Endocrine System

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Composition of Bones

10.4   They may seem hard and lifeless, but bones are made of living tissue. The outer membrane of the bone, called the periosteum, contains many nerves, as well as blood vessels that transport food and oxygen to the bone’s many cells. Inside the periosteum is the bony layer, which contains more blood vessels, bone cells (osteoblasts), and nerves. At the centre of some bones is the marrow, which makes blood cells and stores fat. Bones appear to be hard and dry, like stone. Yet they are lightweight. Mineral salts like the salts of calcium and phosphorus provide the hardness. Without these salts, bones would be as soft and pliable as rubber. Bone is the most active organ in the body. Bones are continuously  reshaped, remodeled and overhauled.

 Bone Marrow

10.5   Bones, just like tubes, are hollow inside and are filled with spongy, red or yellow, bone marrow. Marrow is made up of tissue and fat, tiny arteries and veins, and cells that make new blood cells including red, white and platelets. The red colour of bone marrow is due to a pigment called haemoglobin that is produced there. The yellow bone marrow is mostly fat.

 Bone Growth

10.6   The long bones of the body, such as the thighbone, femur, continue to grow until a person is about 20 years old. In the end of long bones is a band of cartilage called the epiphysis, or growth plate. Here, cartilage forms that will later be replaced by bone. As a young person matures, this cartilage formation slows down and finally stops. The growth plate becomes completely hardened, and growth in the length of the bone is no longer possible. Heavy exercise makes the bones grow larger, heavier and stronger. Inactivity may lead to the loss of minerals from the bones, which makes them weak and brittle.

10.7   At birth, the bones of an infant’s skull are incompletely developed and are separated by areas of soft tissue called fontanels, or soft spots. Fontanels allow for movement of the skull bones as the baby passes through the birth canal. There are no growth plates in these bones. The skull bones simply grow until they meet, closing up the soft spots. Some fontanels close up as early as two months after birth, but the one at the top of the head does not close up until about one year after birth. There are several factors that can interfere with bone growth. If the epiphysis is injured, growth in the bone may stop. Deficiencies in the diet – especially of the vitamins A, C and D – can interfere with bone growth. Steroid use by young people can cause premature closure of the epiphysis.

 Cartilage

10.8   Cartilage is a tough connective tissue found in various regions of the body. In the skeletal system, cartilage connects the ribs to the sternum, helps form the movable joints and is found between the bones of the pelvis. The tip of the nose and the outer ears are made of cartilage, as are the intervertebral disks. These disks serve as cushions that prevent the vertebrae from rubbing and scraping against each other. Sometimes a disk becomes compressed and presses against a spinal nerve, which can be very painful. As people age, a disk’s outer layer may develop cracks. If the inside of the disk protrudes through a crack, a condition called a herniated disk results. Disk damage can also result from misuse of the spine, such as the repeated improper lifting of heavy objects.