Skin, Bone and Muscle

The integument is the outer body covering layer in animals. In vertebrates, it is referred to as skin.

It is a strategically located boundary between the animal and its environment and it acts as protective barrier between the animal and the environment. It shields the internal structures of the animal from a changing and often harsh environment that might otherwise infect the body (bacteria), evaporate its fluid (environments are often drying) or mutate the bodies genes.

Skin is actually a biological cooperative between four tissue types: epithelial, connective, muscle and nerve tissues. Thus skin is actually an organ. Indeed, it is the largest organ in your bodies.

Human skin consists of two distinct layers: the outer epidermis and the inner dermis.

The epidermis is a protective epithelium consisting of several layers of cells. These cells are formed by a layer of cells in the deepest layer of epidermis and move towards the surface.

As these cells get closer to the surface, they become flattened and began to elaborate fair amounts of a kind of filament called keratin. The outermost layer of your epidermis is highly keratinized and dead. At the same time, it is airtight, watertight and quite resistant to bacteria and most chemicals. It provides an amount of protection from the ultraviolet radiation of the sun by possessing a pigment called melanin. Some individuals have more of this pigment than do others.

The dermis lies below the epidermis and is a rather complex layer consisting of dense connective tissue, a rich supply of blood vessels, nerve fibers and smooth muscle cells.

Let's look carefully at the structure of skin.

Blood flow to the dermis can range from a mere trickle when heat must be conserved to as much as 50% of the total blood supply when heat loss is necessary to cool the body. The pinkness that you can see in light-skinned people is due to the presence of blood flowing though the dermis.

What about hair? Hairs consist of dead, keratinized cells similar to that of the outermost layer of skin. Hair is formed from a living follicle of cells. What is hair good for? Mostly it acts as a cover for protecting a body from abrasion and as insulation from heat loss.

Humans probably evolved in a warm, tropical environment and was accompanied by a loss of much body hair. Natural selection operating, selecting those individuals better able to loose excess body heat perhaps. Those body regions where humans retain body hair are enigmatic.

Skin contains a large number glands whose secretions eventually arrive at the surface of the body. They are of two broad types: sebaceous glands that produce a mixture of lipids (sebum) that oil the hair and skin, keeping it pliable and sweat glands wich are distributed over the surface of the body. Sweat glands secrete a dilute salt solution which evaporates and cools the body.

That the skin is an extremely important can be readily seen when catastrophes such as large areas being burned occur. Burns to as little as 20% of the body can be fatal if not treated rapidly. The proximate cause of death in these circumstances is dehydration-- the body just loses too much water.

The Skeleton

Most animals possess some kind of rigid support structures. This can be an exoskeleton or an endoskeleton. We will not entertain the insects or crustaceans with their exoskeletons or animals such as earthworms that have a hydrostatic skelton. You might wish to refer to your texts about these organisms.

The endoskeletons of vertebrates are the most complex and versatile. Vertebrates are supported by internal skeletons composed of two distinct types of connective tissues: bone and cartilage.

Bone has two neat properties-- it is strong and it is light. Bone is strong-- Maybe four times as strong as concrete-- yet it adds only about 14% to the weight of a body.

What gives a bone its strength and lightness. In a few words-- its architecture. It is well designed for its function. The bulk of bone is extracellular matrix-- materials excreted by bone cells (osteocytes) outside of the cells themselves. This extracellular material is complex and consists of collagen (a protein material that is cable-like) and calcium phosphate. Together the protein and mineral produces a material much like concrete and rebars (the metal rods one sees being placed within concrete forms to add strength). Without collagen, bone would tend to be brittle; without calcium, bones tend to be rubbery and bend.

What is osteoporosis? Bones are living, dynamic tissues that require continual maintenance and repair. Maintenance is done in two stages: First, an existing portion of bone matrix is broken down and second, new matrix is deposited. That is, bone is constantly being remodeled.

