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When a lot of us think about what our body is made of, many of us think firstly of our bones. This is rightfully so, as our skeletal system provides the framework for our body, which makes it extremely important. Not only is the skeletal system comprised of bones, but also cartilage, ligaments, and tendons. Cartilage is very important during one’s childhood, as it composes the majority of their skeletal system, and is replaced by bone later on as they mature. Ligaments hold bones together at the joints. Tendons hold bones to muscle, and are similar to ligaments. 

 

Bones have 6 main functions:

  1. Support: bones support the muscles of the body.

  2. Protection: bones create a strong barrier for the brain, spinal cord, heart, and various other parts against the outside world.

  3. Movement: movements such as breathing or moving your limbs are only possible by muscles on the bones.

  4. Electrolyte balance: the skeletal system participates in an intricate system of calcium and phosphate balance.

  5. Acid-base balance: bone tissue is a blood buffer, maintaining a stable pH by balancing alkaline phosphate and carbonic salts.

  6. Blood formation: blood cells are mostly produced in red bone marrow.

 

 General anatomy of bones:

There are different types of bones, such as flat bones (scapula), long bones (humerus), short bones (tarsals), and irregular bones (vertebrae). Bone is mostly comprised of compact, or dense, bone, which is its outer protective shell. Inside the bone is the marrow cavity. In children, the marrow cavity is mostly filled with red bone marrow, whereas adults have mostly yellow (fatty) bone marrow. There is also spongy bone, a loose form of osseous tissue. The diaphysis is the shaft of the bone, while the epiphysis are the ends. 

Bone cells:

There are 4 types of bone cells:

  1. Osteogenic cells: stem cells that give rise to other bone cells.

  2. Osteoblasts: bone-forming cells. They harden the soft matter of bone matrix and secret the hormone osteocalcin, which stimulates the pancreas to secrete insulin.

  3. Osteocytes: osteoblasts that are trapped in the matrix that they deposited in. They maintain bone density, blood concentration, and are strain sensors.

  4. Osteoclasts: bone-dissolving cells. Involved in bone resorption.

 

Osteon: 

An osteon is the structural unit of compact bone, which consists of a central canal surrounded by lamellae. There are also perforating canals that are transverse or diagonal passages, and lacunae, small holes within the osteon. 

Ossification:  

Ossification is the formation of bone. There are two types of ossification:

  1. Intramembranous ossification: produces the flat bones of the skull and clavicle. Osteoid tissue is calcified and osteocytes are entrapped, creating a honeycomb effect of spongy bone. Compact bone is formed at the surface by filling in spaces, leaving the spongy bone in the center. 

  2. Endochondral ossification: bone develops from hyaline cartilage. 

    1. Perichondrium produces chondrocytes and cartilage grows in thickness.

    2. In the primary ossification center, chondrocytes inflate and die, and the walls between them calcify. Osteoblasts begin to produce. 

    3. Osteoclasts hollow the shaft, creating the primary marrow cavity. 

    4. A secondary marrow cavity is created in the epiphysis.

    5. The epiphyses fills with spongy bone during infancy and childhood.

    6. As we reach adulthood, the primary and secondary marrow cavities unite.

 Bone Elongation: 

An epiphyseal plate is a cartilaginous wall that functions as a zone of bone growth and elongation. The elongation of bone can be described in these steps:

  1. Zone of reserve cartilage: this region is comprised of hyaline cartilage with no sign of bone generation.

  2. Zone of cell proliferation: chondrocytes multiply and line up in rows.

  3. Zone of cell hypertrophy: chondrocytes stop multiply and enlarge in size.

  4. Zone of calcification: the cartilage is temporarily calcified for support.

  5. Zone of bone deposition: chondrocytes die and walls are broken down between each lacunae, allowing blood vessels to flow through. Osteoblasts deposit lamellae. 

 

So, how does this relate to height?
When chondrocytes multiply and enlarge, the zone of reserve cartilage is pushed upward towards the end of the bone, causing elongation.

Achondroplastic Dwarfism can happen when the bones of the limbs, such as the tibia, stops growing during childhood. This happens when bone elongation does not occur, as cartilage does not grow. Achondroplastic dwarfism arises from spontaneous mutations in DNA replication. 

 

Get Out and Be Active! Why it’s good for your bones…

Wolff’s law of bone states that bone is formed based on the mechanical stresses of the person. Thus bone adapts to those stresses placed upon it. This means that your bones are unique to your movements. Generally, people who are physically active have a larger bone density and mass than sedentary people. Because activity creates mechanical stress on the body, your bones adapt to become stronger. 

 

Bone physiology

Osseous tissue plays a key role in the homeostasis of the body. Bone utilizes mineral deposition (minerals taken from the blood into the bone) and mineral resorption (dissolving bone to put minerals into the blood). The primary minerals in play are calcium and phosphate. Calcium is involved in neural communication, muscle contraction, and blood clotting. Phosphate is used in acid-base balance of the body. The skeleton serves as a reservoir for these minerals, and are thus deposited when there is an excess supply and released when they are needed. 

Because calcium is so important, three hormones help maintain calcium homeostasis:

  1. Calcitriol: raises blood calcium concentration by increasing calcium resorption.

  2. Calcitonin: lowers blood calcium concentration by inhibiting osteoclasts and stimulating osteoblasts.

  3. Parathyroid hormone: raises blood calcium by increasing osteoclast population.

Why should you keep your bones healthy?

