Life Cycle of Bones
As an infant develops in-vitro, cartilage slowly hardens into bone forming the axial and appendicular structures. At birth, we have close to 300 separate bones that continue to solidify into the 206 bones in the adult body.
Growth hormones from the pituitary gland deep inside the brain prompt the long bones of the arms and legs grow and lengthen, reaching peak size and mass at around age 30. Diet and exercise continue to strengthen bone as well as the ligaments, tendons, and muscles that connect to provide movement.
After a hopefully long and healthy span of years, the skeletal system, like every part of the human body, begins to slow and weaken. Mineral loss from the bones decreases their weight and density and could result in fractures or other age-related disorders. A shrinking of the disks between the vertebrae causes the spinal column to shorten, and a loss of cartilage in the knee and hip joints starts to affect strength and mobility.
Types of Bone
The thyroid gland beneath the larynx monitors levels of calcium and phosphorous in the bloodstream. In response, the outermost layer of bone can either store or release these minerals. This type of bone tissue is cortical, or compact bone.
Cancellous, or spongy bone, is inside the knobby ends of the long bones of the arms and legs, and in the center of the vertebra and pelvis.
Bone marrow generates billions of new red blood cells, white blood cells, and platelets each day
Spongy bone is soft and porous, containing the red bone marrow where blood cell production takes place in the process of hematopoiesis. Stem cells that form in the red marrow can grow into either red blood cells, white blood cells, or platelets. Billions of new blood cells are manufactured each day within healthy bone marrow.
Yellow bone marrow consists mostly of fatty tissue that can rapidly be converted to red marrow to produce blood cells, and it’s found in the diaphysis, the straight mid-section of the long bones. As we age, red marrow is converted into yellow until the two types exist in equal parts in the adult body.
At a cellular level, the osteoclast cells are continually breaking down old bone tissue as the osteoblast cells are reabsorbing vital minerals needed to reconstruct healthy new bones.
A single bone cell is an osteon. An osteon has a hollow core called the Haversian canal, a pathway with nerve and blood vessels that supply the outer layer of bone with oxygen and nutrients. The osteons combine collagen, a flexible connective tissue, with the minerals calcium and phosphate to create bone matrix; a material strong enough to endure stress yet pliable enough that it won’t fracture easily.
Shapes of Bones
- Characterized by a mid-length, the diaphysis, with knobby ends containing spongy bone and bone marrow. Arms and legs, as well as fingers and toes are the long bones
- These are essentially layers of hard bone with a mid-layer of spongy bone. Usually thin and concave, as in the bones of the skull
- The knee caps are sesamoid bones located within ligament structures
- Found in the wrists and ankles, short bones are square shaped and fit together in a way that supports both the weight of the body and the flexibility of the hands and feet
- A thin layer of compact bone surrounds an interior of spongy bone. The vertebrae of the spinal column are irregular bones
Bone breaks can occur at every age. We usually associate broken bones with sudden accidents, but injuries to bone tissue can be caused by strain or repetitive stress as well. Weight-bearing activities like running or weight lifting are beneficial for strengthening the skeletal system, but overexertion could damage bone tissue as well as muscles, ligaments, and tendons.
The instant a bone breaks the healing process starts. Blood vessels in the periosteum are severed causing blood at the outer layer of compact bone to seep into the fracture site. As the blood congeals into a hematoma, a soft callous starts to form. During the next three weeks the callous becomes a cartilage “bridge” that slowly hardens into a bone callous, a process that lasts for weeks or months depending on the type of break and the age and health of the patient.
In the case of a serious accident, physicians can surgically reposition bones using pins and rods. Traction might be necessary during the healing process; putting weight on different parts of the affected bone prevents the muscles from pulling the bones out of alignment. And the last phase of recovery from a bone break is physical therapy, to cautiously put pressure on the new bone tissue and strengthen the surrounding muscles that have been relaxing during the healing process.