Boning Up on Classifications, Structures, and Ossification

By Janet Rae-Dupree, Pat DuPree

Adult bones are composed of 30 percent protein (called ossein), 45 percent minerals (including calcium, phosphorus, and magnesium), and 25 percent water. Minerals give the bone strength and hardness. At birth, bones are soft and pliable because of cartilage in their structure.

As the body grows, older cartilage gradually is replaced by hard bone tissue. Minerals in the bones increase with age, causing the bones to become more brittle and easily fractured.

Various types of bone make up the human skeleton, but fortunately for memorization purposes, bone type names match what the bones look like. They are as follows:

  • Long bones, like those found in the arms and legs, form the weight-bearing part of the skeleton.

  • Short bones, such as those in the wrists (carpals) and ankles (tarsals), have a blocky structure and allow for a greater range of motion.

  • Flat bones, such as the skull, sternum, scapulae, and pelvic bones, shield soft tissues.

  • Irregular bones, such as the mandible (jawbone) and vertebrae, come in a variety of shapes and sizes suited for attachment to muscles, tendons, and ligaments. Irregular bones include seed-shaped bones found in joints such as the patella, or kneecap.

Unfortunately, for students of bone structures, there’s no easy way to memorize them. So brace yourself for a rapid summary of what your textbook probably goes into in much greater detail.

Compact bone is composed of dense layers of lamellae surrounding lacunae, small concavities containing osteocytes, arranged in concentric circles called Haversian systems (structural units also referred to in short as osteons), each of which has a central, microscopic Haversian canal.

A perpendicular system of canals, called Volkmann’s canals, penetrate and cross between the Haversian systems. This network ensures circulation into even the hardest bone structure.

Compact bone tissue is thick in the shaft and tapers to paper thinness at the ends of the bones. The bulbous ends of each long bone, known as the epiphyses (or singularly as an epiphysis), are made up of spongy, or cancellous, bone tissue covered by a thin layer of compact bone.

The diaphysis, or shaft, contains the medullary cavity (medullary derives from Latin for marrow) containing blood cell–producing marrow supplied by a medullary nutrient artery. A membrane called the periosteum covers the outer bone, except for the epiphyseal surfaces, to provide nutrients and oxygen, remove waste, and connect with ligaments and tendons.

Bones grow through the cellular activities of osteoblasts on the surface of the bone, which differentiate into layers of mature bone cells called osteocytes. Osteoclasts are cells that function in the developing fetus to absorb cartilage as ossification occurs and function in adult bone to break down and remove spent bone tissue.

There are two types of ossification, which is the process by which softer tissues harden into bone. Both types rely on a peptide hormone produced by the thyroid gland, calcitonin, which regulates metabolism of calcium, the body’s most abundant mineral. The two types of ossification are

  • Endochondral or intracartilaginous ossification: Occurs when mineral salts, particularly calcium and phosphorus, calcify along the scaffolding of cartilage formed in the developing fetus beginning about the fifth week after conception. This process, known as calcification, takes place in the presence of vitamin D and a hormone from the parathyroid gland.

    The absence of any one of these substances causes a child to have soft bone, called rickets. Next, the blood supply entering the cartilage brings osteoblasts that attach themselves to the cartilage.

    As the primary center of ossification, the diaphysis of the long bone is the first to form spongy bone tissue along the cartilage, followed by the epiphyses, which form the secondary centers of ossification and are separated from the diaphysis by a layer of uncalcified cartilage called the epiphyseal plate, where all growth in bone length occurs. Compact bone tissue covering the bone’s surface is produced by osteoblasts in the inner layer of the periosteum, producing growth in diameter.

  • Intramembranous ossification: Occurs not along cartilage but instead along a template of membrane, as the name implies, primarily in compact flat bones of the skull that don’t have Haversian systems. The skull and mandible (lower jaw) of the fetus are first laid down as a membrane.

    Osteoblasts entering with the blood supply attach to the membrane, ossifying from the center of the bone outward. The edges of the skull’s bones don’t completely ossify to allow for molding of the head during birth. Instead, six soft spots, or fontanels, are formed: one frontal or anterior, two sphenoidal or anterolateral, two mastoidal or posterolateral, and one occipital or posterior.

Once formed, bone is surrounded by the periosteum, which has both a vascular layer (the Latin word for “vessel” is vasculum) and an inner layer that contains the osteoblasts needed for bone growth and repair.

A penetrating matrix of connective tissue, called Sharpey’s fibers, connects the periosteum to the bone. Inside the bone, a thin membrane called the endosteum (from the Greek endon, meaning “within,” and the Greek osteon meaning “bone”) lines the medullary cavity.

Following are the basic terms used to identify bone landmarks or surface features:

  • Process: A broad designation for any prominence or prolongation

  • Spine: An abrupt or pointed projection

  • Trochanter: A large, usually blunt process

  • Tubercle: A smaller, rounded eminence

  • Tuberosity: A large, often rough eminence

  • Crest: A prominent ridge

  • Head: A large, rounded articular end of a bone, often set off from the shaft by a neck (articular is an adjective describing areas related to movement between bones)

  • Condyle: An oval articular prominence of a bone

  • Facet: A smooth, flat, or nearly flat articulating surface

  • Fossa: A deeper depression

  • Sulcus: A groove

  • Foramen: A hole

  • Meatus: A canal or opening to a canal