The human body is an incredible machine. It runs so well most of the time that we don't pay much attention to any of the life-sustaining systems that keep it humming.
Right now, your body is performing vital and complicated tasks too numerous to comprehend. Fortunately, our bodies don't demand our comprehension in order to pump the heart, oxygenate blood, regulate hormone production or keep us standing.
Speaking of standing, the human skeleton prevents us from puddling on the floor, but what else do bones do? Bones rebuild themselves, they produce blood cells and bone tissue, they protect our brains and our organs, and bones also help maintain a steady supply of calcium in our bodies.
And, even if after you depart this world, your bones will stick around for a long time afterward.
But before we even talk about leaving behind our skeletal remains for future generations, we should first talk some basics about bones: What are bones made of? What happens when they break? And how many bones are in the human body?
Bone is a honeycomb-like grid of calcium salts located around a network of protein fibers. These protein fibers are called collagen.
When you patch a hole in a piece of drywall, you usually cover it with tape that has a gummy fibrous grid, and then cover that with wall compound mortar. Bone is made in much the same way. Collagen fibers are gummed together by a kind of shock-absorbing glue. Then, all of this is covered and surrounded by calcium phosphate, which hardens everything into place. Not only do bones use calcium for strength, they also keep some stored in reserve. When other parts of the body need a calcium boost, the bones release the needed amount into the bloodstream.
There are three types of bone tissue: cortical tissue (the outer layer), cancellous tissue (the inner layer) and subchondral tissue (the ends of bones).
Cortical bone tissue is covered by a fibrous membrane called the periosteum. Think of the periosteum as a utility vest that fits over the bone — it has brackets and places for muscles and tendons to attach. The periosteum contains capillaries that are responsible for keeping the bone nourished with blood. Cortical bone makes up 80 percent of bone mass and is dense, strong and rigid.
Cancellous bone tissue is the inner layer of bone and is much less dense than cortical bone. It's formed by trabeculae, which are needlelike structures that create a meshwork. However, instead of a network of bone structure with periodic gaps, cancellous bone is more like a network of connecting spaces with periodic structure. The latticework of tiny chambers is filled either with bone marrow or connective tissue. New blood cells are produced within these marrow-filled spaces.
Though cancellous bone only makes up about 20 percent of the body's bone mass, it plays important roles in body function. It provides structural stability and acts as a kind of shock absorber inside the bone, but without adding too much to the overall weight of the body.
Inside the cavities of cancellous bone is soft, fatty tissue composed of an irregular network of blood vessels and cells called bone marrow. There are two types of marrow: red and yellow.
Red marrow contains stem cells, unspecialized cells that can grow into different types of specialized cells. They're responsible for replenishing and replacing damaged or lost cells in the body. There are two types of stem cells found in red marrow:
Hematopoietic stem cells (HSCs): This type of stem cell is creates billions of new blood cells daily, including red blood cells (which carry oxygen throughout the body), white blood cells (which fight infections and kill bacteria) and platelets (which help your blood clot).
Stromal stem cells: This type of stem cell provides nutrients in both the secondary lymphoid organs and non-lymphoid tissues.
Yellow marrow is mostly fat and as we age, it can be found in places where red marrow once resided — some of the bones in our arms, legs, fingers and toes, for instance. If the body needs more blood cells, yellow marrow can transform back into red marrow and produce them.
Bone marrow can become diseased. Chronic myeloproliferative disorders (MPDs) are blood cancers that cause the overproduction of immature cells from the marrow. Disorders such as aplastic anemia and myelodysplastic syndromes (MDS) hinder the marrow's ability to produce enough blood cells.
Several marrow diseases can be treated through stem cell transplants, which introduce healthy stem cells to the patient's body to replace the diseased cells.
How Many Bones Are in the Human Body?
There are 206 bones in the human skeleton, including the tiniest bones in the middle ear to the longest bone the femur.
Infants are typically born with around 270 bones. As they age, some of those bones fuse together to become 206 separate bones. This number can vary, though, because injury could cause some people to lose ribs, vertebrae or digits.
Bones are divided into two categories: axial and appendicular. There are 80 axial bones that make up spine, chest and head. They protect delicate organs like your heart and brain.
Axial bones include:
The skull: The skull bones are comprised of 22 interlocking cranial and facial bones. These cranial plates and oddly shaped bones are held together by joints, though these joints don't allow for movement (except for the mandible or jawbone). Deep in your ear is the smallest bone in your body, the stirrup. It's about the size of a grain of rice.
The sternum: The sternum — or breastbone — protects your heart, lungs and portions of your major arteries from external forces. The sternum starts off as different sections that fuse over time into one unified piece. The sternum also provides stability to the ribs.
