How Many Bones Are in the Human Body and Other Skeletal Facts

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:

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.

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:

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:

The 206 bones in the human body can be roughly divided into four categories: long, short, flat and irregular.

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).

Although bone is very strong, it can break with enough force pushing, pulling or twisting it. Here are some of the more common breaks:

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.

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:

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:

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.

Contributing factors of osteoporosis include:

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.