Bone

A bone is a hard organ of the skeletal system. Its most prominent component is bony tissue, but bones are also partially composed of other tissues, including cartilaginous tissue, connective tissue, and bone marrow (within the medullary cavity).

Types

Bones are usually classified by their location and their shape

Classification of bones by location

• Bones of axial skeleton
  • Cranial bones
  • Bones of vertebral column
  • Bones of thoracic skeleton
• Bones of appendicular skeleton
  • Bones of upper limb
    • Bones of pectoral (or shoulder) girdle
    • Bones of arm and forearm
    • Bones of hand
  • Bones of lower limb
    • Bones of pelvic girdle
    • Bones of thigh and leg
    • Bones of foot

Classification of bones by shape

• Long bones consist of a long shaft with one or two bulky ends (e.g., most of the limb bones: humerus, radius, ulna, femur, tibia, fibula, metacarpals, metatarsals and phalanges). long bones are primarily compact bone but may have a large amount of spongy bone at the ends.
• Short bones are roughly cube-shaped with vertical and horizontal dimensions approximately equal (e.g., carpal and tarsal bones). They consist primarily of spongy bone, which is covered by a thin layer of compact bone.
• Flat bones are thin, flattened, and usually curved (e.g., some cranial bones, sternum, ribs, scapula, and hip bone).
• Irregular bones have a peculiar or complex shape (e.g., mandible, palatine and zygomatic bones, hyoid, and vertebrae). They are primarily spongy bone that is covered with a thin layer of compact bone.
• Pneumatized bones contain sinuses or air cells (e.g., ethmoid, sphenoid, maxilla, and temporal and frontal bones).
• Sesamoid bones are formed in tendons. The patella is the only bone among the set of 206 named bones, but other sesamoid bones may develop within individuals.

Description

All bones have surface markings and characteristics that make a specific bone unique. There are holes, depressions, smooth facets, lines, projections, and other markings. These usually represent passageways for vessels and nerves, points of articulation with other bones, or points of attachment for tendons and ligaments.
Although bones are unique individually, they have a similar histology.

Bony part

The most prominent part of a mature bone is bony tissue. Mature bony tissue is histologically described as lamellar bone because the bone matrix is laid down in orderly layers. (Bone tissue with haphazardly-arranged matrix is called woven bone. Woven bone is present in bone development and repair, and in some diseases, e.g., Paget Disease.)
The architecture of the bony tissue includes compact bone and spongy bone.

• Compact bone is dense, closely-packed bony tissue. Compact bone is present in the parts of a bone that appear thick and solid on cross section, for example, the cortical bone of long bone shafts. Blood vessels are present in canals within compact bony tissue, and the bony tissue forms lamellae around these canals.
• Spongy bone, also called cancellous or trabecular bone, is lighter than compact bone. Spongy bone appears like a latticework to the naked eye, and is commonly found in parts of the medullary cavity of mature bones. The trabeculae of spongy bone may appear to be arranged in a haphazard manner, but they are organized to provide maximum strength similar to braces that are used to support a building. They are organized to follow the lines of stress and can realign if the direction of stress changes.

Cartilaginous part

In mature bones, cartilaginous tissue is found at the sites of some joints. This tissue is articular cartilage, a specialized form of hyaline cartilage.
Some bones develop completely, or partially, from a cartilaginous template. During endochondral ossification, the cartilaginous template is replaced by bony tissue, except at articulation sites. By the time pubertal growth ceases, most of the cartilaginous part has been replaced.

Membranous part

The external surface of a bone is covered by periosteum, except over the articular surfaces. In developing bones that have cartilaginous tissues, the covering over the cartilaginous parts is called perichondrium.
The histology of periosteum and perichondrium is similar. Each has two layers: an inner cellular layer adjacent to the bone, and a outer fibrous layer. The inner layer is called the osteogenic layer in periosteum, and the chondrogenic layer in periochondrium. The fibrous layer is dense irregular connective tissue that is continuous with similar fibrous layers that cover joints.

Medullary cavity

Bones have a central hollow (the medullary cavity) that is filled with bone marrow. Trabecular bone may also occupy part of the medullary cavity. Endosteum, which is histologically similar to the cellular layer of the periosteum, lines the medullary cavity. (In other words, endosteum covers the internal surfaces of bone, including the inner surfaces of compact bone and the surfaces of trabecular bone.
There are two types of bone marrow, which usually cohabit. Red bone marrow is hematopoietic tissue. Yellow bone marrow is adipose tissue. The ratio of red to yellow bone marrow generally decreases with age.

Bone Development

All bones form from mesenchyme (embryonic connective tissue). As a general rule, the axial skeleton forms from paraxial mesoderm (somites and somitomeres), and the appendicular skeleton forms from parts of the somatic lateral plate mesoderm that contribute to development of the limb buds. The most important exception to this rule is that many bones of the ventral head and neck form from neural crest-derived ectomesenchyme.
Bony tissues form either directly or indirectly from mesenchyme. Direct ossification of mesenchyme is called intramembranous ossification. Indirect (or endochondral) ossification involves creation of a cartilaginous template (or model) of the bone, and then replacement of the cartilaginous tissues with bony tissues.
Bones have one or more ossification centers. The first site of bone formation in a bone is referred to as the primary ossification center. Other sites are called secondary ossification centers. Because most bone have more than one ossification center, some bones have parts that were formed by intramembranous ossification, and other parts formed by endochondral ossification.
Ossification begins at the end of the embryonic period. The primary ossification centers of the skull, long bones and vertebrae have formed by 12 weeks post-conception. Ossification continues until the end of puberty.