The ribs: These flat bones form a protective shield around your internal organs. There are 24 ribs, 12 on each side of your body. They come in three different types. From the top, the first seven sets of ribs are true ribs. They connect in the back to the spine and connect in the front to the sternum. The next three pairs are the false ribs. They connect in the back to the spine, but in the front, they attach to the seventh true rib, which is the last rib that connects to the sternum. Last are the floating ribs, and these two pairs attach to the spine like all the others, but "float" in the front, meaning they're not attached to the sternum or any other rib.
While the axial bones form the vertical axis of the body, the appendicular bones are the bones that connect to this axis.
Appendicular bones include:
Bones of the shoulder: The bones that make up your shoulder girdle connect your arms to your sternum and rib cage for stability and support. You have two clavicles (collarbones) that attach on one end to the breast plate and, on the other end, support the shoulder blades, or scapulas. Each shoulder blade provides a point of contact for many muscles and the bone of each upper arm.
Bones of the arm and hand: The arm has three basic components: the upper arm, the lower arm and the hand. The upper arm is one long bone, the humerus. The top fits neatly into the scapula, and the lower end is connected by the elbow joint to the two bones of the lower arm: the ulna (the bone on the same side as your little finger) and the radius (the bone on the side of your thumb). The radius plays a larger role in your mobility and function, while your ulna provides more stability. Both the ulna and the radius connect to the wrist bones in the hand. Each hand has an impressive 27 bones: eight carpal bones that make up the wrist, five metacarpal bones that extend the length of your palm, and 14 phalanges that form four fingers with three bones each along with a single two-boned thumb.
The pelvic girdle: This tough pair of hip bones protects lower organs such as the bladder and, for women, a fetus when she's pregnant. The dimensions of the pelvis differ significantly for men and women.
Bones of the thigh, leg and foot: Connecting the pelvic girdle to the lower leg is the femur, the longest and strongest bone in the body. The femur connects through the knee joint (which is covered and protected by the patella, or kneecap) to the shin bone (tibia). Slightly smaller than the tibia is the other bone in the leg, the fibula. Each foot has 26 bones: seven tarsal bones that make up the ankle, five metatarsal bones that make up the body of your foot, and 14 phalanges that form four toes with three bones each with a big toe that has two bones.
The Shapes of Bones
The 206 bones in the human body can be roughly divided into four categories: long, short, flat and irregular.
Long bones: Long bones have a slightly curved shaft capped on both sides with hyaline cartilage and are longer than they are thick. They're made mostly of compact bone, allowing them to support great amounts of weight and withstand pressure. The femur is an excellent example of the strength of a long bone. Its hollow cylindrical design allows it to provide the most possible strength without being too heavy. The hollow inside of many long bones is where the marrow is located. Long bones grow from both ends, and have a cartilage plate (also known as epiphyseal plates) between the bone shaft and each bone end. These plates continue growing through adolescence.
Short bones: Short bones consist mainly of spongy bone with a protective covering of compact bone. Short bones are neither long nor thick, but rather cubelike. Your kneecaps (patellae), wrists (carpals) and some of the other bones in your feet and ankles (tarsals) are short bones. Short bones aren't designed for much movement, but they're sturdy, compact and durable. The short bones in the wrist and ankle are also known as sesamoid bones. Sesamoid bones (usually classified as short or irregular bones) are placed within tendons in parts of the body where a tendon must cross a joint. These bones hold the tendon slightly away from the joint to provide better range of motion when the tendon tightens.
Flat bones: These bones are thin and flat. Flat bones have a middle layer of spongy bone located between two protective layers of compact bone. Two examples include breast bones and skull bones. Flat bones contain marrow that produces more red blood cells than any other bone.
Irregular bones: The bones that don't fit in the other three categories are irregular bones. Vertebrae in the spine and the jawbone (mandible) are irregular bones. These bones usually have very specialized functions and are made of mostly spongy bone with a thin layer of compact bone around them.
How Bones Form
Before we talk about bone formation, we need to discuss how cartilage turns into bone. When you're floating around in the womb, your developing body is just beginning to take its shape, and it's creating cartilage to do so. Cartilage is a tissue that isn't as hard as bone, but much more flexible and, in some ways, more functional. Cartilage is pretty good stuff to use if you're going to mold a human — good enough for the finer work, especially, such as your nose or your ear.
A large amount of that fetus cartilage begins transforming into bone, a process called ossification. When ossification occurs, the cartilage begins to calcify; that is, layers of calcium and phosphate salts begin to accumulate on the cartilage cells. These cells, surrounded by minerals, die off. This leaves small pockets of separation in the soon-to-be-bone cartilage, and tiny blood vessels grow into these cavities.
