Ossification: Growing Bones
If it weren’t for ossification, you’d be a soft mound of blood, water, and flesh. The process of creating and growing bones is complicated, interesting, and chock-full of vocabulary that you need to know to get through an anatomy or physiology class.
Compact bone is a dense layer made up of structural units, or lacunae, arranged in concentric circles called Haversian systems (or osteons), each of which has a central, microscopic Haversian canal. A perpendicular system of Volkmann’s canals penetrate and cross between the Haversian systems, ensuring circulation into even the hardest bone structure.
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 and blood cell–producing marrow. A membrane called the periosteum covers the outer bone 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 produce layers of mature bone cells called osteocytes. Osteoclasts are cells that function in the developing fetus to absorb cartilage as ossification occurs and in adult bone to break down and remove spent bone tissue.
There are two types of ossification. Both types rely on the thyroid hormone calcitonin, which regulates metabolism of calcium. The two types of ossification are
- Endochondral or intracartilaginous ossification: Occurs when mineral salts 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, resulting in a disorder 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 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 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, two sphenoidal, two mastoidal, and one occipital.