A Skeleton Made Of Sugar Sucks;… A Flexible Water Skeleton That Can Suck Or Blow;… Some Seek Shape And Shelter Within Their Own Metabolic Wastes;… Your Dry Bones;… Shock Absorbers Reduce Structural Stress;… Collagen Pulls While Cartilage Sucks;… You Were Patterned In Cartilage;… Perichondrium;… Bone Formation;… Your Inner Rocks;…
Bones Rarely Break;… Bone Remodeling Reduces Risk Of Fracture While Controlling The Calcium Concentration In Your Blood;… Avoid Osteoporosis, Rhubarb And Spinach.
Strong as it is, collagen alone could never hold you up. To stand tall, or even short, you also need a rigid incompressible framework that can resist gravity and other accelerations. So just as tent poles bring structure to a floppy cloth tent, your bony skeleton provides functional form to your otherwise soft and shapeless substance.
A Skeleton Made Of Sugar Sucks
Support alone does not guarantee mobility, of course, for otherwise trees would wander in and out of the shade as you do. Instead trees are immobile strongly rooted porous laminations of polar (thus water-loving) cellulose fibers bonded by lignin. Those sturdy and wet cellulose fibers enclose glucose-filled osmotically active plant cells that draw great masses of polar water out of barely moist soils.
Entrapped water then unfurls and supports many new branches and leaves in order to capture more solar energy in reduced carbon (glucose) form. Their water burden makes larger trees enormously heavy—about as heavy as the same volume of water.
That is why freshly cut live-tree logs other than balsa do not float very high. Lum- ber shrinks as it dries, losing both weight and flexibility.
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A Flexible Water Skeleton That Can Suck Or Blow
A squid actively traps water for support within its tough cone-shaped mantle- muscle-surrounded body cavity. The weight of this trapped water is no deterrent to streamlined travel through the equally dense surrounding seas. Indeed the inertia of its internal water column ordinarily helps the squid move slowly astern with minimal effort while also stiffening it into proper squid shape. But for an urgent departure, the squid forcefully ejects much of that skeletal water through a jet nozzle that can swivel in almost any direction. Its reduced size and inertia then allow the just-contracted squid to accelerate more rapidly. Caterpillars rely upon internally generated hydraulic pressures for their support and locomotion since they too lack rigid internal or external support structures. As a result, their locomo- tion on land is slow and energetically costly. However, one soft-bodied legless fruit fly larva (maggot) is able to proceed more efficiently by jumping 12 cm (5 inches) at a time. It achieves that mode of muscle-assisted legless hopping by curling tightly and then utilizing the elastic recoil of sudden release to help it evade predation by ants.
Some Seek Shape And Shelter Within Their Own Metabolic Wastes
For over 600 million years, single and multicelled organisms have sought sup- port and shelter from predators within sturdy external deposits composed of their own waste CO2 co-precipitated with spare calcium as calcium carbonate (CaCO3).
Even giant clams find such shells economical, functional and reproductively ad- vantageous, but a similarly proportioned structurally sufficient limestone exoskeleton (external skeleton) for you would weigh many hundred pounds. To achieve easy mobility, animals must excrete or abandon all non-essential weight.
Your Dry Bones
By weight you are one-half to three-fourths water. That water fills out and even inflates various of your tissues and organs. But your bony skeleton is a rather dry composite of strong collagen fibers interlocked with metabolically essential (and potentially toxic) calcium phosphate salts. Being quite insoluble and more dense than water, bones sink to the bottom of your soup. Within their hollow cores, your bones either contain fat cells (yellow bone marrow) or blood-forming cells (red marrow). Most humans enclose enough body fat plus gases in lungs and bowels to barely float their bony burden in fresh water or more easily in heavier salt water. Thus live people are nearly of water density (which is about 1 gram per ml.
or 2.2 pounds per liter or a bit over a pound to the pint). Bird-bones enclose air but live seabirds float high or low depending upon how much air is trapped within their lightly oiled and therefore water repellent plumage. Unfortunately those non- polar insulating feathers readily absorb spilled hydrocarbons—heavily oiled birds that do not become hypothermic (cold) or drown are soon poisoned by oil ingested during self-grooming. Your relatively lightweight but rigid endoskeleton (internal skeleton) provides sturdy support for your tissues. To allow mobility, that incom- pressible skeleton is broken into hundreds of independent metabolically active bones.