Specialized cells called osteoblasts begin traveling into the developing bone by way of these blood vessels. These cells produce a substance consisting of collagen fibers, and they also aid in the collection of calcium, which is deposited along this fibrous substance.
Eventually, the osteoblasts become part of the mix, turning into lower-functioning osteocytes. This osteocyte network helps form the spongelike lattice of cancellous bone. Cancellous bone isn't soft, but it does look spongy. Its spaces help transfer the stress of external pressures throughout the bone, and these spaces also contain marrow. Little channels called canaliculi run all throughout the calcified portions of the bone, enabling nutrients, gases and waste to make their way through.
Before turning into osteocytes, osteoblasts produce cortical bone. One way to imagine this process is to picture a bricklayer trapping himself inside a man-sized brick chamber of his own construction. After forming the hard shell (cortical bone), the bricklayer himself fills the chamber. Air makes its way through the brick and decays the bricklayer.
In bone, this part of the process is accomplished by osteoclasts, which make their way into the calcifying cartilage and take bone out of the middle of the shaft, leaving room for marrow to form. Osteoclasts do this by engulfing and digesting the bone matrix using acids and hydrolytic enzymes. So, our bricklayer (osteoblast) made the tomb (cortical bone), died inside the tomb (became an osteocyte), decayed over time (dissolved by osteoclasts) and left behind his remains that formed a network of mass and space inside the brick tomb.
Eventually, all the cartilage has turned to bone, except for the cartilage on the end of the bone (articular cartilage) and growth plates, which connect the bone shaft on each side to the bone ends. These cartilage layers help the bone expand and finally calcify by adulthood.
So, right now in your body, there are osteoclasts hard at work absorbing old bone cells and osteoblasts helping to build new bone in its place. This cycle is called remodeling. When you're young, your osteoblasts (the builders) are more numerous than the osteoclasts, resulting in bone gain. When you age, the osteoblasts can't keep up with the osteoclasts, which are still efficiently removing bone cells, and this leads to loss of bone mass (and a condition called osteoporosis, which we'll discuss shortly).
How Bones Break
Although bone is very strong, it can break with enough force pushing, pulling or twisting it. Here are some of the more common breaks:
Stress fracture: This type of fracture is the result of sustained force on a bone, like running or jumping. Most stress fractures occur in the lower body, and you can have stress fracture without feeling any pain.
Open fracture: Unlike closed fractures in which all portions of the broken bone remain within the skin, open fractures result in a piece of bone puncturing and piercing the skin.
Complete fracture: This is when the bone breaks neatly into two pieces.
Single fracture: This is a break in which the bone has only one damaged area.
Comminuted fracture: Comminuted fractures are bones that have been crushed or broken into more than two fragments.
Greenstick fracture: With this type of fracture, the bone has cracked on one side, but not all the way through. These breaks normally occur in children.
Pathologic fractures: These fractures may be caused by external forces, but the underlying cause is a bone that has been weakened by disease or infection, such as bone cancer.
Displaced fractures: The two broken ends of the bone don't line up and require repositioning before they're set in place.
Simple transverse: This type of fracture is an even, perpendicular break to the bone. (Imagine if someone chopped your femur bone in half by neatly striking it from the side at a right angle.)
Oblique fracture: An oblique fracture would be a diagonal fracture running lengthwise along the bone. (Think greenstick fracture, but all the way through the bone.)
Spiral fracture: Spiral fractures occur when the bone has been twisted past its maximum point of resistance.
How Broken Bones Heal
When bones are fractured, the body immediately initiates the first phase of healing, the reactive phase. Ruptured blood vessels gather at the site of the break and form a clot, or hematoma. This clot contains fibroblasts, which are connective tissue cells that produce collagen proteins. When this clot forms, it lays the groundwork for what will be a full-scale restoration of the bone.
In a few days, the broken ends of the bone produce new blood vessels that grow into the clot that now bridges the separation in bone caused by the fracture. White blood cells arrive with these new blood vessels and begin carting away unneeded material from the site of the break. Now, the fibroblasts begin to multiply and secrete collagen fibers that form a matrix that replaces the blood clot.
In the reparative phase, specialized cells — osteoprogenitor cells — located in the periosteal membrane that covers most of the bone begin transforming into different types of needed cells. Some of these cells — chondroblasts — produce cartilage, while others — osteoblasts — produce uncalcified bone called callus. The new cartilage and callus bridge the separated pieces of bone, and the cartilage begins to ossify into trabecular bone.
In the third phase, the remodeling phase, osteoclasts begin removing the trabecular bone while osteoblasts replace it with compact bone. Once this phase is finished, the fractured bone has healed.
How Joints Connect Your Bones
Each time you lean forward, pick up a cup of coffee, raise it to your lips and put it back down, your bones, joints, muscles and other tissues are all working together to make this effort possible.
