The human skeletal system

Skeletal system gives mechanical support to our body from inside and enables free movement of body . With out this system our body is mere sac of flesh that collapses down. As this system gives support from inside, it is called endoskeleton. Insects like crab, Scorpions have tough shield like plates made of Chitin externally,  giving mechanical strength and protection from out side, that version is called exoskeleton.  Caterpillars, Starfishes have fluid inside them hence called hydrostatic skeleton. However the skeletal system of man is made up of bones containing Calcium phosphate, calcium carbonate and chlorides and fluorides of calcium and magnesium. The  Skeleton has its own supply of blood vessels and nerves.

The main responsibilities of the skeletal system include

1. To provide protection to all the internal organs of the body. For example, the skull protects the eyes, brain and ears. The spinal cord is protected by the vertebrae.

2.  To provide the body shape and structure. The lungs are supported by the rib cage.

3. To act with the muscles for moving the body parts.

4. To produce blood cells (WBC’s, RBC’s and Platelets).

5. Bones store minerals and are the greatest deposit of calcium.

The human skeletal system is composed of 206 bones. These bones are divided in following two categories.

1. Axial Skeleton- The bones of the Skull, Thoracic Girdle/Rib Cage and Vertebral Column, form the axial skeleton of the body.

2.  Appendicular skeleton- The bones of the Shoulder Girdle, Upper Limbs, Pelvic Girdle and Lower Limbs form the appendicular skeleton of the body.

Axial skeleton

The Human Skull

It is composed of two parts.

The Neurocranium- It is the cranial vault (or a cranial/bone protection layer) surrounding the brain and the brain stemIt is also called brain box. .   The cap of the skull is called Skull Cap or the Calvaria. The Calvaria is made up of Occipital Bones, Parietal Bones and the Frontal Bones. Bergama is the point where the 2 parietal bones and the frontal lobe meet. Labda is the point where 2 parietal and occipital bones meet. The inner surface of the skull cap has depressions for housing the cerebrum and its convulsions. The frontal lobes occupy the Upper-Front region of the Cranium. The frontal lobe and the parietal lobes are separated by Central Sulcus. From the temporal lobe, the frontal lone is separated by the Lateral Sulcus. 

Viscerocranium- The bones that together form the bones are called Viscerocranium. These bones are formed from the Pharyngeal Arches. The first pharyngeal arch forms the jaw. The second pharyngeal arch forms the jaw support and the hyoid (a horse shoe shaped bone). Likewise, there are other pharyngeal arches as well (total 6 arches). But it is not necessary that all of them form a bone or a cartilage. For example arch 5 does not form any bone. The bones of the Viscerocranium are the Inferior Nasal Concha, Mandible, Lacrimal Bone, Nasal Bone, Maxilla, Vomer, Palatine Bone, and Zygomatic Bones.

Bones of the skull are joined by the Synarthrodial Joints (provide for little or no movement) and Sutures (fibrous joints). The skull also has Sharpey’s Fibers (connective tissue) that imparts flexibility to the movements. The human skull has 22 main bones and 7 associated bones.

The Rib Cage or the Thoracic Girdle.

It has 122 pairs of ribs and sternum. It encloses the heart and lungs. The human rib cage supports the Pectoral Girdle and surrounds the Thoracic Cavity. There are 24 ribs in the human thoracic girdle. It thus houses 12 Thoracic Vertebrae, The Sternum and the Coastal Cartilages.The 24 ribs (12 pairs) can be either true or false ribs. The true ribs are connected with the sternum through the arteries. The false ribs are connected with the sternum indirectly through a cartilage. The first seven ribs are True Ribs.

The 8th to 12th pairs are False Ribs.All ribs are composed of a head, a neck and a shaft. The head is close to the spine. All the human ribs are attached to the thoracic vertebrae posteriorily. The neck of the rib is approximately 2.5 cm long. It extends laterally form the rib’s head. The shaft is located at the end of the rib.Sternum- The sternum is an elongated bone. It is placed in the thorax at its interior wall. The cartilages of the first 7 pairs of ribs or true ribs articulate with the sternum through the sternum’s margins. The upper end of the sternum supports the collarbone or the clavicles.

