HUMAN SKELETAL SYSTEM

Skeletal system is the system of bones, associated cartilages and joints of human body. Together these structures form the human skeleton. Skeleton can be defined as the hard framework of human body around which the entire body is built. Almost all the hard parts of human body are components of human skeletal system. Joints are very important because they make the hard and rigid skeleton allow different types of movements at different locations. If the skeleton were without joints, no movement would have taken place and the significance of human body; no more than a stone

Components of Human Skeleton:Human skeleton is composed of three main components; Bones, Associated cartilages and Joints.

Bones: Bone is a tough and rigid form of connective tissue. It is the weight bearing organ of human body and it

is responsible for almost all strength of human skeleton. Cartilages:

Cartilage is also a form of connective tissue but is not as tough and rigid as bone. The main difference in the cartilage and bone is the mineralization factor. Bones are highly mineralized with calcium salts while cartilages are not. For more details visit: Basic anatomy article; “Cartilage and its types“.

Joints: Joints are important components of human skeleton because they make the human skeleton mobile. A

joint occurs between “two or more bones”, “bone and cartilage” and “cartilage and cartilage”

What are your bones made of?

Cross section showing osteons. The large dark spots are passages for blood vessels and neves. The little black spots are osteocytes.

Cross section of a bone.

 Compact bone

Compact bone is the heaviest, hardest type of bone. It needs to be very strong as it supports your body and muscles as you walk, run, and move throughout the day. About 80% of the bone in your body is compact. It makes up the outer layer of the bone and also helps protect the more fragile layers inside.

If you were to look at a piece of compact bone without the help of a microscope, it would seem to be completely solid all the way through. If you looked at it through a microscope, however, you would see that it's actually filled with many very tiny passages, or canals, for nerves and blood vessels. Compact bone is made of special cells called osteocytes. These cells are lined up in rings around the canals. Together, a canal and the osteocytes that surround it are called osteons. Osteons are like thick tubes all going the same direction inside the bone, similar to a bundle of straws with blood vessels, veins, and nerves in the center.

Looking at compact bone (A) under a microscope reveals tube-like osteons (B) made up of osteocytes (C). These bone cells have long branching arms (D) which lets them communicate with other cells. 

Spongy bone

Close up view of spongy bone.

Spongy bone is found mostly at the ends of bones and joints. About 20% of the bone in your body is spongy. Unlike compact bone that is mostly solid, spongy bone is full of open sections called pores. If you were to look at it in under a microscope, it would look a lot like your kitchen sponge. Pores are filled with marrow, nerves, and blood vessels that carry cells and nutrients in and out of the bone. Though spongy bone may remind you of a kitchen sponge, this bone is quite solid and hard, and is not squishy at all.

Bone marrowThe inside of your bones are filled with a soft tissue called marrow. There are two types of bone

marrow: red and yellow. Red bone marrow is where all new red blood cells, white blood cells, and platelets are made. Platelets are small pieces of cells that help you stop bleeding when you get a cut. Red bone marrow is found in the center of flat bones such as your shoulder blades and ribs. Yellow marrow is made mostly of fat and is found in the hollow centers of long bones, such as the thigh bones. It does not make blood cells or platelets. Both yellow and red bone marrow have many small and large blood vessels and veins running through them to let nutrients and waste in and out of the bone.

When you were born, all of the marrow in your body was red marrow, which made lots and lots of blood cells and platelets to help your body grow bigger. As you got older, more and more of the red marrow was replaced with yellow marrow. The bone marrow of full grown adults is about half red and half yellow.

DIVISIONS OF HUMAN SKELETON:o Axial Skeleton:

Axial skeleton forms the axis of human body. It consists of Skull, vertebral column and thoracic cage. Skull: Skull is that part of human skeleton that forms the bony framework of the head. It consists of 22

different bones that are divided into two groups: bones of cranium and bones of face. Vertebral Column: It is a flexible column of vertebrae, connecting the trunk of human body to the skull

and appendages. It is composed of 33 vertebrae which are divided into 5 regions: Cervical, Thoracic, Lumbar, Sacral, and Coccygeal.

Rib Cage: It is a bony cage enclosing vital human organs formed by the sternum and ribs. There are 12 pairs of ribs that are divided into three groups: True ribs, False ribs, and Floating ribs. 

o Appendicular Skeleton:It is the skeleton of appendages of human body. It consists of Shoulder girdle, Skeleton of upper

limb, Pelvic girdle and Skeleton of lower limb.

Shoulder Girdle: It attaches the upper limb to body trunk and is formed by two bones: clavicle and scapula.Clavicle is a modified long bone and is subcutaneous throughout its position. It is also known as the beauty bone. For more details on clavicle, visit:”"Scapula is a pear shaped flat bone that contains the glenoid fossa for the formation of shoulder joint. It possesses three important processes: Spine of scapula, Acromion process and Coracoid process. For more details, visit “”.

