Introduction To Bones

An Introduction To Bones

Bones are the rigid parts of the skeleton that support and protect the body’s organs. They are made of living tissue that can grow and repair itself. Bones also produce blood cells and store minerals such as calcium and phosphorus.

There are different types of bones, such as long bones, short bones, flat bones, irregular bones, and sesamoid bones. Each type has a different shape and function. For example, long bones are found in the arms and legs and help with movement, while flat bones are found in the skull and ribs and protect vital organs.

Bone Growth

At birth bones are made of cartilage. As growth takes place Calcium is deposited causing the cartilage to harden and eventually develop into bone. This process is called Ossification. Once a bone is fully developed it has a hard, compact outer layer, with a honeycomb type centre.

The contents of the bones are constantly being replaced throughout our lives. Osteoblast are cells that create new bone. Osteoclasts are cells that remove existing bone.

Bone Types

There are five types of bones in the human body, based on their shape and structure.

They are:

• Long bones: These are mostly compact bones with little marrow and include most of the bones in the limbs. They tend to be longer than they are wide and have a long shaft called diaphysis and two ends called epiphyses. They help with movement and support body weight. Examples are the humerus, femur, radius, ulna, tibia, fibula, metacarpals, metatarsals, and phalanges
• Short bones: These have a squat, cubed shape and include bones of the wrist and ankle. They are mostly spongy bones covered by a thin layer of compact bone. They provide stability and some movement. Examples are the carpal and tarsal bones
• Flat bones: These have a flat, broad shape and include bones of the skull, ribs, and sternum. They are composed of two layers of compact bone with spongy bone in between. They protect vital organs and provide surfaces for muscle attachment. Examples are the occipital, parietal, frontal, scapula, and rib bones
• Irregular bones: These have complex shapes that do not fit into any of the other categories. They are mostly spongy bone with a thin layer of compact bone. They protect internal organs and have various functions depending on their location. Examples are the vertebrae, pelvic bones, sphenoid, ethmoid, and hyoid bones
• Sesamoid bones: These are small, round bones that develop within tendons near joints. They reduce friction and increase leverage for muscles. They vary in number and location among individuals. The most common examples are the patella (kneecap) and the pisiform (a carpal bone)

Structure of a Long Bone

Long bones are one of the five types of bones in the human body, based on their shape.

They are defined as those that are longer than they are wide. Long bones are mainly found in the arms and legs, but also in the fingers and toes. They have several functions, such as supporting body weight, acting as levers for movement, providing strength and structure to the limbs, and producing blood cells.

Long bones have a common structure that consists of two main parts: the diaphysis and the epiphyses. The diaphysis is the central shaft of the bone, made of hard compact bone that surrounds a hollow cavity called the medullary cavity. The medullary cavity contains yellow bone marrow that stores fat and is lined by a thin membrane called the endosteum. The endosteum helps the bone grow, remodel, and heal. The epiphyses are the two ends of the bone, made of spongy bone that contains red bone marrow. The red bone marrow produces blood stem cells that give rise to red blood cells, white blood cells, and platelets. The epiphyses are covered by a layer of hyaline cartilage called the articular cartilage, which reduces friction and absorbs shock at the joints.

The diaphysis and the epiphyses are connected by a narrow region called the metaphysis. In a growing bone, the metaphysis contains a growth plate called the epiphyseal plate, which is a layer of cartilage that allows the bone to elongate. When the bone reaches its adult length, usually between 18 and 21 years of age, the cartilage is replaced by bone and the growth plate becomes a thin line called the epiphyseal line.

The outer surface of the long bone is covered by a fibrous membrane called the periosteum. The periosteum contains blood vessels, nerves, and lymphatic vessels that nourish and innervate the bone. It also provides attachment sites for muscles and ligaments. The only parts of the long bone that are not covered by the periosteum are the epiphyses where they articulate with other bones.

Long bones are subject to various injuries and diseases that can affect their function and cause pain or disability. Some common examples are fractures (breaks in the bone due to trauma or osteoporosis), infections (inflammation of the bone or marrow due to bacteria or fungi), tumours (abnormal growths of cells in or around the bone), osteoarthritis (degeneration of the articular cartilage and bone spurs), osteoporosis (loss of bone density and strength), osteomalacia (softening of the bone due to vitamin D deficiency), rickets (deformity of the bone due to vitamin D deficiency in children), Paget’s disease (abnormal remodelling of the bone), achondroplasia (short stature due to defective growth plate), and Marfan syndrome (long limbs due to defective connective tissue).

