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Intro to Anatomy 6: Tissues, Membranes, Organs
Intro to Anatomy 6: Tissues, Membranes, Organs
The Lone Ranger
Published by The Lone Ranger
01-07-2007
Default Connective Tissue


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.

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