Bones are often used as symbols of fear and death, such as the ever popular skull and cross bones. Sadly, this gives bones a bad reputation at times. Bones may appear simple, but they are quite fascinating and serve many important functions. They have a life all their own.

Function

Bones are typically thought of as having two functions: structural support and storage of minerals such as calcium and phosphorous. While this is true and a vital function for all vertebrates (we would be nothing but piles of goo without them), that is not all bones are capable of.

Bones also offer protection, such as our skull encasing our brains or our ribs protecting our heart and lungs. Bones work together with muscles, tendons and ligaments to allow movement.

Yet another important function that occurs in our bones is hematopoiesis (blood production), which occurs in the red marrow of bones and yellow marrow stores fat. A lesser known function of bones is pH balance by the release or absorption of alkaline salts.

General Bone Structure

Bone structure can vary depending on the type of bone, but there are some generalizations. Bones consist of three different kinds of cells. Vertebrate bones are composed of a substance called hydroxyapatite, which is made of calcium phosphate. All of the bones in our body have three types of cells: osteoblasts, osteocytes and osteoclasts.

Osteoblasts are often referred to as the “bone builders”, which is easy to remember because of the “b” in blast. Osteoblasts produce a matrix that is eventually mineralized, or hardened, to become bone. When this occurs, the osteoblasts become osteocytes.

Osteocytes are mature bone cells that are inside the matrix. To be more specific, osteocytes are inside lacunae.

Lacunae are spaces that are located between the lamellae, or bone layers. Osteoclasts are considered bone destroyers, which may sound bad, but it is an essential function.

In conjunction with the osteoblasts, osteocytes allow for longitudinal growth in childhood and constant remodeling of bone in adulthood. Other structures exist that are also important for bones.

First are the canaliculi. These are small canals where extensions of the osteocytes reach one another and are able to communicate. Haversian canals are also important parts of all bones. These canals are tubes that allow blood vessels and nerves to pass through bone.

Multiple Haversian canals form a Haversian system, which is also called an osteon. Haversian canals are found in compact bone, but not in cancellous bone.

Osseous tissue (bone tissue) comes in two forms: compact and cancellous. Compact bone is denser and forms the outer layer. Cancellous bone forms the inner layers. This layer is not dense and is made up of narrow plates called trabeculae, and in some bones, red marrow is located here. Cancellous bone resembles a sponge, and is often referred to as spongy bone. And it can also be referred to as trabecular bone.

Another way to distinguish between the two is where they are located. In long bones, compact bone is found in the epiphysis and cancellous bone is found in the diaphysis. Cancellous bone can also be found in the epiphysis of other types of bones such as flat, irregular and short.

Bones have two membranes, the periosteum and the endosteum. The periosteum lines the outer surface and the endosteum lines the medullary cavity of the bone. Both membranes are made of connective tissue.

The periosteum has two layers. The fibrous layer contains fibroblasts and the cambium layer produces osetoblasts from progenitor cells. Sharpey’s fibers connect the periosteum to the bone and they are connected to the lamellae, which are the layers of bone. The periosteum also has nerve endings, which makes it more sensitive.

Types of Bones

First up are the long bones. The long bones have a few distinguishing characteristics. A long bone consists of a diaphysis (the shaft of the long bone) and the epiphysis, which refer to the ends of the long bones. They are separated by the epiphyseal line, which is made of cartilage and is an area where growth occurs. Long bones also have a medullary cavity, which is located in the center of the bone and stores the marrow.

Next you have short bones, which include the bones of the ankle and wrist (tarsals and carpals). Short bones are cuboidal shaped.

The scapula (shoulder blade) and ribs are examples of flat bones. Flat bones are very thin and have a slight curve, such as many bones in the skull.

Next are the irregular bones such as the vertebrae and mandible. They are called irregular because of their different shapes and they do not fit into any of the other categories of bones.

Finally, you have the sesamoid bones. The best example of a sesamoid bone is the kneecap, or patella. Sesamoid bones are special because they are embedded in tendons, which connect muscle to bone.

Then, we have our joints. Joints are not necessarily a type of bone, but rather an area where two or more bones meet. Joints are vital to how we move. Without them, we would not be able to bend our arms and legs. There are many types of joints and methods of classifying them, which I will include in a separate article to prevent this one from being too verbose.

Bone Development

Bones begin to develop when we are a fetus, and they undergo two types of development (ossification in this case): intramembranous and endochondral.

Intramembranous development occurs in flat bones and originates from mesenchymal (stem) cells. This kind of ossification begins with sheets of connective tissue, which contains our bone builders, the osteoblasts. They secrete their matrix, which forms cancellous, or spongy bone. As the matrix begins to harden, the osteoblasts become trapped and become osteocytes.

Endochondral ossification forms compact bone from hyaline cartilage. The cartilage serves as a model for the shape bones need to be, but before ossification can occur, the chondrocytes must die. Blood vessels then enter the cartilage. At the same time, the chondrocytes begin their death in the diaphysis. Spongy bone begins to form, which creates the primary ossification site. From this site, the osteoblasts move to transform the cartilage to spongy bone at the ends of the newly forming bone. Afterwards, osteoclasts destroy some of the new spongy bone for the medullary cavities. Cartilage remains at the epiphysis and continues to increase in length.

After birth, ossification of the epiphysis at the secondary ossification site begins, which is the same as in the diaphysis, except spongy bone is not broken down. A small region of cartilage remains between the diaphysis and epiphysis, called the epiphyseal plate. Bone growth continues until early adulthood by a process like endochondral ossification. After early adulthood, bones cease to increase in length, but do increase in diameter. Hormones secreted by the pituitary gland and the testes or ovaries regulate bone growth.

Bones are pretty amazing, huh? They are far more ornate than their physical appearance leads us to believe.