There are approximately 360 joints in the human body, and each joint comprises several elements. Among them are:
Bones: The articular cartilage at the end of the bones prevents the bone ends from being damaged by contact with each other. Cartilage itself can be harmed by infection, injury, disease or simple wear and tear. This damage may lead to pain, inflammation and stiffness, a condition known as arthritis.
Skeletal muscle: Skeletal muscles attach to bones and appear striped when viewed closely, earning the name "striated muscle." Unlike your cardiac muscle or the muscle in the walls of your stomach, skeletal muscles can be voluntarily moved and lie at rest when not consciously activated. These muscles connect to bones through tendons.
Tendons: When skeletal muscle contracts or lengthens, it pulls on bone through an attached tendon, a tough, flexible tissue.
Ligaments: These tissues are pretty similar to tendons, except they connect bone to bone, ensuring they come together to form a joint that will stay in place.
Synovial membrane: This layer of connective tissue exists around each joint, providing protection and producing synovia, a fluid that lubricates the joint and nourishes the cartilage.
Bursa: Similar to the synovial membrane, the bursa is a small sac that provides a lubricant to ease the movement of muscle against muscle or muscle against bone.
Not every joint moves. The skull, for example, consists of several bone plates that join together, but the fibrous tissue connecting these plates don't move.
Freely moveable joints fall into one of several different categories. The different types of joints are:
Pivot joints (known also as rotary joints): These joints allow rotation around an axis. There is a pivot joint near the top of your spine that allows your head to move from side to side.
Hinge joints: These joints open and close like a door. Your elbow is a hinge joint.
Gliding joints (known also as plane joints): These joints include two bone plates that glide against one another. The joints in your ankles and wrists are gliding joints.
Ball and socket joints: These are the most maneuverable types of joints. They have a connection between one bone-end equipped with a protrusion that fits into the receptive space at the end of the other bone in the joint. Your shoulder is a type of ball and socket joint.
Saddle joints: These joints allow two different types of movement. For instance, a saddle joint allows your thumb to move toward and away from your forefinger (as when you spread all five digits out, then bring them all together side-by-side) as well as cross over the palm of your hand toward your little finger.
Conyloid joints: These are like ball-and-socket joints, just without the socket (the "ball" simply rests against another bone end).
Osteoporosis and Other Bone Diseases
Bones, like any other part of the body, are susceptible to disease, the most common being osteoporosis. Osteoporosis is the diminishing of bone mass, leaving it structurally brittle and physically porous.
Though any bone can be affected by osteoporosis, the hip, spine and wrist bones are the most common.
Sex: Women are more likely than men to get osteoporosis. But it also affects men and young people.
Age: The likelihood of getting osteoporosis increases as you age.
Diet: A poor diet will result in not getting enough vitamin C, calcium, phosphorus, magnesium and vitamin D — all important elements of good bone health.
Low estrogen: Women with higher estrogen levels tend to have higher bone density.
Family history: Having a parent or sibling with osteoporosis puts you at greater risk.
Race: People who are white or of Asian descent also are at greatest risk of osteoporosis.
Another disease that can affect the bones is bone cancer. Bone cancer most often spreads to the bone from other parts of the body, but it can also start in the bone. When it begins in the bone, it's known as primary bone cancer. Primary bone cancer is rare and accounts for just 1 percent of all cancers diagnosed — about 3,450 new cases are diagnosed each year. Bone cancer can be treated surgically or with chemotherapy, radiation therapy or cryosurgery (killing cancerous cells by freezing them with liquid nitrogen).
Osteonecrosis is a degeneration of the bone due to little blood supply. Treatments include corticosteroids, immunosuppressant medications, chemotherapy and surgical intervention.
Osteogenesis imperfecta is an inherited disease that causes bones to be especially brittle. A faulty gene leaves the body unable to produce collagen normally.
Paget's disease causes bones to grow too large and structurally unstable. While the disease can affect any bone, it most often affects the pelvis, skull, spine and leg bones.
Lots More Information
DNA Researchers Call on Bone Hoarders to Share Bone Access
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Hou, Chuanqiang; Wu, Xuejun; Jin, Xing. "Autologous Bone Marrow Stromal Cells Transplantation for the Treatment of Secondary Arm Lymphedema: A Prospective Controlled Study in Patients with Breast Cancer Related Lymphedema." Sept. 5, 2008. http://jjco.oxfordjournals.org/cgi/content/abstract/38/10/670
Houston Museum of Natural Science. "Body Facts." Jan. 23, 2009. http://www.hmns.org/exhibits/special_exhibits/bodyworlds/bodyworlds_body_facts.asp?r=1
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