• Diaphysis: the shaft of the bone, dense compact bone covered by a hard outer casing.
• Periosteum: hard outer casing of the bone.
• Epiphysis: the bone end, less dense spongy bone covered by hyaline cartilage.
• Epiphyseal plates: situated between the diaphysis and epiphysis, promote bone growth until late teens or early twenties, during which time the cartilaginous plate is replaced by bone cells and the diaphysis and epiphyseal plate fuse together.
• Compact Bone: bones grow in length initially as a result of the ossification of the diaphysis (shaft) and compact bone is formed.
• Cancellous (spongy) bones: contains bone marrow, less dense than compact bones usually found in the epiphysis.
• Hyaline cartilage: covers the bone ends and allows the bones to move more freely over each other and protects the bones beneath from possible friction.
• Red bone marrow: produces red and white blood cells.
• Medullary cavity: hollow space in the middle of the bone where bone marrow is stored.

Classification Of Bones

Bones are classified according to their shapes.

There are five types of bones in the human body: long bones, short bones, flat bones, irregular bones, and sesamoid bones.

Long Bones

Long bones are a type of bone that have a long and slender shape, as the name suggests. They are found mainly in the arms and legs, but also in the fingers and toes. Long bones have several functions, such as supporting the body weight, acting as levers for movement, and providing strength and structure to the limbs.

Long bones are composed of several parts, such as:

• The diaphysis, which is the central shaft of the bone. It is made of hard and dense compact bone, and contains a hollow cavity filled with yellow bone marrow that stores fat.
• The epiphyses, which are the two ends of the bone. They are made of spongy bone that contains red bone marrow that produces blood cells. The epiphyses are covered with a layer of cartilage that cushions the joints and reduces friction.
• The metaphysis, which is the narrow region between the diaphysis and the epiphysis. It contains a growth plate that allows the bone to grow in length during childhood and adolescence.
• The periosteum, which is a membrane that covers the outer surface of the bone, except for the parts covered by cartilage. It contains blood vessels, nerves, and lymphatic vessels that nourish and innervate the bone.

Long bones can be affected by various diseases or disorders, such as fractures, infections, tumours, or congenital defects. Some people may also take supplements or undergo surgery to enhance or lengthen their long bones.

If you want to learn more about long bones, you can check out these web pages: Anatomy of a Long Bone, Long Bone – Wikipedia, or Long Bones – MedlinePlus Medical Encyclopaedia.

Short Bones

Short bones are a type of bone that have a similar width and length, giving them a cube-like shape. They are mainly found in the wrist and ankle, where they provide support and stability with little to no movement. They are made of spongy bone tissue, except for a thin layer of compact bone tissue on the surface. Some examples of short bones are the carpal bones in the wrist and the tarsal bones in the ankle. The patella or kneecap is sometimes considered a short bone, but it is more accurately classified as a sesamoid bone, which is a special type of bone that forms within a tendon. Short bones can be affected by some genetic disorders, such as brachydactyly or achondroplasia, which cause abnormal shortening of the bones in the fingers, toes, or limbs.

Flat Bones

Flat bones are a type of bone in your body that have a thin and flat shape, sometimes with a slight curve. They serve as either protection for your internal organs or as attachment points for your muscles. Some examples of flat bones are the bones of your skull, your sternum, your ribs, your scapula, and your coxal bone. Flat bones have a different structure than other bones, such as long bones. They consist of two layers of compact bone tissue enclosing a layer of spongy bone tissue that contains red bone marrow. The red bone marrow is responsible for producing blood cells. Flat bones can be affected by some diseases or disorders, such as fractures, infections, or genetic conditions that cause abnormal shortening or shape of the bones.

Irregular Bones

Irregular bones are bones that have a peculiar form and cannot be classified as long, short, flat or sesamoid bones. They serve various purposes in the body, such as protection of nervous tissue and attachment for skeletal muscles.

Some examples of irregular bones are the vertebrae, sacrum, coccyx, temporal, sphenoid, ethmoid, zygomatic, maxilla, mandible, palatine, inferior nasal concha, and hyoid.

Sesamoid Bones

Sesamoid bones are small bones embedded within a tendon or a muscle1. They can help reduce friction, change the direction of the tendon, or increase the leverage of the muscle. The largest sesamoid bone is the patella in the knee. Other common sesamoid bones are found in the hand and the foot.

The function of sesamoid bones is to act like pulleys, providing a smooth surface for tendons to slide over, increasing the tendon’s ability to transmit muscular forces. They can also modify pressure, diminish friction, and alter the direction of a muscle pull. They also help to protect the tendon from wear and tear.

Some examples of injuries related to sesamoid bones are:

• Turf toe: an injury of the soft tissue surrounding the big toe joint, usually caused by hyperextension of the toe.
• Fracture: a break in a sesamoid bone, either acute (from trauma) or chronic (from overuse).
• Sesamoiditis: an inflammation or irritation of the tendons surrounding the sesamoids, usually from repetitive impact.
• These injuries can cause pain, swelling, redness, bruising, and difficulty in bending or bearing weight on the affected foot.