Bones are obviously the core structure of the skeletal system. There are many different shapes, types and classifications of bone that warrant some discussion. These various bone types can serve different functions and may support other anatomical functions of the body in different ways.
The composition and structure of individual bones is also relevant to our studies. Understanding why bones can be painful in one location but not in another is an interesting and useful bit of information. Appreciating how bones grow, diminish in size, and produce various components of blood may help you understand how to achieve or improve your overall health. This information may also help you maintain strong and health bones as you age.
As you will discover there is much more to an individual bone than you may have considered. These are highly structured, complex, and essential living organs of the body. We will begin our discussion by examining bone tissue.
Contrary to what many people think, your bones are not composed of solid mineral deposits. They are quite active living tissue. In fact, each bone in your body is considered to be a separate and distinct organ. Throughout every bone you will find blood supplies and nerves vital to the health of your bone tissues. Your bones are all very much alive.
Bones are composed of different types of tissue. Each bone typically has some component of each tissue type. The hard bone surfaces are typically made of compact bone tissue. Some portions of a bone, especially where it needs to be somewhat resilient, is composed of spongy bone tissue. The core of all bones is normally reserved for bone marrow tissue, which is vital to the production of blood cells and the storage of fat.
Osteoblasts, Osteons, Osteocytes, and Osteoclasts
Osteoblasts are bone producing cells that generate new bone tissue. They are located on the periphery of bones and produce new bone tissue on the outer layers of an existing bone. New bone tissue (bone matrix) is produced in layers that run generally parallel to the length of the bone.
Osteocytes are osteoblasts that have become surrounded or entrapped by the bone matrix they have produced. Osteocytes represent approximately 95% of the living cells in an adult skeleton. Adjacent osteocytes are able to influence the behavior of nearby osteocytes, osteoclasts and osteoblasts by extending tentacles called dendrites through channels called Canaliculi. Hormonal secretions allow osteocytes to affect the behavior of nearby osteocytes, osteoclasts, and osteoblasts.
This behavior may be influenced by the mineral content and hormones in the blood supply and by mechanical stresses upon the bone. Osteocytes signal when it is necessary to slow or accelerate the production of new bone tissue. Mature bone typically has a relatively slow rate of new bone production. This may change if a bone is under stress (for example, you start taking martial arts classes), or when a fracture or other bone injury occurs.
An osteon is a cylindrical shaped component of compact bone. An osteon is also commonly called a harversian system. At the center of each osteon is a core, called the Haversian canal, through which blood vessels supplying bone cells in the osteon are routed. Blood supplies are routed to this canal via an artery that links multiple concentric osteon together via what is called the Volkmann’s canal.
Each osteon is built up from multiple layers of dense bone material called Lamellae. The lamellae eventually surround and encapsulate the osteoblast that created them, turning the osteoblast into an osteocyte.
Osteoclasts are cells that break down and absorb bone tissue. This is part of the process called bone remodeling where bone tissue is constantly undergoing changes. A balance between bone production and bone resorption is how your body regenerates and keeps your skeleton healthy and sound. As bones age older sections of bone are reabsorbed by the osteoclasts and new bone is produced through the action of osteoblasts and osteocytes. This is a continual ongoing process that keeps a human skeleton strong and vibrant.
Outer Bone Layers
All bones are surrounded by a dense outer layer of hard compact bone. Internally the structure of bones can differ significantly, depending on the bone’s location and its primary functions.
All bones are covered by a thin layer called the periosteum. This layer has an abundance of nerves and contains blood vessels that convey blood into and out of the deeper bone tissues. Periosteum covers the outer layer of every bone (except at joints where cartilage covers the bones). It is often referred to as the “skin of the bones.” Periosteum contains an abundance of stem cells that can be converted into the osteoblasts and other cells required to generate or maintain bone tissue.
Throughout a person’s life their bones are constantly being remodeled or modified to meet new growth demands, repair bone injuries, or to adjust to the person’s changing lifestyle conditions. Periosteum plays a vital role in helping to remodel and rebuild bone as required.
Compact bone, also called cortical bone, is composed of hard, mineral rich cellular connective tissue that grows in concentric layers. The bone grows in a manner similar to how tree rings are formed. Each new layer grows on the outside of previous layers. But bone does not have an annual growth pattern like trees.
Compact bone tissue surrounds all bones in the body and functions primarily to provide structural support to the body. It also serves as a strong and broad connection point for ligaments and tendons. About eighty percent of the skeleton in a typical adult is composed of compact bone.
Spongy bone, also called cancellous bone, is named because of its resemblance to the structure of a sponge. The bone is porous with many small columns of bone with small cavities throughout much of a bone’s tissue. These small cavities are filled with bone marrow and are a vital component of blood production in the body.
