Skeletal System Notes-Part 2

Bone Formation Ossification – The process of bone formation.

Bones at Birth By birth or shortly after, most hyaline cartilage models have been converted to bone except for two regions: 1. The articular cartilages that cover the bone ends. These persist for life, reducing friction at joint surfaces. 2. The epiphyseal plates. Provide for longitudinal growth of the long bones during childhood.

Hyaline Cartilage Remodeling “New” cartilage is formed continuously on the external face of the articular cartilage and on the epiphyseal plate surface that is farther away from the medullary cavity. At the same time, the “old” cartilage abutting the internal face of the articular cartilage and medullary cavity is broken down and replaced by bony matrix.

Widening of Bones Growing bones must also widen as they lengthen.  Osteoblasts in the periosteum add bone tissue to the external face of the diaphysis.  Osteoclasts in the endosteum remove bone form the inner face of the diaphysis wall.  Since these two processes occur at about the same rate, the circumference of the long bone expands and the bone widens.

Widening of Bones Appositional Growth – The process by which bones increase in diameter. The growth of long bones is:  Controlled by hormones (growth and sex hormones).  Ends when the epiphyseal plates are converted to bone (during adolescence). Bone Remodeling is essential if bones are to retain normal proportions and strength during long-bone growth as the body increases in size and weight.

Bone Tissue Bone is a dynamic and active tissue.  Bones are remodeled continually in response to changes in two factors: 1. Calcium levels in the blood. When blood calcium levels drop, osteoclasts break down the bone matrix and release calcium ions into the blood. When blood calcium levels are too high, calcium is deposited in bone matrix as hard calcium salts. 2. The pull of gravity and muscles on the skeleton. Bedridden or physically inactive people tend to lose bone mass and to atrophy because they are not subjected to stress. Bones become thicker and form large projections to increase their strength in areas where bulky muscles are attached.

Rickets Rickets – A disease of children in which the bones fail to calcify.  As a result, the bones soften and a definite bowing of the weight-bearing bones of the legs occur.  Usually due to a lack of calcium or vitamin D in the diet.  Not seen very often in the US.  Foods are fortified with vitamin D and most children drink enough calcium-rich milk.  It remains a problem in other parts of the world.

Bone Fractures Fractures – Breaks of the bone.  Two types of fractures: 1. Closed (Simple) Fracture – A fracture in which the bone breaks cleanly and does not penetrate the skin. 2. Open (Compound) Fracture – A fracture in which the bone ends penetrate through the skin.

Treatment of Fractures A fracture is treated by reduction, which is the realignment of the broken bone ends.  In closed reduction, the bone ends are coaxed back into their normal position by the physician’s hands.  In open reductions, surgery is performed and the bone ends are secured together with pins or wires.

Treatment of Fractures After the broken bone is reduced, it is immobilized by a cast or traction to allow the healing process to begin.  Healing time for a simple fracture is 6- 8 weeks, but is much longer for large bones and for the bones of elderly people.

The repair of bone fractures involves four major events: 1. A hematoma is formed. Blood vessels are ruptured when the bone breaks. As a result, a blood-filled swelling called a hematoma occurs. Bone cells deprived of nutrition die. 2. The break is splinted by a fibrocartilage callus. CT cells of various types form a mass of repair tissue called the fibrocartilage callus. Contains some cartilage matrix, some bony matrix, and collagen fibers. Acts to “splint” the broken bone, closing the gap.

Repair of Bone 3. The bony callus is formed.  More osteoblasts and osteoclasts migrate into the area and multiply.  Fibrocartilage is gradually replaced by a callus made of spongy bone, called the bony callus. 4. Over the next few months, the bony callus is remodeled.  It is remodeled in response to the mechanical stresses placed on it, so that it forms a strong permanent “patch” at the fracture site.

Axial Skeleton The axial skeleton forms the longitudinal axis of the body.  It can be divided into three parts: 1. The skull 2. The vertebral column 3. The bony thorax

The Skull The skull is formed by two sets of bones:  The cranium  Protects the fragile brain tissue.  The facial bones  Hold the eyes in an anterior position and allow the facial muscles to show our feelings through smiles or frowns.

The Skull All but one of the bones of the skull are joined together by sutures.  Sutures – Interlocking, immoveable joints. Only the mandible (jawbone) is attached to the rest of the skull by a freely moveable joint.

Normal Open Sutures in Infant Skull

Cranium The boxlike cranium is composed of 8 large, flat bones. Except for two paired bones (the parietal and temporal), they are all single bones.

The Eight Cranial Bones 1. Frontal Bone – Forms the forehead, the bony projections under the eyebrows, and the superior part of each eye’s orbit. 2. & 3. Parietal Bones – The paired parietal bones form most of the superior and lateral walls of the cranium.

The Eight Cranial Bones 4. & 5. Temporal Bones – Lie inferior to the parietal bones

The Eight Cranial Bones 6. Occipital Bone – Most posterior bone of the cranium.  Forms the floor and back wall of the skull.

The Eight Cranial Bones 7. Sphenoid Bone – Butterfly- shaped; Spans the width of the skull and forms part of the floor of the cranial cavity.  The central part of the sphenoid bone is riddled with air cavities, the sphenoid sinuses.  Part of the sphenoid can be seen exteriorly forming part of the eye orbits and the lateral part of the skull.

The Eight Cranial Bones 8. Ethmoid Bone – Very irregularly shaped; lies anterior to sphenoid.  Forms the roof of the nasal cavity and part of the medial walls of the orbits.