20.1: What is the Skeletal System?
The adult human skeleton comprises 206 bones that are connected through cartilage, tendons, and ligaments. The skeleton provides a rigid framework for the human body, protects internal organs, and enables movement and locomotion. The human skeletal system consists of the axial and appendicular skeletons. Bone tissue is continuously built up and chewed away by specialized bone cells which are essential to overall health. Dysregulated bone cells and incorrect levels of chemical compounds in the blood lead to bone diseases.
The axial skeleton consists of 80 bones and is divided into three regions: the skull, the vertebral column, and the rib cage. The upper portion of the skull—the cranium—consists of eight bones that enclose the brain, while the lower part consists of 14 bones. The vertebral column consists of 33 vertebrae: seven cervical, 12 thoracic, five lumbar, five fused sacral vertebrae, and four fused coccygeal vertebrae.
The rib cage adds stability to the vertebral column and also protects the lungs and heart. It consists of 12 pairs of ribs, which attach to the thoracic vertebra via the costovertebral joint. The anterior portion of the rib cage attaches to the sternum—the flat bone at the center of the front of the chest—via the costal cartilages. The first seven ribs on each side are known as true ribs, as their cartilages attach directly to the sternum. Ribs eight through 12 are called false ribs because they do not directly attach to the sternum. However, ribs eight through 10 join with the sternum via the costal cartilages of the ribs above. Contrarily, ribs 11 and 12 are referred to as floating ribs since they are only attached to the vertebral column in the back but have no connection to the sternum at all.
The appendicular skeletal system consists of the 126 bones of the limbs and girdles as they are appended to the axial skeleton. The appendicular skeletal system is made up of several different types of bones.
Bone Tissue and Cells
Bone is known as osseous tissue. The cells of osseous tissue are dispersed in the matrix—the substance that provides strength and hardness to bones. The matrix consists of organic components—mainly collagen—and inorganic components consisting of crystallized mineral salts such as calcium phosphate, calcium hydroxide, and magnesium fluoride.
There are three major types of bone cells—osteoblasts, osteoclasts, and osteocytes. The osteoblasts and osteoclasts have opposite functions. Osteoblasts build up the bone matrix, whereas osteoclasts chew it up. Both functions continue throughout life, and both are essential for good health. Building up bone is important to keep bones strong; chewing up bone allows maintenance of calcium levels in the bloodstream (which is vital for the health of other organs, such as the heart).
Osteocytes are cells which have matured from osteoblasts and are now surrounded by matrix. Osteocytes communicate with the blood through microscopic channels in the bone, sensing the levels of calcium and other minerals in the blood. They then control the functions of the osteoblasts and osteoclasts by secreting substances that affect the activity of these cells.
Many diseases of the skeletal system have a common feature: weak bones due to poor mineral content. Osteoporosis, for example, is characterized by decreased bone mineral density. It occurs most commonly in postmenopausal women but can occur in men and premenopausal women as well. In this disease, there is more osteoclast than osteoblast activity. Patients with osteoporosis have greatly increased risk of fractures, especially of the spinal column, hip, and wrist.
Another widespread bone disease is renal osteodystrophy. This disease is a part of a larger condition known as mineral and bone disorder in chronic kidney disease. The kidneys perform many functions, including regulation of calcium, phosphorus, and vitamin D, which are all critical to bone health. When the kidneys are not functioning correctly, such as in diabetic kidney disease, the bones may become weakened and painful, and the joints may become painful as well.