In turn, each of those appropriately sized collagen-reinforced shatter-resistant skel- etal struts and levers is strengthened, aligned and securely positioned to transfer all reasonable forces safely and effectively—which allows you to range widely and avoid predation as you seek water, steal solar energy from plants and devour other herbivores (animals that prey upon plants) or carnivores (such as fish).
Shock Absorbers Reduce Structural Stress
Heavy collagenous restraining straps and hinges at each of your joints (where two or more bones meet) place strict limits on the movements of your bony struts with respect to each other, for an excessively mobile bone could easily injure adja- cent tissues or allow catastrophic collapse due to loss of leverage. Furthermore, the strong muscles (and their tendons) that surround and spring those joints, markedly reduce the effect of sudden strains or impacts by resisting or giving way slowly and elastically. But only your soft water-filled cartilages can provide essential hydraulic cushioning to absorb shocks where rock-hard bones contact and move across one another. Clearly you are held in shape by an endless array of interwoven collagen fibers, threads, strings, ropes and cables ranging from the fine meshwork of your loose connective tissues and skin to the taut fibers that transfer the pull of each muscle fiber into the thick collagen bundles of your tendons. In turn, those ten- dons penetrate the fibrous periosteal wrapping of your bones to meld with the organized collagen bundles of your skeleton. Obviously there is much more to life than just collagen, but if you could somehow dissolve away their other molecules while keeping each collagen fiber intact and in place, your best friends would remain readily recognizable. They might appear a bit pale, ghostly and worse for the wear and tear, but their entire structure, even teeth and eyes, would remain fully formed.
Collagen Pulls While Cartilage Sucks
In common with other connective tissues, cartilage consists mostly of col- lagen (and numerous other fibrous proteins), various proteoglycans and jellied (trapped) extracellular water containing the usual solutes. Cartilage maintains a higher osmotic pressure than surrounding tissues through its increased concentra- tions of highly anionic and hydrophilic proteoglycans and the many Na+ cations that these proteoglycans attract. So the extracellular matrix of cartilage tends to imbibe water until it becomes tightly distended—that is, until its collagen fibers are fully stretched out and tense. Therefore the firmness and compressibility of your cartilages reflects their high osmotic pressure. Where joints require only lim- ited flexibility (between your vertebrae, for example), both bones can be securely bonded to a single intervening fibrous (collagen-rich) cartilage. But the more mo- bile synovial joints of your extremities depend upon the separate bearing surfaces of each bone end being individually cushioned by a smooth hyaline (more watery/less fibrous) cartilage cap. The slippery gas-and-nutrient-rich synovial fluid secreted by surrounding synovial membranes lubricates your hyaline joint spaces. Hyaline cartilages require far less collagenous reinforcement than fibrous cartilage because their opposing surfaces ordinarily slide so smoothly that hyaline cartilage encoun- ters only compressive (and not shear or friction) forces. However, fully compressed hyaline cartilage resists sliding so a hyaline cartilage forcefully pinned into position can be torn by severe shear stress—the meniscus cartilage of the knee is commonly injured in this fashion when football players are hit while their foot is firmly planted.
And badly torn cartilage cannot heal properly, for chondrocytes (the cells that first deposit and then live on within your cartilage) generally reside too far from their own blood supply to engage in vigorous healing. Indeed, chondrocytes are sus- tained by the rather slow diffusive exchange of gases and solutes with adjacent tissue capillaries (often via the synovial fluid).
That diffusion of gases and nutrients is greatly speeded by the repeated carti- lage compressions of ordinary use. So cartilage tends to atrophy (shrink) with disuse or hypertrophy (flourish and thicken) as a result of ongoing vigorous but not exces- sive joint activity. However, the persistent and extreme stresses applied to various joints by gymnasts and distance runners can eventually disrupt cartilage, as can ordinary wear and tear (by the time you reach old age) or any local bacterial infec- tions or ongoing damage consequent to autoimmune (misdirected at self) antibodies.
The repair of joint damage generally involves inflammation, enhanced circulation and deposition of scar tissue by fibroblasts—all three may interfere with smooth
and painless joint function, especially since any ingrowth of blood vessels encour- ages cartilage conversion to bone. A multitude of generally painful joint dysfunctions are descriptively grouped as various forms of arthritis. Although cartilage rebounds from an impact (brief compression), it tends to creep (deform progressively) under ongoing pressure. This accounts for your half centimeter (about 1⁄4 inch) loss of height during an ordinary day’s upright activities. You regain that height at night when those decompressed cartilages are again free to imbibe fluid (weightless space- persons may gain a temporary inch in height by that same mechanism). It seems that all vertebrates must walk (swim, fly) on trapped water in this fashion, for without cartilage there would be nothing to prevent your bones from cracking and grinding together until eventually they became solidly fused (connected through their surfaces by scar tissue and calcium phosphate deposits), leaving you an im- mobile pushover.