The remodeling process is still not well-understood beyond that it is regulated by a number of factors (remember calcitonin, parathyroid hormone, mineralocorticoids, growth factors and even vitamin D).

Bone is disassembled by cells called osteoclasts which emerge from bone marrow, migrate to particular sites and begin to disassemble bone. Osteoclasts secrete acid which dissolves the calcium salts of the bone matrix and collagenase, which chews up the protein collagen. In about 10 days, an osteoclast has produced a crater that is ready to be filled by a newly arriving osteoblast (a young bone cell).

One reason that we are very interested in how bone formation occurs is so that we may develop better treatment for a condition called osteoporosis, a bone weakening condition common in older women. A person with osteoporosis is often seen to have a so-called "Dowager's hump," and is very susceptible to bone fractures, particularly of the hip or vertebrae. Osteoporosis occurs when bone breakdown occurs at a more rapid rate than bone replacement.

Osteoporosis is particularly common in women who undergo early menopause or how have had their ovaries removed early in life. The cessation of estrogen production seems to be involved as a major contributing factor in the onset of osteoporosis. At the present time, osteoporosis seems to be best treated by the administration of estrogen. Estrogen binds to receptor sites on osteoblasts and encourages them to increase bone deposition. But the details of this process are simply not well-understood and truly need further research.

Bone is alive and concrete is not. Within the solid mass of bone matrix, one finds living osteocytes.

Each bone has a solid, quite hard portion called compact bone. This usually surrounds a region of more honeycombed region of bone called spongy bone. The hollows within the spongy region of bone are filled with red marrow, a soft tissue that produces red blood cells.

Let's look carefully at the structure of bone.

Compact bone consists of osteocytes embedded or engulfed in an extracellular matrix which has been deposited as a matrix of concentric cylinders called lamellae which results in a greatly strengthened structure overall.

Osteocytes obtain their necessary nutrients and gaseous exchange from blood vessels that are threaded though channels in the extracellular matrix. The channels are called Haversian canals. These larger Haversian canals communicate with small canaliculi-- microscopic channels which in turn communicate with the osteocytes themselves.

Spongy bone consists of thin, bony elements that surround marrow filled chambers.

Long bones such as the femur (thigh bone) grow in size during childhood and adolescence. At the ends of these bones are epiphyseal plates and represent places where the bone is not yet mineralized-- where the bone can still grow in length. These epiphyseal plates are the last portions of long bones to become mineralized in adulthood.

The presence of these epiphyseal regions is the reason why contact football for still growing boys is not a good idea. If these plate regions are damaged, bone growth can be affected adversely.

Finally, bone is covered by a connective tissue sheath, the periosteum.

The constant interplay between bone disassembly and reassembly allows our bodies to strengthen those bones that are receiving the most use and worry less about those bones in less demand. Disuse of your body tends to cause an atrophy (wasting away) of bone. Even a few weeks in a cast diminishes the overall size of a bone.

Like bone, cartilage consists of living cells that secrete an extracellular material that envelopes cells. Cartilage provides strength and resilience but its lack of mineral deposits allows it to retain flexibility. If you want to feel some cartilage, reach up and wiggle your nose or ear lobe with your fingers. Lots of cartilage in both structures.

By the time you are born, most of the cartilage present in the early embryo has been converted into about 350 partially hardened bones. As growth progresses, many of these bones fuse to finally yield the 206 odd bones that you possess as an adult.

These bones articulate through various kinds of joints.

Consider the axial skeleton which consists of bones aligned along the long axis of the body. These bones are mostly for protection. There is a precious three pound mass of tissue called the brain that resides in a bony structure called the skull. The skull consists of the cranium (eight bones) plus 11 additional bones. At birth, the individual bones of the cranium are held together by flexible membranes. These allow the skull some flexibility so that passage through the birth canal is easier. Examination of a newborn's head often reveals an odd shape that "fixes" itself after a bit of time.

You may have felt these "soft spots" on a baby's head. They are spots of vulnerability but will be replaced with hard bone by the end of the a child's second year.

