Skeletal System

The skeletal system is the body system composed of bones and cartilage and performs the following critical functions for the human body:

"Gross Functions" are functions that are observable by simple observation

The Gross functions of the skeletal system are Support, Movement, and Protection.

Support

Support is the most obvious function of the skeletal system. Your skeletal system is the frame on which your body is structured. Without it you would be a squishy blob.

Movement

Many of us think of muscles powering our body's movements but it is our skeletal system that facilitates those movements. The bones give the muscles something to attach to and to pull against. Think of your bones as the lever and your joints as the fulcrum. Almost all of our movements involve a muscle pulling on a bone causing it to bend over a joint.

Protection

Bones are also used as protection. Our skull protects our sensitive and valuable brain. Our rib cage protects our lungs and heart. Your spin protects your spinal cord. All of these organs are valuable and tender requiring protection.

On a metabolic level bone tissue is important for Mineral Storage, Energy Storage, and Hematopoieses  

Mineral Storage

The bone matrix acts as a reservoir for calcium and phosphorus. Whenever the body is low on calcium or phosphorus, these minerals are released from the bone tissue into the blood stream. 

Calcium ions are important for muscle contractions and for nerve impulses. 

Energy Storage and Hematopoieses

The middle of bones is called bone marrow. There are two types of bone marrow; yellow marrow and red marrow.

Yellow Marrow - is in the center most part of the bone and is made of adipose tissue. This adipose tissue contains triglycerides that can be used as an energy source when needed.

Red Marrow - is where blood cells are made. Red blood cells, white blood cells, and platelets are all made in the red marrow of the bone. This is called hematopoieses

Bone Classification

The 206 bones that compose the adult skeleton are divided into five categories based on their shapes 

Long Bones

A long bone is one that is cylindrical in shape, being longer than it is wide. Keep in mind, however, that the term describes the shape of a bone, not its size. Long bones are found in the arms (humerus, ulna, radius) and legs (femur, tibia, fibula), as well as in the fingers (metacarpals, phalanges) and toes (metatarsals, phalanges). Long bones function as levers; they move when muscles contract.

Short Bones

A short bone is one that is cube-like in shape, being approximately equal in length, width, and thickness. The only short bones in the human skeleton are in the carpals of the wrists and the tarsals of the ankles. Short bones provide stability and support as well as some limited motion.

Flat Bones

The term “flat bone” is somewhat of a misnomer because, although a flat bone is typically thin, it is also often curved. Examples include the cranial (skull) bones, the scapulae (shoulder blades), the sternum (breastbone), and the ribs. Flat bones serve as points of attachment for muscles and often protect internal organs.

Irregular Bones

An irregular bone is one that does not have any easily characterized shape and therefore does not fit any other classification. These bones tend to have more complex shapes, like the vertebrae that support the spinal cord and protect it from compressive forces. Many facial bones, particularly the ones containing sinuses, are classified as irregular bones.

Sesamoid Bones

A sesamoid bone is a small, round bone that, as the name suggests, is shaped like a sesame seed. These bones form in tendons (the sheaths of tissue that connect bones to muscles) where a great deal of pressure is generated in a joint. The sesamoid bones protect tendons by helping them overcome compressive forces. Sesamoid bones vary in number and placement from person to person but are typically found in tendons associated with the feet, hands, and knees. The patellae (singular = patella) are the only sesamoid bones found in common with every person.

Student Activity:

Create a chart of the 5 bone shapes. The chart should take up a whole sheet of paper. In one column should be the bone shape, the next column a description of that shape, the next column its function, and the last column example bones.

Types of Bone Cells

There are four types of bone cells

-Osteoblast - is the bone cell that forms new bone. Osteoblasts are found in the sections of bone that are growing.

Osteocyte - Once an osteoblast is done growing it calcifies and becomes an osteocyte. These are the main bone cells. The cannot reproduce and they no longer grow. 

Osteogenic cell - Since osteoblasts and osteoctes are not capable of mitosis, osteogenic cells are the bone cells that go through cell division. Osteogenic cells divide into new osteoblast cells.

Osteoclast - Osteoclast cells come from white blood cells. Osteoclast cells are found all over the surface of bone. Osteoclast cells are constantly dissolving old bone for repair or for calcium release. It is the constant breaking down of bone by osteoclast and the constant building of bone by osteogenic cells that reshape bones. 

Compact and Spongy Bone

There are two different types of bone tissue depending on function and location. Compact bone is denser and stronger and used for protection. Spongy bone has more give and flexibility to it. Its lattice shape allows for strength but makes it lighter for muscle to pull. Spongy bone also allows for room for the red and yellow marrow. 

There are small openings at the ends of bones that allow arteries , veins, blood vessels, and nerve to enter. These arteries and veins spread throughout the spongy bone providing the cells with blood, oxygen, and nutrients.

Watch this video on the skeletal system

Student Activity:

Open your coloring book to page 17 and follow the coloring instructions.

Students Read:

Skeletal System: Paget’s Disease

Paget’s disease usually occurs in adults over age 40. It is a disorder of the bone remodeling process that begins with overactive osteoclasts. This means more bone is resorbed than is laid down. The osteoblasts try to compensate but the new bone they lay down is weak and brittle and therefore prone to fracture.

While some people with Paget’s disease have no symptoms, others experience pain, bone fractures, and bone deformities. Bones of the pelvis, skull, spine, and legs are the most commonly affected. When occurring in the skull, Paget’s disease can cause headaches and hearing loss.

What causes the osteoclasts to become overactive? The answer is still unknown, but hereditary factors seem to play a role. Some scientists believe Paget’s disease is due to an as-yet-unidentified virus.

Paget’s disease is diagnosed via imaging studies and lab tests. X-rays may show bone deformities or areas of bone resorption. Bone scans are also useful. In these studies, a dye containing a radioactive ion is injected into the body. Areas of bone resorption have an affinity for the ion, so they will light up on the scan if the ions are absorbed. In addition, blood levels of an enzyme called alkaline phosphatase are typically elevated in people with Paget’s disease.

Bisphosphonates, drugs that decrease the activity of osteoclasts, are often used in the treatment of Paget’s disease. However, in a small percentage of cases, bisphosphonates themselves have been linked to an increased risk of fractures because the old bone that is left after bisphosphonates are administered becomes worn out and brittle. Still, most doctors feel that the benefits of bisphosphonates more than outweigh the risk; the medical professional has to weigh the benefits and risks on a case-by-case basis. Bisphosphonate treatment can reduce the overall risk of deformities or fractures, which in turn reduces the risk of surgical repair and its associated risks and complications.