3. The Vertebral Column or  Spine

Each vertebra is formed of a bone and a hyaline (transparent) cartilage. A vertebra is irregular in shape. The spinal cord or vertebral column consists of 24 vertebrae that are joint with each other. Another 9 that are fused in the coccyx (the final segment of the vertebral column) and the sacrum (a pelvic structure). The naming of the articulating vertebrae is done on the basis of their location in the spine. The Presacral vertebrae include the 7 Cervical Vertebrae, 12 Thoracic and 5 Lumbar Vertebrae. The sacrum consists of 5 fused sacral vertebrae and the coccyx consists of 4 fused Occygeal Vertebrae. The Presacral Vertebrae are flexible in the sense that they provide for movements.The spinal cord has both convex and concave curvatures. The convex curvatures (anteriorly) are the thoracic and sacral. The concave curvatures (posteriorily) are cervical and lumbar.The naming of spinal vertebrae is done as follows-

Cervical vertebrae- The seven vertebrae of the neck.

Atlas- The upper most/first vertebrae.

Axis- The second vertebrae located just below the atlas.

Thoracic Girdle – There are 12 vertebral  pairs in the Thoracic Vertebrae. These form 24 ribs.Lumbar Vertebrae- There are 5 vertebras in this region. The region is located between the thorax and the sternum.Sacrum- 5 fused vertebrae are located in this region.Coccyx- 4 vertebrae are located in this region.

The Appendicular Skeleton

This section of skeleton consists of 126 bones. 

 Shoulder girdle/Pectoral Girdle- It provides locomotion to the axial muscles and the upper limbs. The set of bones which connect the upper limb (arm, maxilla and shoulders) to the axial skeleton is called the pectoral girdle. It is formed of the Clavicle and Scapula.

Clavicle (collar bone) - It is a flat bone. It has a rod shape. It absorbs compressions and is located between the scapula and the sternum. It lies horizontally inside the body unlike other long bones that lie vertically. It is a part of the pectoral girdle and the shoulder.

Scapula- The scapula connects the Upper Arm (Humerus) to the Collar Bone (Clavicle). The front of the scapula consists of subscapular fossa. The back of the scapula has the supraspinous fossa. The supraspinous fossa lies beneath the infraspinous fossa. At the side of scapula the shoulder summit point is located.

The Pectoral Girdle consists of 5 joints. 3 of the joints are called True Anatomical Joints and 2 are called Physiological Joints. The joints are linked and contribute to the movement of shoulder to variable degrees. These joints are-

Anatomical Joints

• Glenohumeral join- The joint between scapula and the Humerus.
• Acromioclavicular joint- The joint between clavicle and the scapula.
• Sternoclavicular joint- The joint between clavicle and the sternum.
• Physiological joints
• Sternoclavicular Joint- It is joint between Clavicle and the Sternum
• Scapulocostal Joint- The joint between scapula and the rib cage.

Upper limb- It includes the bones of arms, shoulders and maxilla.

Upper limb (includes the shoulder girdle)

Arm

Upper arm- The region forms the shoulders of the elbow is called the upper arm. It is composed of elbow joint and Humerus.

Elbow- It is formed of 3 joints. The Superiod radioulnar joints, the Humerolunar joint and the Humeroradial joints.

Forearm- It is made of the bones radius and ulna.

Wrist- It is made up of Carpal Bones. The wrist joints are called the Radiocarpal and the Carpometacarpal  Joints.

Hand - The finger bones are called phalanges and the bones of the hand are called metacarpals.

The pelvic girdle

It has the shape of basin and is also called pelvis. It connects the upper limbs, thoracic girdle and axial skeleton system , the upper part of the body ( trunk), to the legs. It supports the urinary bladder, intestines and internal sex organs.  It also supports and balances the trunk. The pelvic girdle has 2 hip bones. 

The hipbone is made of Ilium (make the width of hip), Ischium and the Pubis. The three unite to form Acetabulum during early adulthood. The three bones are found in the pelvic cavity. The pelvic floor forms the lower region the pelvic cavity.

Lower limbs

The thigh is formed of a single bone called femur. The two long bones of the legs are called Tibia and Fibula. The knee is the largest joint of the body. The fibula attaches   muscles to the leg. The Tibia or the Shin Bone forms the inside of the leg. The Tibia is connected to the Patella or the foot bone. The Talus or the ankle bone is also linked with the Tibia and Patella (a sesamoid bone). The Patella helps in flexing and extending of foot.The foot is formed of 26 bones. The region is divided into ankle or Tarsus, the Instep or Metatarsus. The third region is the toe or the phalanges. The ankle has 7 bones. The largest of them is the heel (also called as the calcaneous bone). The instep has 5 Metatarsal Bone.

