Structure of Bone

The Anatomic Structure of bone is best studied using a typical long bone. Each long bone consists of the Epiphysis, the diaphysis and the metaphysis.

• Epiphysis: It is the widened or broad area found at the each ends of the long bone and is primarily composed of cancellous (Spongy) bone, where the red marrow fills the spaces within the bone. The wide epiphysis allows for greater weight distribution and provides stability to the joint. The Epiphysis also provides space for attachment of the muscles, the articular cartilage covers the ends of the epiphysis to provide smooth surface for Joint movement.
• Diaphysis: The diaphysis is the main shaft or body of the bone. It is composed of compact bone and provides structural support; the tubular structure of the diaphysis allows to easily withstand more twisting and bending forces.
• Metaphysis: The metaphysis is the flared area between the epiphysis and diaphysis. Like epiphysis, it is also composed of cancellous bone. Each Epiphysis attaches the diaphysis by a narrowed portion called the metaphysis, through a layer of transparent hyaline cartilage that contains the epiphyseal plate, present between the metapysisand epiphysis. It is also known as the growth Zone, as it actively produces bone cells to allow longitudinal growth in children. When the bone stops growing at the early adulthood (around 18-21 years) the cartilage is replaced by osseus tissue and the epiphyseal plate becomes the epiphyseal line

LAYERS OF THE BONE:

Bone consists of two layers called the Periosteum and Endosteum

• Periosteum: The periosteum is composed of fibrous connective tissue that covers the outer surface of the bone except the articular cartilage. Tiny blood vessels penetrate the periosteum that provides Nutrition to the underlying bone. The outer layer of the periosteum is anchored to the musculotendinous fibers and ligaments. The inner layer of the periosteum is attached to the bone by bundles of collagen. The region where the epiphysis is not covered with periosteum, a thin layer of cartilage called the articular cartilage covers the epiphysis which helps to reduce friction and acts as a shock absorber.
• Endosteum (Medullary Cavity): Endosteum is the delicate membranous lining present inside the bone composed of medullary cavity that contains either the red or yellow bone marrow. In the growing child the red bone marrow is actively involved in blood cell production called as hematopoiesis. In adult, the medullary cavity of long bones contains yellow bone marrow, which is mainly composed of Adipose tissue. Yellow bone marrow at times may be involved in hematopoiesis, only when there is greater need for blood cell. Blood cell production in adults, normally occurs in the red bone marrow of the skull, ribs. Sternum, vertebrae, pelvis and shoulder bones

GROSS STRUCTURE OF FLAT BONES:

Flat bones vary extensively in their surface features depending on the principal function and location in the body. These bones are expanded into broad flat plates that provide surfaces for muscle attachment. The flat bones in the body are the bones of the cranium or skull (Occipital, Parietal, frontal, Nasal, Lacrimal &vomer; the coxal or hip bones (Ilium, Ischium & Pubis) and the sternum, ribs and scapula

Flat bones are composed of two thin layers of compact bone (lined on either side) enclosed in between a variable quantity of cancellous bone called as diploe, which is the location of red bone marrow. In cranial bones, the layers of compact tissue are usually termed as tables of the skull, the outer layer is thick and tough and the inner layer is thin and brittle, hence termed as vitreous (glass like tables)

MICROSCOPIC STRUCTURE OF BONE

Bone contains both Organic material (Collagen) and inorganic material (Calcium & phosphate). The internal and external growth and the remodeling of the bone are ongoing process. Based on the structure bones are classified as cortical (Compact and dense) and cancellous (Spongy bone)

• Cortical (compact) bone:

Compact bone is stronger & denser bone tissue that is found interior to the periosteum of the long bones. It is composed of cylindrically shaped structural units that fit closely together in the compact bone, creating a dense bone structure called the Osteon or haversian systems. At thecentre of each osteon, central canals run parallel to the bone’s long axis called as the haversian canals that contains blood vessels, nerves and lymphatic vessels. These vessels and nerves branch off at right angles through a canal, called the volkmann’s canal that travels to the bones interior from the periosteum and extends to the endosteum. The haversian canals are surrounded by concentric rings of calcified matrix called the lamellae, which comprises the mature bone. Smaller canals, called canaliculi extend from the haversian canals to the lacunae, where the mature bones are embedded.

• Cancellous (Spongy) bone:

Cancellous bones contain osteocytes surrounded by lacunae, but they lack the organized structure because the lamellae are not arranged in concentric circles as in cortical or compact bone. Instead the lacunae and osteocytes are found like a network of bone matrix spikes called as trabeculae, which are filled with red or yellow bone marrow. Each trabecula forms along the line of stress to provide strength to the bone. The spaces in the trabecular network provide balance by distributing the weight to the dense and heavy compact bone by making the bone lighter and facilitate easy movement of the muscles.