that’s as slippery as is synovial fluid.

Meninges: Meninges (meningeal membranes), like synovial membranes, contain only connective tissue. These membranes surround the organs of the dorsal cavity – that is, the brain and the spinal cord.

There are three layers of tissue in the meninges: the pia mater, the arachnoid mater, and the dura mater. The pia mater (the name means “tender mother”) lies closest to the brain and the spinal cord. Blood capillaries within the pia mater supply the brain and spinal cord with oxygen and nutrients. The arachnoid mater is named for its “spiderweb-like” appearance. The space between the arachnoid mater and the pia mater (the subarachnoid space) is filled with cerebrospinal fluid, which cushions and lubricates the brain and spinal cord. The dura mater (“hard mother”) is a tough and relatively inflexible outer membrane that surrounds and protects the central nervous system. It fuses to the underside of the skull and to the inside of the neural canal in vertebrae. The dura mater contains larger blood vessels that split into the capillaries of the pia mater.

A bacterial or viral infection of the meninges can cause inflammation and swelling of these membranes. This is meningitis, and the increased pressure can cause damage to the delicate underlying tissues of the brain or spinal cord.

A subdural hematoma occurs when veins bridging the dura mater and arachnoid mater are torn. This is typically a result of head trauma, such as may occur during an automobile accident, for instance. Blood leaks into the space between the the dura mater and arachnoid mater, and the increased pressure can cause severe brain damage.

An epidural hematoma is similar to a subdural hematoma, except that it’s caused by tearing of arteries in the dura mater. This causes blood to fill the space between the dura mater and the skull. As in a subdural hematoma, the increased pressure can quickly cause severe brain damage.

The Cutaneous Membrane: The cutaneous membrane is the skin. It consists of an outer layer of stratified squamous epithelium underlain by a thicker layer of loose connective tissue and/or adipose tissue. We’ll discuss the cutaneous membrane in some detail in the next chapter.

Organs and Organ Systems: A tissue, as you recall, consists of cells of the same general type that perform a common function. An organ is a structure made of two or more tissue types that together perform a common function.

For example, the stomach contains an inner layer of epithelium that is supported by connective tissue. Layers of smooth muscle surround the epithelial and connective tissue layers. Nervous tissue relays messages from the central nervous system to the muscles and glandular tissue of the stomach.

Groups of organs that work together to perform a common function make up an organ system. For example, the stomach is part of the digestive system. For the sake of convenience, the various organ systems are often divided into various categories, according to what they do.

Organ systems that are important in support and movement of the body include the integumentary system, the skeletal system, and the muscular system.

The nervous system and the endocrine system allow the integration and coordination of body functions.

Transport of substances throughout the body and immunity against disease are the responsibility of the cardiovascular system and of the lymphatic system.

Absorption of nutrients and oxygen and excretion of metabolic wastes is the responsibility of the digestive system, the respiratory system, and the urinary system.

Finally, the reproductive system is responsible for … well … reproduction.

In the pictures below, you can see many of the components of the various organ systems and their positions in the body. The remaining chapters of this series will focus on the individual organ systems. We’ll start with the one organ system you can observe without taking a scalpel to your acquaintances – the Integumentary System.

Superficial Body Structures
Here you can see the skin (the cutaneous membrane), the mammary glands,
and some of the major superficial muscles.

Superficial and Deep Muscles
Here you can see many of the superficial muscles (mostly on the subject's right) as well as deeper muscles (mostly on the subject's left). Some of the more superficial veins and arteries can be seen as well. The femoral nerve can be seen extending into the leg.

The Viscera
Here, most of the overlying muscles have been removed, revealing the underlying viscera. The ribcage surrounds and protects the heart and lungs, as well as most of the stomach and liver. The diaphragm is a muscle that separates the thoracic cavity and the abdominopelvic cavity. The mesentery is a layer of tissue that hangs down over the intestines like an apron; it secretes fluid that lubricates the intestines.

The Internal Organs
The ribcage and mesentery have been removed, exposing most of the organs of the thoracic and abdominopelvic cavities. Most of the respiratory system can be seen, as can most of the organs of the digestive system. Most of the major veins and arteries can be seen. Most of the male reproductive system is visible.

Deep Body Structures
The internal structure of the heart and lungs is visible. The ureters, which transport urine from the kidneys to the urinary bladder are visible. Most of the female reproductive system (which lies much deeper than does the male reproductive system) is visible. Some of the deeper veins and arteries are visible. The deep muscles of the upper leg are visible.

Deeper Body Structures
The esophagus can be seen, lying deep to the trachea. The kidneys can be seen, lying behind the main portion of the abdominopelvic cavity. Most of the major deep blood vessels can be seen. More of the deep muscles of the leg can be seen.

Really Deep Body Structures
Here you can see the back of the ribcage. You can see how the esophagus, the vena cava (the body's major vein) and the aorta lie almost as far back as do the vertebrae. Many of the bones that form the framework of the body are visible.