Skeleton of Upper limb: The skeleton of each upper limb consists of 30 bones. These bones are: Humerus, Ulna, Radius, Carpals (8), Metacarpals (5), Phalanges (14).Click on the name of any bone for more details.

Pelvic Girdle: There are two pelvic girdles (one for each lower limb) but unlike the pectoral girdles, they are jointed with each other at symphysis pubis. Each pelvic girdle is a single bone in adults and is made up of three components: Ileum, Ischium and Pubis. For more details, visit “Hip Bone“.

Skeleton of Lower limb: The skeleton of each lower limb consists of 30 bones. These bones are; Femur, Tibia, Patella, Tarsals (7), Metatarsals (5), Phalanges 

Functions of human skeleton:Human skeleton performs some important functions that are necessary for survival of human beings.

1. STRENGTH, SUPPORT AND SHAPE: It gives strength, support and shape to the body. Without a hard and rigid skeletal system, human body cannot stand upright, and it will become just a bag of soft tissues without any proper shape

2. PROTECTION OF DELICATE ORGANS: In areas like the rib cage and skull, the skeleton protects inner soft but vital organs like heart and brain from external shocks. Any damage to these organs can prove fatal, therefore protective function of skeleton is very important

3. LEVERAGE FOR MOVEMENTS: Bones of the human skeleton in all parts of body provide attachment to the muscles. These muscles provide motor power for producing movements of body parts. In these movements the parts of skeleton acts like levers of different types thus producing movements according to the needs of the human body.

4. PRODUCTION OF RED BLOOD CELLS: Bones like the sternum, and heads of tibia have hemopoeitic activity (blood cells production). These are the sites of production of new blood cells.

BONE HEALING PROCESS

Step 1: Inflammation

When a bone fractures, white blood cells move in to the area to clean up debris created by the break. This creates inflammation, which triggers the growth of new blood cells — the first stage of healing.

Step 2: Soft callus

As blood cells divide and multiply near the break, new blood vessels develop to fuel the repair process. The body also begins to create cartilage around the bone fracture to bridge the gap in the bone. Called the soft callus, this cartilage is simply fibrous tissue.

Step 3: Hard callus

Eventually, the body replaces the soft callus with a hard callus, connecting the bone fragments more solidly. This stronger callus, which creates a bulge at the site of the fracture, can generally be seen in X-rays just a few weeks after the bone fracture occurs.

Step 4: Remodeling

In the final stage of bone fracture healing, the body replaces old bone with new bone in a continual process called remodeling. Remodeling makes bones stronger and more compact and blood circulation in the bone improves.

BONE REMODELING-is a continuous process of bone resorption and formation for the purpose of maintaining normal bone

mass. Normal bone mass indicates healthy bones that are strong and free from problems like osteoporosis. This process goes on inside the human body as long as the person is living. Cells that play important roles in it are osteoclasts, which are responsible for bone resorption; osteoblasts, which are vital in the formation of bones; and osteocytes, which send the signals that bones are being exposed to stress or injury.

Constant remodeling allows bones to perform their many functions, including structural support to the whole body and important storage sites of calcium. With bone remodeling, the body is also able to repair small bone fractures that occur from daily physical activities. Old bone is being replaced by new bone during the remodeling cycle. In adults, this occurs at a rate of about 10% each year. This is a natural process to ensure maintenance of normal bone mass as a person ages.

The remodeling cycle usually starts when injury or mechanical stresses occur in bones. Growth hormones stimulate the production of osteoclasts, which then release enzymes capable of dissolving the bone matrix, creating pits in most bone surfaces. Their lifespan is approximately two weeks, and then they die naturally through a programmed process of cell death, or apoptosis.

BONE OSSIFICATIONSometimes referred to as osteogenesis, ossification is the development of bone within the osseous

system. The term is used to refer to the natural formation of bone, such as in the development of a fetus and during the first years of life. At the same time, the term can also be applied to the occurrence of irregularities in bone development that lead to health issues in children and adults.

It is not unusual for some people to confuse ossification with the process of calcification. Essentially, calcification involves the formation of calcium crystals and salts within cells and tissue. This means that calcification occurs as one part of the process of ossification. However, it does not account for the entire process, and thus cannot be properly considered synonymous with osteogenesis.