You Were Patterned In Cartilage
Every surface cartilage of your many fitted synovial joints seems perfectly sculpted for its setting. Detailed design specifications need not clutter your DNA, however, for cartilage tends to develop and assume proper shape in response to conditions at sites where it is needed. Even a fractured (broken) bone that fails to mend properly may develop a psuedarthrosis (false joint) at the break site. Such a psuedarthrosis is commonly associated with pain and malfunction since any weight placed upon the false joint tends to cause buckling with forceful displacement of broken bone ends into surrounding soft tissues. Interestingly, both bony ends of a chronically non-healed fracture tend to develop smooth cartilaginous caps of the sort one might find in an ordinary synovial joint. Similarly, when a congenitally displaced hip (without a normal hip socket) is properly positioned soon after birth, it causes a fully functional well-shaped hip joint to develop—complete with carti- lage and ligaments—at the site of persistent contact with pelvic bone. Easily modified flexible cartilage skeletons allow sharks and rays of all sizes to prosper—however, adult fish and land-dwelling animals gain reproductive advantage by replacing the cartilaginous fetal skeletal pattern with a more rigid structure. Most larger land- based herbivores must soon be ready to flee for their lives so they are born with quite advanced bone formation. In contrast, the primarily cartilaginous construc- tion of human infants reduces risk of injury to mother and child while still providing sufficient support for effective screaming, eating, excreting and smiling.
Perichondrium
An external collagen-fiber-reinforced envelope surrounds cartilage except at its bony or free (synovial joint) surfaces. This firmly attached perichondrial (around cartilage) layer carries nutrient blood vessels that serve the underlying avascular (lacking in blood vessels) cartilage—perichondrium also produces new cartilage.
Forty or fifty years ago, wrestlers competing without ear protectors often suffered repeated mat friction (shear) injuries that separated a patch of ear perichondrium from its underlying cartilage. Repeated episodes of new cartilage formation along the inner surfaces of such displaced perichondrium (over blister-like collections of blood) eventually led to unsightly “cauliflower ears”. Similarly, when rib cartilage is surgically removed but some of its surrounding perichondrium remains in place, new and irregular cartilage or even bone will soon regenerate from those disrupted perichondrial surfaces. But do not be misled by how readily the tasty perichon- drium or periosteum (around bones) comes away from those shiny barbecued cartilage or rib surfaces at a picnic—that only occurs because the heat and moisture of your skillful barbecuing has converted their sturdy collagen to gelatin.
Bone Formation
During normal growth and development, the human fetal skeleton ossifies (turns to bone) under the influence of transforming growth factors as blood vessels invade the cartilage. Because this process follows a standard schedule, the current stage of skeletal maturation is referred to as the skeletal or bone age of the fetus, infant or child. With each defined in terms of the other, bone age and chronologi- cal age are normally about the same. So a skeletal X-ray examination should reveal the approximate age of any youngster. Chondrocytes give way to osteoblasts (bone- producing cells) under the influence of a variety of related bone-growth proteins (that may originate in the kidneys) and other signal or support molecules. These osteogenic proteins or morphogens also encourage other tissues to become bone. While size, shape, location and use all affect their final structure, bones generally develop a hollowed-out (often cross-braced) interior surrounded by a sturdy dense-bone cortex. Ordinary tensile and compressive forces act mainly upon outer bone edges so this more-or-less hollow design reduces weight without much loss of strength.
Dense cortical bone is mostly laid down in tiny side-by-side cylinders—each surrounding a central nutrient blood vessel derived from periosteal vessels. The thin outermost bone layers that surround all of these concentrically layered cylin- drical Haversian systems are deposited just under the periosteum. Once fully enclosed within bone by their own activity, osteoblasts become known as osteocytes (a bit like
changing your name after painting yourself into a corner). Individual osteocytes (osteo means bone, blast means producer, cyte means cell) transfer nutrients, wastes and gases to neighboring cells via thin protoplasmic extensions that pass through tiny canaliculi (interconnecting bone passages).