Bone transverse section

The transverse section of the bone consists of :-

Osteon- It is the fundamental unit of bone which functions. Osteon is cylindrical and consists of concentric layers. The osteon has a .2 mm diameter. The lamellae (or concentric layers) of the tissues of the bone surround around a central canal. This canal is called Harvesian canal. The circumferential lamellae arrange themselves parallel to the surface of the bone. The space between two osteon is filled by interstitial lamellae which are remnants of destroyed osteons. The osteons interact with other cytoplasmic organelles and processes through Canaliculi.

Harvesian Canal- The Harvesian Canal is vascular and is connected by blood vessels and nerves. The region of the bone is formed of lamellae and is also called Cortical Bone. Besides interacting with the nerves, the cortical bone region also supports the body, causes movements, protects the underlying organs and helps in maintain the element composition constant and healthy.

Osteocyte- Osteocytes are derived from the Osteoblasts. The osteocytes make up more than 90% of volume of bone cells. They have the shape of a star. They have a length of 15µm.  Their diameter is 7µm. The osteocytes control the activity of both Osteoblasts and osteoclasts within the BMU or Basic Multicellular Unit. The BMU is the basic structure (temporary) where the bone remodeling i.e. absorption of bone cell and manufacture of new bone cell takes place. For inhibiting blood formation, the osteocytes secrete Sclerostin, a hormone. The osteocytes die when they age or when apoptosis or osteoclastic engulfment occurs.

Canaliculi- The bone canaliculi are canals (microscopic in dimensions) found between two Osteocytes.

Bone Structure

  
If we section a long bone, we will find the following parts-

Diaphysis- The elongated shaft located in the central region.

Epiphysis- The large round ends of the bones are termed as epiphysis.

Metaphysis- The area of the bone that lies between the epiphysis and diaphysis is called metaphysic.

Ephiphyseal plates- These plates are made of cartilage. They are also called the growth plates as the bone elongation is caused through these plates.

The hyaline cartilage covers the end of bones. The cartilage doe not have any vascular supply but aids in shock absorption.

Composition of bone

The bone is extremely hard with the great tensile and compressional strenths. The organic components of the matrix of bone are mainly Chodroitin-4-sulphate, hyaluronic acid and large number of collagen fibres; they form not more than 35% of the weight of fresh bone. The inorganic components form at least 65% of the weight of fresh bone; of this 85% calcium phosphate, 10% calcium carbonate and remaining 5% is formed of chlorides and fluorides of calcium and magnesium. Major proportion of phosphates of calcium in the bone matrix occurs as complex salt called hydroxyapatite (Ca(Po4)6(OH)2) in the form of needle like crystals.

It is interesting to know that bones are formed from other tissues in special conditions. Ex:- Sesamoid bone (Patella) is formed from tendon
Os cordis in the heart of some mammals like horses, Os penis in the penis of some mammals like bats.

Anatomy of bone

If we cross section a bone, then we will study the below given areas:

Periosteum- The Periosteum is a thin layer made up of connective tissue. The Periosteum is the point where the muscles attach to the bone. It also provides blood supply to the bone.The Periosteum can again be divided into:

Cambium Layer- It contains the progenitor cells which develop into osteoblasts. Osteoblasts are the cell responsible for bone formation.

Fibrous Layer- It contains fibroblasts.   

Compact Bone- it is located beneath the Periosteum and is also called the cortical bone. It provides strength to the bones. They make up around 80% of the bone weight.

Spongy Bone- It is also called the cancellous bone. They are found at the ends of the long bones but are located inside the compact bone in flat and irregular bones. They make around 20% of the bone weight.

Medullary cavity- It is the center of the bone shaft. It contains both the red and the yellow bone marrow.

Types of bone joints

The points where 2 or more bones meet are termed as bone joints. There are basically 3 types of bone joints. They are:

Fibrous joints- When the bones are held together by a ligament, then the joint is called Fibrous Joint. A ligament is a fibrous connective tissue.

