Skeletal System and Bone Structure

Bones

The adult skeleton is composed of approximately 206 bones
Bones are made of vascularized dense connective tissue
Bones are living tissue.
The skeletal system is divided into two parts: the front and the back.
The front part of the skeletal system includes: Skull, Mandible, Clavicle, Scapula, Humerus, Sternum, Ilium, Femur, Patella, Fibula, Tibia, Ulna, Radius, Carpus, Metacarpals, Phalanges, Tarsus, Metatarsals, Pubis
The back part of the skeletal system includes: Vertebral column, Sacrum, Ischium, Coccyx
Bones are classified according to their structure, shape, and type of ossification.

Bone Structure

Compact Bone:
- Forms the outer layer of flat and short bones as well as the cylinders of long bones.
- Characterized by its solid and compact nature with compressed bone lamellae.
- The arrangement of these lamellae is related to the distribution of blood vessels.
Cancellous (Spongy) Bone:
- Found within the diploe (flat bones, such as the skull)
- Makes up the inner mass of short bones.
- Located at the terminal ends of long bones.
- Consists of thin irregularly arranged branching bone trabeculae (corresponding with lines of maximal tension), separated by bone marrow spaces.

Bone Shape

Long Bones:
- Examples include the clavicle, humerus, radius, ulna, femur, fibula, and tibia.
- Have a cylindrical shape with two ends (upper and lower) and a shaft.
- Ends (epiphysis): Composed of cancellous bone trabeculae covered by a shell of compact bone.
- Shaft (diaphysis): Consists of compact bone surrounding the medullary cavity, which contains bone marrow.
- Metaphysis: The part of the diaphysis near the epiphysis, separated from it by the epiphyseal plate of cartilage.
- Shaft is covered in periosteum (membrane covering bone), which is sensitive and contains blood vessels and nerves.
Short Bones:
- Examples include the carpal and tarsal bones.
- Vary in form and lack bone marrow cavities.
- Red marrow may be present in the interstitial spaces (between the trabeculae).
Sesamoid Bones:
- Example: Patella (the largest sesamoid bone).
- Special type of short bone found in specific locations throughout the body.
- Embedded within the tendons of some muscles.
- Function to modify pressure and reduce friction at their origin.
Short Long Bones:
- Examples: Metacarpal, metatarsal, and phalanges.
- Short cylindrical bones with one end and a bone marrow cavity.
Flat Bones:
- Examples: Vault of the skull, ribs, and scapula.
- Thin and broad bones.
- Composed of outer and inner plates of compact bone, with an intermediate layer of cancellous bone.
Irregular Bones:
- Examples: Vertebrae and hip bones.
- Characterized by ill-defined shapes due to the presence of bony processes and extensions that diverge in different directions.
Pneumatic Bones:
- Examples: Paranasal sinuses (found in some bones of the face and around the nose). These bones contain small air cavities inside:
  - maxillary air sinuses
  - frontal air sinuses
  - sphenoid air sinuses
  - ethmoid air sinuses

Bone Ossification

Cartilaginous Ossification:
- Cartilage is gradually replaced by bone.
- Example: Long bones.
Intra-membranous Ossification:
- Membrane is converted to bone.
- Example: Flat bones.
- Growth occurs predominantly in thickness (sub-periosteal deposition of bone).

Bone Marrow

Red Bone Marrow:
- Responsible for producing red blood cells.
- Found in: skull, vertebrae, ribs, sternum, hip, upper end of humerus, and femur.
Yellow (Inert) Bone Marrow:
- Composed of fatty tissue.

Functions of Bones

Provide the body with its form, contour, and width.
Support overlying soft tissues.
Protect vital, delicate internal organs (skull for the brain, vertebrae for the spinal cord, scapula and ribs for the heart and lungs).
Serve as attachment points for muscles.
Act as levers during movements.
Enable the body to assume different positions (e.g., standing, sitting).
Essential for various activities such as walking, running, jumping, playing, and climbing stairs.
Store calcium salts that can be used during times of need (e.g., pregnancy, aging).
Produce red blood cells via the bone marrow.

Blood Supply of Bones

Nutrient Artery: Supplies the bone marrow and branches into the metaphysis (its branches are end arteries).
Epiphyseal Artery: Supplies the epiphysis.
Metaphyseal Artery: Supplies the metaphysis.
Periosteal Artery: Supplies the periosteum.
Note: The cartilaginous epiphyseal hyaline cartilage has NO blood supply. Blood vessels penetrate the epiphyseal ossification centers as ossification begins.

Cartilaginous Ossification

Existing hyaline cartilage models of bones are gradually destroyed and replaced by bone.
Steps of Cartilaginous Ossification:
- Each bone is represented by small cartilaginous models in the embryo.
- A primary center of ossification appears in the shaft of the cartilage during intrauterine life.
- Secondary centers of ossification emerge at the ends of long bone epiphysis around the time of birth.
- Bone occupies the entire cartilaginous model except at the epiphyseal plate of cartilage.
Therefore, a long bone is comprised of:
- A shaft (diaphysis).