Spongy bone is found near the end of all long bones. It is useful in these locations because long bones are subject to substantial stress at these joint locations. The structure of spongy bone allows it to absorb and distribute shocks that commonly occur near the end of long bones. Bones that are more rigid might succumb to these stresses and break, so spongy bone helps reduce the chances of fracture at the ends of long bones.
In addition to its location at the ends of long bones, spongy bone is found at the core of flat bones such as the scapula and skull. These bones are the primary production sites for red blood cells and also require the impact and stress resiliency afforded by spongy bone.
Because spongy bone is porous it has a very high surface area. This porous structure provides spongy bone with strength and resiliency. It also provides innumerable cavities in which red bone marrow can develop. These cavities are created by columns of bone called Trabeculae which are aligned to provide added strength in the direction in which a bone must support loads.
Spongy bone is found inside an outer layer of compact bone. As a result spongy bone is not directly exposed to contact stresses.
Bone marrow (also called myeloid tissue) is a soft gelatinous structure that develops inside spongy bone. There are two types of bone marrow in the human skeleton. The first is red blood marrow. This bone marrow is comprised of a rich network of blood vessels that supply nutrients to the marrow, remove waste, exchange hormones, and absorb new blood cells as they are produced. This rich blood network (highly vascular network) gives red bone marrow its color.
Red bone marrow, also called hematopoietic marrow, contains hematopoietic stem cells which reproduce through cell division. Through a process called hematopoiesis these stem cells are transformed into red blood cells, white blood cells or platelets as required by the body. A barrier exists in the marrow that prevents immature cells from entering the blood stream. Once a cell transformation is completed the newly formed blood cell is allowed to enter the blood stream in the marrow where it is carried away to become part of the blood stream.
The second type of blood marrow is yellow blood marrow, or Stroma. Yellow marrow has fewer blood cells and blood vessels, so it does not have a red color. Instead it has, as the name suggests, a yellow color which is derived from the high concentration of fat in the marrow. The stroma produces and stores fat cells. Stromal stem cells are converted to fat cells when the body needs to store energy. These fat cells are then stored in the bone marrow itself. Yellow marrow fat cells are the last cells to be utilized by the body during periods of starvation. The fat stored in bone marrow is different from adipose fat cells that are stored in soft tissue areas of the body.
In addition to producing fat cells the yellow marrow also produces muscle, cartilage and bone. Yellow marrow contains mesenchymal stem cells which can differentiate (specialize) into fat cells, osteoblasts, or cartilage cells (chondrocytes). Some studies suggest these cells may also differentiate into muscle cells (myocytes), skin cells, and other cell types, but there does not seem to be conclusive proof nor agreement among researchers about this type of differentiation occurring from mesenchymal stem cells.
There are several different types and classifications of the bones in the human body. It is possible for a single bone to be classified as two different types of bone. In this section we will discuss these various types.
Flat bones generally protect internal organs and are composed of spongy bone surrounded by compact bone. Flat bones include the skull, clavicle, ribs, ilium, sternum, ischium, and pubis. These bones are also common locations for the attachment of powerful ligaments and tendons. The powerful muscles of the arm, back, and legs are often attached to flat bones via strong tendons.
Long bones generally involve bones of the appendicular skeleton and include the femur, radius, ulna, tibia, fibula, humerus, metacarpals, metatarsals, and phalanges. These bones commonly have some spongy bone near the joint areas. This is particularly true of the femur.
The primary purpose of these bones is to support weight. This load bearing ability is made possible due in large part to their long narrow, columnar shape. These bones are also used to facilitate movement. Powerful muscles make connection with most long bones so that the bones can be used as a lever such that movement at a joint location can be accomplished.
Short bones are generally about as long as they are wide. The seldom experience significant movement. The carpals and tarsals are the primary locations for short bones. The patella (knee cap) is sometimes classified by some medical and anatomical authorities as a short bone.
An irregular bone is any bone that has a specialized shape. The sacrum is a classic example. The vertebrae are also considered to be irregular bones. So are the ilium, pubis, and ischium (which are also classified as flat bones).
Sesamoid bones are typically short but stout bones that are embedded within a tendon. The patella is the most often cited example. It sits within and is a part of the tendon that connects the quadriceps to the tibia (shin bone). The pisiform bone in the wrist is another sesamoid bone that develops in late childhood (it is not present at birth).
Sesamoid bones are normally found at joint locations. They serve to keep the associated tendon(s) elevated above the joint so that the tendon can exert more leverage across the joint. Some sesamoid bones may develop in response to sprains.
Sesamoid bones are commonly found at the distal end of the first and second metacarpal bones. The first metatarsal bone often has two sesamoid bones that aid in movement of the big toe.
Since sesamoid bones can develop as the result of injury or repetitive stresses different individuals may have difference sesamoid bones. As a result it is possible for an adult to have in excess of 206 bones.
Sutural bones are tiny bones that develop in the sutures in the cranial bones. The number and location of these small bones varies greatly between individuals.