The vertebrae are another part of the axial skeleton and afford protection to the spinal cord as well as lending flexibility to the axial body. Vertebrae are cushioned from one another by a cartilage cushion.

The rib cage, made up of the costal bones forms the chest cavity and protects the organs therein. Ribs extend from the vertebrae and connect to a sternum or breast plate.

Then consider the appendicular skeleton which is composed of the moveable limbs that are connected to the axial skeleton.

The pectoral girdle holds the arms to the axial skeleton. Two bones, the radius and ulna in your forearm allow rotation of the hand. Similarly in the your leg, two bones, the tibia and fibula allow rotation of the foot.

A truly amazing collection of bones are those that lie at the end of your wrist. The hand has strength as well as dexterity and fine control.

The pelvic girdle is where the weight of the upper body is received and transmits this to either the bones of the legs or the object upon which you are sitting. And, although not as dextrous as hands, the 26 bones that make up each foot are designed with an arch in mind to accept the tremendous forces that walking and jumping bring to them.

Joints allow mobility. While a lack of joints would lend greater strenth to our skeleton, such a lack would also leave us with much less mobility and dexterity. In general, the more mobile a joint, the weaker it is.

Articulating bones are held together at a joint by long straps of connective tissue called ligaments. These ligaments can be torn or otherwise injured and are sometimes repair with by means of orthoscopic surgery. Torn cartilages in joints are also sometimes "repaired" by the same sort of surgery.

Muscle

Muscle is a specialized tissue with one particular function-- it generates a pulling force. Large muscles of the human body pull on individual bones.

But muscles do more. In humans, muscles move eyelids, pump fluids though vessels, propel stuff through an alimentary tract, discharge wastes and suck oxygen into the lungs.

Muscles require tremendous amounts of ATP to fuel their activities. A person running as fast as possible burns about 1,000 Calories an hour. A quality chocolate bar contains about this much caloric value.

Muscles are quite efficient in their use of ATP, converting from 35-50 % of the energy associated with ATP into the mechanical energy of movement. This is about 5 times more efficient than an automobile engine. What happens to rest of the energy? It appears as heat.

Chemically, muscles are composed mostly of two proteins-- actin and myosin. Muscles contract by a molecular sliding of actin over myosin filaments in the presence of ATP.

About half of the body weight of a vertebrate animal is muscle consisting of three types-- skeletal, smooth and cardiac muscle. These three muscle types perform quite differently.

Skeletal muscle responds to voluntary control-- that is, you can consciously call skeletal muscle into play. About 40% of a man's body and about 23% of a woman's body is skeletal muscle. Skeletal muscle derives its name from the fact that most of it is attached to bones that these muscles move.

Skeletal muscles taper at their ends becoming a dense connective tissue cord or tendon.

Muscles can only shorten and pull. They cannot push. An opposing muscle is needed to provide movement in the opposite direction. The biceps muscle is a large skeletal muscle of your upper arm. When you contract it, it causes your forearm to move towards your upper arm.

The biceps muscle cannot cause your forearm to move away from your upper arm. The muscle that does this is the triceps, a muscle on the lower part of your upper arm. Contraction of the triceps causes your forearm to move away from your upper arm. The biceps-triceps muscle pair are said to be antagonistic pairs of muscles. Most skeletal muscles are arranged in antagonistic pairs.

Smooth muscle is involuntary muscle and is independent of voluntary control. Smooth muscle in your urinary bladder, for instance, automatically contracts when it receives pressure from being full. Fortunately, humans have a skeletal muscle under voluntary control that acts as a valve so that urinating at inopportune times can be avoided.

Smooth muscles also have control over the diameter of blood vessels and the diameter of the pupil of the eye.

Cardiac muscle is also an involuntary muscle but has structural properties more like that of skeletal muscle. Cardiac muscle, however, cannot function anaerobically like skeletal muscle. Without oxygen, cardiac cells rapidly die.

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