Bone Structure and Types: A Comprehensive Guide
Bones
Types of Bones
Long Bones
Long bones are characterized by their length and consist of:
- Diaphysis: The middle or body of the bone contains a cavity called the medullary cavity. Its hollow, cylindrical shape and compact bone within this region make it well-suited for providing strong support without excessive weight.
- Epiphysis: Refers to the ends of the long bone. Its bulbous shape provides ample space near the joints for muscle attachment and joint stability.
- Articular Cartilage: A fine layer of cartilage covering the epiphysis surface. Its elasticity cushions shocks and impacts.
- Periosteum: A dense, white fibrous membrane covering the bone except for the articular surfaces, which are covered by articular cartilage.
- Medullary Cavity: A tubular cavity found within the shafts of long bones.
- Endosteum: A thin epithelial membrane lining the marrow cavity of long bones.
Short Bones
Short bones are nearly equal in length, width, and height, providing them with great strength but limited mobility.
Flat Bones
In flat bones, the thickness is considerably less than the other dimensions. They often protect delicate organs.
Irregular Bones
Irregular bones have unique and varied shapes. Vertebrae and ear bones are classic examples.
Bone Classification by Composition
Bones are classified into two types based on their composition:
- Compact Bone: Contains numerous cylindrical structural units called osteons or Haversian systems.
- Spongy Bone: Unlike compact bone, spongy bone does not contain osteons. Instead, it has irregular plates called trabeculae.
Bone Cell Types
- Osteoblasts: Bone-forming cells.
- Osteoclasts: Cells that resorb bone.
- Osteocytes: Mature bone cells.
Bone Marrow
Bone marrow is a specialized type of soft connective tissue called myeloid tissue. It produces blood cells and is found in the marrow cavity of long bones and within the spaces of spongy bone.
Bone Tissue
Bone tissue, like other connective tissues, is composed of cells, fibers, and extracellular material (matrix). However, the extracellular components in bone are hard and calcified. The extracellular matrix predominates in bone. It is much denser than bone cells and contains many collagen fibers (the most abundant protein in the body). The rigidity of bone allows it to act as support and protection.
Composition of the Bone Matrix
- Inorganic Salts
- Organic Matrix: Collagen fibers and a mixture of proteins and polysaccharides called ground substance.
Fontanelles
Fontanelles are soft spots in the skull present at birth due to incomplete ossification. They allow for compression of the skull during childbirth and are important for determining the position of the head before delivery.
- Anterior (or Frontal) Fontanelle: Located at the intersection of the sagittal and coronal sutures (union of the frontal and parietal bones). It is diamond-shaped and the largest fontanelle, usually closing around 18 months of age.
- Occipital (or Posterior) Fontanelle: Located at the intersection of the sagittal and lambdoid sutures (union of the parietal bones and the occipital bone). It is triangular and usually closes around the second month of age.
- Sphenoid (or Anterolateral) Fontanelle: Located at the junction of the frontal, parietal, temporal, and sphenoid bones.
- Mastoid (or Posterolateral) Fontanelle: Located at the junction of the parietal, occipital, and temporal bones. It typically closes around the second year of age.
Lower Jaw Bone (Mandible)
The lower jaw bone is horseshoe-shaped and highly mobile, as its primary function is mastication.
Nasal Bones
These two small bones join together at the midline. They articulate with the maxilla and the frontal bone, forming the bridge of the nose.
Malar Bones (Zygomatic Bones)
These two bones are roughly square-shaped and form the cheekbones.
Teeth
The first set of teeth, called deciduous or milk teeth, consists of 20 teeth in each jaw, distributed as follows: 4 incisors, 2 canines, and 4 molars. They begin to appear a few months after birth. The second set of teeth, or permanent teeth, typically starts erupting around 6-7 years of age and is usually complete by 20-30 years with the emergence of the last molars, or wisdom teeth. The average adult has 32 teeth, distributed as follows: 4 incisors, 2 canines, 4 premolars, and 6 molars in each jaw.
Spine (Vertebral Column)
- Cervical Portion: The upper region of the spine, consisting of seven vertebrae. These are the smallest and most mobile vertebrae, supporting the neck.
- Thoracic Portion: Located below the cervical vertebrae, it consists of 12 vertebrae. They are larger than the cervical vertebrae and have less mobility. They correspond to the back area.
- Lumbar Portion: Situated between the thoracic spine and the sacrum, it consists of five vertebrae. These are the largest vertebrae and have some mobility. They are located in the abdominal area.
Intervertebral Discs
Intervertebral discs allow for flexion, extension, and lateral rotation of the spine. They act as elastic cushions.
Rib Cage
The rib cage consists of all the bones in the chest. Its boundaries are:
- Anterior: Formed by the sternum and costal cartilages of the ribs.
- Lateral Surfaces: Formed by the ribs.
- Posterior: Consisting of the vertebrae and ribs.
- Upper Boundary: Formed by the progressive narrowing of the costal arches.
- Lower Boundary: Bounded by the diaphragm (a muscular structure, not bone).