Overview of the Skeletal System

Our body is made up of many organ systems formed by organs, tissues and cells. The skeletal system is the main framework that provides proper structure, protection of different vital organs and helps in the movement of our body. The skeletal system includes bones, cartilage, and ligaments. Bone is a rigid organ with dense connective tissue. Cartilage is like the bone, but are more flexible and stiffer than muscle. Ligaments are bands of dense and fibrous connective tissue that are key to the function of joints.

The human skeleton can be divided in two: (Figure1)1

1. Appendicular skeleton: This facilitates the movement of our body. eg. girdles and limbs

2. Axial skeleton: Thisprotect internal organs of our body eg. skull, vertebral column, and thoracic cage.

Bone formation and development2&3

We all know the start of human life begins as an embryo, in the womb of his/her mother.The embryo has only three layers with the size of the tip of a pen.(Figure 2)

1. The top layer is the ectoderm

2. The middle layer is the mesoderm

3. The last layer is the endoderm

The mesoderm isresponsible for the development of nearly all the connective tissues of the musculo- skeletal system. Most of the bony skeleton forms by the process of endochondral ossification, that replaces a developmental cartilage template with bone. The actual ossification or bone formation, starts during the 14th week of development.(Figure 3).

When the child grows, the new center of ossification (secondary) occurs, which helps in development of adult bone and similarly the mature joint supply of nutrients to the bone comes from blood through nutrient arteries and the vessels. The long bone grows in length by a continuous growth at epiphyseal plate (the end part of long bone) and the girth of bone increase by the new bone deposition. The shafts get longer, and bone gradually replaces the cartilage epiphyses. At the end of the growth period, that is in the adult phase, the epiphysis fuses with diaphysis (the central part of bone) and the growth of bone length stops or ceases there. Between 17 and 25 years, normal growth stops. Here, it is important to notice that the bone "remodeling" and age-related bone breakdown takes place throughout life. Remodeling means the ability to alter its size, shape and structure in response to any type of stress.

Joint formation and development3

The bones in the skeletal system are linked by number of joints with the help of the cartilage, muscles and the ligaments.

During fetal development, joint cavity formation (cavitation) occurs along planes of the future articular surfaces of synovial joints(Figure 4). First, the condensation of chondrocytes occurs where jointspecification ismarked byinduction ofinterzone. A number of different markers like

hyaluronan and hyaluronan synthase have been shown to be present in the interzone at the time of cavitation. Then, after the 9th week, on both the ends, the tissue differentiates into the articular cartilage, which are separated by a region of dense connective tissue. This, articular cartilage which is mainly responsible for future arthritis, is composed of cells known as chondrocytes, forming a smooth, flexible surface.The connective tissue of this central region gives rise to the internal elements of the joint. Fibroblast-like cells and adjacent chondrocytes with uridine-diphospho glucose dehydrogenase (UDPGD) activity contribute to glycosaminoglycan levels. The developing cells of bones requires the steady supply of nutrients from the blood stream like calcium, phosphorus, vitamin E, B6, C and A.4 The maturation of the synovial joint of the knee occurs during fetal development and continues in early postnatal (after birth) life.