Cartilaginous joints- When the bones are held together by a cartilage, then the joint is called cartilaginous joint. For example, the vertebrae found in human spine are cartilaginous.

Synchondroses joints- These joints are found only in children and are temporary in nature.

Synovial joints- They are the most common of all bone joints and are highly movable. They consist of a synovial capsule that surrounds the entire joint, a synovial membrane that forms the inner layer of the synovial capsule, and a fluid by the name synovial fluid. The synovial fluid provides lubrication to the entire structure. The synovial joint also has a hyaline cartilage. The cartilage articulates the bones. The different types of synovial joints are Hinge, Ball and Socket, Pivot, Condyloid and Gliding synovial joints. 

Joints can be classified on the basis of movability as follows. 

1.Freely movable joints (Diarthroses)

These joints allow free movements, they are further classified as

i.Ball and socket joint(Enarthroses)

The head of one bone is ball like and fits into a socket or concavity of other. This joint allows free movement in more than one plane. 

Example: Shoulder joint between head of humerus(bone of upper arm) and glenoid cavity, (concavity in pectoral girdle) at shoulder.

Hip joint between head of femur (thigh bone) and acetabulum (concavity in pelvic girdle) at hip.

ii.Hinge joint

A protruberance (condyle) at the end of one bone fits into a corresponding depression of the other, which may be flexion or extension involving increase or decrease of the angle.

Example: Elbow joint between radiul-ulna of fore arm and humerus of upper arm at elbow.

Knee joint between tibia-fibula of shank and femur of thigh at knee

Joints of phalanges of digits that is fingers and toes.

iii.Pivot joint (Rotatoria)

One of the bones is fixed in tis place and has a peg like elevation called pivot. The other bone fits over the pivot by a concavity and rotates freely around. E.g In neck, the joint between the atlas (first vertebra) and axis (second vertebra). Axis has a peg like odontoid process on which atlas (bearing the skull) rotates freely.

iv.Saddle joint

These are almost like ball and socket joints. But ball and socket are poorly developed. The ball bearing bone can be rotated around inefficiently. Example: Joint between carpal and metacarpal of thumb. Hence the thumb enjoys greater freedom of movement than the other fingers.

v.Gliding joint(Arthrodia)

The articulating bones can slide upon one another at the joint. This joint allows restricted movement in different planes.
E.g., Joints between carpals(wrist bones)
       Joints between tarsals (ankle bones)
       Joints between the zygapophyses of successive vertibrae.

2.Imperfect movable joints (Amphiarthroses)
 Synovial capsule and synovial fluid are absent. The cartilage tipped adjacent bones are directly articulated with one another. These joints allow very little movements e.g., Pubic symphysis of pelvic girdle of females.

3.Immovable joints (Synarthroses)

Articulating bones does not allow movements. These are the following.
a. Sutures: These are the zigzag lines of the junctions of edges of flat bones. E.g joints between the bones of cranium of skull.

b. Gomphoses: A bony projection of one bone fits into a socket of other . e.g. teeth on maxilla and premaxill of jaw.

c. Shindylases: One bone fits into a slit in another bone. e.g. articulationof ethmoid bone with the vomer.

If we section the bone transversely, we will find the following structures.

Osteon- It is the fundamental unit of bone which functions. Osteon is cylindrical and consists of concentric layers. The osteon has a .2 mm diameter. The lamellae (or concentric layers) of the tissues of the bone surround around a central canal. This canal is called Harvesian canal.  The circumferential lamellae arrange themselves parallel to the surface of the bone. The space between two osteon is filled by interstitial lamellae which are remnants of destroyed osteons. The osteons interact with other cytoplasmic organelles and processes through Canaliculi.

Harvesian Canal- The Harvesian Canal is vascular and is connected by blood vessels and nerves. The region of the bone is formed of lamellae and is also called Cortical Bone. Besides interacting with the nerves, the cortical bone region also supports the body, causes movements, protects the underlying organs and helps in maintain the element composition constant and healthy.

Osteocyte- Osteocytes are derived from the Osteoblasts. The osteocytes make up more than 90% of volume of bone cells. They have the shape of a star. They have a length of 15µm.  Their diameter is 7µm. The osteocytes control the activity of both   Osteoblasts and osteoclasts within the BMU or Basic Multicellular Unit. The BMU is the basic structure (temporary) where the bone remodeling i.e. absorption of bone cell and manufacture of new bone cell takes place. For inhibiting blood formation, the osteocytes secrete Sclerostin, a hormone. The osteocytes die when they age or when apoptosis or osteoclastic engulfment occurs.