Because bones are such an integral part of your body, its important to keep them in tip top shape! If not, various things can happen such as: 

  1. Fractures: Weak bones can easily lead to stress fractures, which are usually caused from falls or while participating in athletic sports. 

  2. Osteoporosis is the most common bone disease. It occurs when your bone severely loses bone density, leading to porous-looking bones. Bones become very brittle, and can break from the simplest of tasks! Osteoporosis can cause pathological fractures in the hips, wrist, and vertebral column. 

Now… How do I keep my bones healthy?

  1. Eat healthy foods rich in calcium. As stated earlier, calcium is integral in the formation and homeostasis of bone. Thus, make sure you always have a good amount of calcium in your body, or else mineral resorption will happen and your bones will weaken. Aim for low-fat dairy products such as milk. Also, always try to consume any calcium foods with vitamin D: it’ll help absorb the calcium better! If you are unable to consume enough calcium in your diet, consider taking supplements.

  2. Stop smoking! Studies have shown that smoking causes traumatic losses of bone mass, even in younger individuals! This leads to higher risks of fractures. Furthermore, women who smoke also experience menopause earlier, which increases bone loss.

  3. Exercise: As stated earlier with Wolff’s Law of Bone, exercising will make bones stronger. By creating mechanical stresses, bones produce more osteoblasts that strengthens it, increasing bone mass and density. By exercising, you will help to maintain a healthy body weight, which will also decrease chances of osteoporosis!

Now that we’ve talked about the microscopic side of the  skeletal system, lets talk about the macroscale side!
The skeleton is divided into two parts: the axial skeleton, which is the central supporting axis, and the appendicular skeleton, which forms the appendices and girdle. While the average adult has 206 bones, we all started out with 270 at birth! How is this so? Over time, some bones fuse with each other, thus decreasing the count. 

 Skeletal Anatomy: 

Bones are comprised with various bumps, ridges, and depressions that make each unique. Each different marking is important because it indicates what that bone does in relation to the body. It may show what that bone is connected or attached to.

 The Skull:

The skull consists of 22 bones, and is easily the most complex part of the skeleton. The skull is immovable and each bone is connected by sutures. 

The most important bone markings of the skull  are its cavities. The cranial cavity, for example, houses and encloses the brain. 

 

 Important Bones:

Cranial bones

  1. Frontal bone

  2. Parietal bones

  3. Temporal bones

  4. Occipital bone

  5. Sphenoid bone 

  6. Ethmoid bone

Facial bones

  1. Maxillae

  2. Palatine bones

  3. Zygomatic bones

  4. Lacrimal bones

  5. Nasal Bones

  6. Inferior nasal conchae

  7. Vomer

  8. Mandible

 Vertebral column:

The vertebral column, commonly known as the spine, supports the skull and trunk. It consists of 33 vertebrae bones with intervertebral discs in between them. The vertebrae are divided into 5 groups: the cervical, thoracic, lumbar, sacrum, and coccyx. The vertebrae is supposed to be slightly S-shaped, with four bends called the cervical, thoracic, lumbar, and pelvic curvatures.

Spine Curvatures and You: How They Can Go Awry

Scoliosis is an abnormal lateral curvature, and occurs when the body fails to develop on one side of the vertebra. While it is non-preventable, back braces and surgery may lessen the scoliosis.

Kyphosis is an exaggerated thoracic curvature which usually occurs in old age. It is a common result of osteoporosis or boys who engage in spine-loading sports such as weightlifting or wrestling. Prevent kyphosis by maintaining strong, healthy bones without overworking them too much!

Lordosis is an exaggerated lumbar curvature common in pregnancy and obesity. Prevent lordosis by maintaining a healthy diet and weight to prevent obesity, as stomach fat pulls the pelvis forward. Stretch frequently to help maintain correct posture and to keep our hip flexors in place.

 Each region of the vertebra has unique characteristics which aid in its form and function. For example, the cervical vertebrae are smaller. As they support the head, vertebra C1, or the atlas, is shaped to support the head and thus does not look like a typical vertebra. The thoracic vertebrae has concave spots called costal facets to allow the ribs to attach. Lumbar vertebrae have thick, stout bodies with a square spinous process for lumbar muscles to attach. Furthermore, the articular process are arranged differently to resist twisting of the lower spine. 

 

 The thoracic cage encloses the lungs, hearts, and other important internal organs. It is integral in breathing, as it expands to draw air in, and compresses to expel air. 

Like the vertebrae, ribs are also shaped differently to meet their form and function. 

 

 The pelvic girdle consists of two hip, or coxal, bones and the sacrum, which create a ring.

 

 The pectoral girdle supports the arm, linking it to the axial skeleton. It consists of the scapula and clavicle.

 

 

The upper limbs contain 30 bones per limb. They include the humerus,  radius, ulna, carpal bones, metacarpal bones, and phalanges.

 

 The lower limbs is similar to the upper limb, and also contains 30 bones per limb. However, they are slightly adapted to bear weight and move, and are thus shaped a little differently. The bones include the femur, patella, tibia, fibula, tarsal bones, metatarsal bones, and phalanges.

 

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