There are two general classes of ossification or bone tissue formation that have to do with the normal process of bone development. Endochondral ossification as well as intramembranous bone formation identifies various aspects of the normal growth of bones throughout the body, both in terms of the development of cells within the bones as well as the proper development of the outer surface of the skeletal structure. A third class, known as heterotopic ossification, refers to situations where there is some type of atypical or abnormal bone development taking place

Endochondral ossification is a process where bone replaces cartilage. It occurs during fetal development and throughout childhood as the bones of the body grow. When people experience fractures, endochondral ossification is part of the healing process, with the body first forming cartilage known as a callus and later replacing it with bone. This method for bone formation relies on replacing a model made from cartilage with fully ossified bone.

Heterotopic ossification" refers to the growth of bone material in the soft tissues of the body, including muscles, tendons and fascia. The severity of the condition varies; some patients have only small nodules of excess bone that can be noted on X-rays, but others suffer severe and debilitating pain. The cause of this condition is not fully understood, and the most effective treatment for the condition is aggressive surgery, although some doctors have had success with radiation. The word "heterotopic" essentially means "wrong place," and "ossification" refers to the formation of bone. Originally, heterotopic ossification was grouped under the heading "myositis ossificans," along with an assortment of similar conditions. This term is no longer widely used in reference to heterotopic ossification, because the problem is not confined to the muscles. A related condition, ossifying fibromyopathy, usually confines itself to the fibrous tissue of the body, and periarticular ossification can be found in the region around the joints

CLAVICLE | COLLAR BONE:Clavicle is a modified long bone having two curves. The medial 2/3 of the bone has a convex curve and

the lateral 1/3 has a concave curve as seen from front. It is the only long bone of human body that lies horizontally in its natural position. Like all long bones, it has two ends. The lateral end articulates with the acromion process of scapula and the medial end articulates with the sternum and first costal cartilage. Towards the medial side, the shaft of the bone is rounded while towards the lateral side, it is flattened and forms a superior and inferior surface. The clavicle is subcutaneous throughout its length and can easily be seen in all subjects. Clavicle has two borders and two surfaces. The borders are anterior and posterior and the surfaces are superior and inferior.

Muscles and Ligaments attached to clavicle:Lateral 1/3 of clavicle: Lateral 1/3 of clavicle gives attachment to following important muscles and

ligaments;

Trapezius muscle: It is inserted into the posterior border of the lateral 1/3 of clavicle. Deltoid muscle: It originates from anterior border of the lateral 1/3 of clavicle. Coracoclavicular Ligament: As the name indicates (Coraco = coracoids process, clavicular =

clavicle) it is a ligament attached to the coracoids process of scapula and the clavicle. It has two points of attachment on clavicle. The conoid part is attached to the conoid tubercle and the trapezoid part is attached to the trapezoid ridge. The conoid tubercle and trapezoid ridge lie at the inferior surface of lateral 1/3 of clavicle.

Function of coracoclavicular ligament: It is a strong ligament and is responsible for bearing most of the weight of the hanging upper limb. The upper limb can hang from the clavicle because of this strong ligament.

Medial 2/3 of clavicle: Lateral 1/3 of clavicle gives attachment to following important muscles and ligaments;

Sternocleidomastoid muscle: it originates from the superior surface of medial 2/3 of clavicle. Pectoralis major muscle: It originates from the antero-superior part of the medial 2/3 of clavicle. Subclavius muscle: It is inserted into the middle of the posterior surface of clavicle. It extends in

both later and medial parts of clavicle. Costoclavicular ligament: As the name indicates (Costo= Rib and clavicular = clavicle),

costoclavicular ligament is attached to the clavicle and first costal cartilage. The attachment on the clavicle lies at the posterior surface near the medial end of the bone.

JOINT CAPSULES:The lateral end of clavicle forms a sliding synovial joint with acromion process of scapula. It provides

attachment to the joint capsule of this joint. The medial end of clavicle forms a double plane synovial joint with manubrium sterni and provides attachment to the joint capsule of this joint.

FUNCTIONS OF CLAVICLE:Clavicle acts as a strut to hold the upper limb laterally away from the body. It also bears a part of the

weight of upper limb (the remaining is borne by the scapula). By being laterally away from the body, the functional efficiency of upper limb increases greatly.

CLAVICLE FRACTURES:Because the clavicle transmits forces from the upper limb to the trunk, it may itself be damages by

these forces. In fact clavicle is the most commonly fractured bone in the human body. The most common reason of fracture is a fall on the shoulder or outstretched hand. The weakest point of the clavicle where it gets fractured most easily is the junction of the convex medial 2/3 and concave lateral 1/3. After the fracture occurs at this point, the lateral fragment of the bone is displaced downward by the weight of upper limb while the medial fragment is pulled upward by the force of sternocleidomastoid muscle.

The clavicle fracture may result in compression of brachial plexus, subclavian artery and subclavian vein causing immense pain over the side of the neck.