Canaliculi- The bone canaliculi are canals (microscopic in dimensions) found between two Osteocytes.

BONE MARROW AND ITS FUCTIONS

The interior of bones contain two types of tissues that are collectively called Bone Marrow. The two types are:

The Red Marrow- It consists of Hematopoietic tissue or the Haematopoietic Stem Cells. They synthesize the blood cells of the body. The blood cells which originate here are Platelets, Red Blood Cells (RBC’s) and most of the White Blood Cells (WBC’s). At the time of birth most of the bone marrow is red. The red marrow slowly changes into yellow marrow with age. The flat bones of the body contain the red marrow. The flat bones specialize in offering protection or provide surfaces for the muscles to attach themselves. Flat bones include sternum, cranium, pelvis, ribs, scapulae and vertebrae, femur, Humerus etc.

The Yellow Marrow- It is made up of fat cells. These fat cells are found in the Medullary Cavity.  The medullary cavity is the central cavity of a bone. The walls of the cavity are composed of spongy bones. Endosteum (connective tissue) lines the spongy bones.

Stroma

Some cells of the red bone marrow and the yellow bone marrow are collectively called stroma. Stroma is that part of the bone which does not produces blood cells. It takes an indirect part in the process of hematopoiesis. It is the provider of hematopoietic environment which facilitates the blood cell formation process. The stroma provides Colony Stimulating Factors or Glycoproteins which bind to the hemopoietic stem cells and activate the process of blood cell formation. The stroma consists of osteoblasts, osteoclasts, adipocytes, fibrobalsts and endothelial cells.

Osteoblasts- These cells synthesize bones. Osteon is made up of these osteoblasts which combine as a unit with the help of other cells. The Osteoblasts are formed from the mesenchymal stem cells.

Osteoclasts- They reabsorb the bone tissues and remodel the bone. The bones formed after osteoclasts remodeling are stronger than Aluminium! The osteoclasts have a multinucleated cell (5 nuclei). The diameter of osteoclasts is 150 to 200µm. They are formed from the haematopoietic stem cells.

The major function of the bone marrow is to produce blood cells. Osteoblasts and osteoclasts together regulate the amount of bone tissue that remains in the body.

Other cells found in the bones are:-

Fibroblasts- They synthesize collagen and extracellular matrix.

Macrophages or WBC’s - During the production of RBC’s in the stem cells, they provide iron.

Adipocytes- They is the fat cells or lipocytes.

Endothelial cells- They are simple squamous cells. They form endothelium which lines the interior surface of the blood vessels found in the bone marrow.

Hematopoietic stem cells- They synthesize the WBC’s (leukocytes), platelets (thrombocytes) and RBC’s (erythrocytes). They are formed from the mesoderm. They are found in the red bone marrow.

The blood vessels of the bone marrow form a barrier. This bone marrow barrier does not allow the outflow of immature blood cells from the red bone marrow.

Ossification- The process of bone formation

The main bone forming cells or the osteoblasts arise from the Mesenchymal Cells. The Periosteum (dense network of connective tissue), that is found on the outer surface of the bones, contain these Mesenchymal cells. Mesenchymal cells are found abundantly in the bone marrow as well. The osteoblasts differentiate to form the progenitor cells. These osteoprogenitors form the osteocytes, the cells that make up the bulk of bones. The osteocytes last for around 25 years.

Diseases of bone and bone marrow

Pagel’s disease of bone- Occurs because of formation of giant osteoclasts.

Osteoporosis- When the balance of bone formation and bone re-absorption is disturbed then osteoporosis occurs. In this disease bone loss occurs resulting in frequent fractures.

Multiple myeloma- It is a cancer found in plasma cells (WBC’s).  It damages the bone marrow and is a fatal disease.

Aplastic anemia- It is caused by exposure to infection, immune disease, drugs, radiation etc.  Heredity can also be a cause. In this disease the bone marrow and blood stem cells are damaged. There is no or lesser production of the blood cells.

Osteogenesis Imperfecta- It is a genetic disorder and results in bone weakening.