Following ossification of the original skeletal cartilage, bone growth contin- ues circumferentially from periosteum and lengthwise (in long bones) from transversely positioned cartilaginous growth centers. These epiphyseal growth plates are located where each end of the diaphysis (bone shaft) widens out to become the proximal or distal epiphysis. By widening near each joint surface, long bones are better able to resist any oblique compressive strains transferred across the adjacent synovial joint—those larger joints are more easily stabilized and cushioned. The expanded epiphyses sufficiently reduce pressures applied to joint surfaces so that bony epiphyses can be built of lighter cancellous (spongy or porous) bone. In con- trast, the compact bone of which the narrow diaphyseal shaft is made permits some slight twisting and bending that can improve resistance to temporary overloads (a hollow cylindrical bone-substitute of uniform diameter and equivalent strength would be totally inflexible and far heavier). Some flat bones of the skull form di- rectly from periosteum through intramembranous bone formation rather than via endochondral bone formation (by conversion of a cartilage model into bone).
Where flexibility has advantages over rigidity, cartilage may persist in the final structure. So cartilage gives form to your ears and the tip of your nose as well as those anterior (front) rib cartilages—although the latter do ossify slowly as you age.
The growth rate of your skeleton (and soft tissues) is subject to the influence of various growth and steroid hormones. Too much growth hormone secreted by a pituitary gland tumor during childhood or adolescence (before those epiphyseal plates mature into solid bone and finally terminate longitudinal growth) can lead to excessive height and size. Anabolic steroids and testosterone (male sex hormone) have similar effects on bone and muscle growth, causing an epiphyseal-plate-based growth spurt in children and adolescents. Unfortunately, anabolic steroids are fre- quently utilized (abused) by athletes seeking improved performance or greater muscle mass. But steroid-induced skeletal-growth-spurts markedly accelerate bone matu- ration (just as your own adolescent growth spurt soon tapered off ) so the net result is premature closure (maturation into bone) of all epiphyseal plates and cessation of bone growth (at least in length). Thus the use of anabolic steroids can bring about a shorter adult stature than would otherwise have resulted. More important ad- verse effects of anabolic steroids on brain, liver, testicles and immune system are discussed in later chapters.
Your Inner Rocks
Hydroxyapatite or Ca10(PO4)6(OH)2 is the crystalline mineral that stiffens your bones and teeth. Like most rocks, hydroxyapatite is rather insoluble in plain water.
The flexibility of a bone varies with its mix of collagen fiber and calcium phos- phate. A high concentration of collagen allows deer antler bone to absorb repeated severe impacts with low likelihood of antler fracture (or of severe headache that might delay mating). On the other hand, whale ear bones are mostly rigid mineral in order to transmit sound vibrations with minimum loss and high fidelity (so they are easily broken by nearby underwater explosions). In all cases, collagen provides an essential site where bone deposition can begin, with the first tiny hydroxyapatite seed crystals being laid down within those 400 A° gaps between the tips of neigh- boring tropocollagen fibers. By thus interlocking strong collagen fibers with sturdy calcium phosphate crystals, the advanced composite material known as bone has provided life-saving strength and resilience for several hundred million years. But tiny changes can occasionally cause big trouble—as in Osteogenesis Imperfecta (“brittle bone disease”)—this well-known, generally lethal bone condition is so strongly selected against that its prevalence depends entirely upon the rate of new genetic mutations. The fact that this disease is associated with frequent bone fractures simply demonstrates the crucial importance of adequate collagen fiber reinforce- ment to bone strength. One common cause of this disease is a single nucleotide error (T for G) that leads to cysteine (a larger amino acid) replacing a glycine near one end of each collagen polypeptide. A number of other substitutions and even exon deletions can produce a similar partial unfolding of the collagen triple helix—
leading to marked weakness of those abnormal collagen fibers (see Collagen).
Bones Rarely Break
To split firewood, you must initiate and then advance a crack that parallels the majority of structural fibers. By forcefully prying apart the two sides, the enter- ing wedge or axe head delivers much of its force at the point of the crack. But a split may not progress if too much of the force is dissipated by excessive flexibility of the sides—that is why a harder strike is far more effective, and frozen or dried wood so much easier to split. A knot (old branch crossing deep inside the wood) can simi- larly stop a split as tension at the crack tip then spreads out along (and is resisted by) those knotty cross-fibers. This explains why your axe is now stuck in the log, although it may not help you to extract it. And that short crack in your automobile windshield may soon lengthen as stresses or movement between the two sides exert