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Chapter 6- Skeleton System. I. Structure of bones. A. Functions 1. The skeleton has many functions, but the most obvious is supporting the weight of the body. .
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I. Structure of bones A. Functions 1. The skeleton has many functions, but the most obvious is supporting the weight of the body.
2. The skeletal system includes the bones of the skeleton and the cartilages, ligaments, and other connective tissues that stabilize or connect them. This system has five primary functions:
a.Support- Provides structural support for the entire body. Individual bones or groups of bones provide a framework for attachment of soft tissue organs. b.Storage-The calcium salts of bones are very important to bones. Also, bones store lipids in areas of yellow marrow as energy reserve.
c. Blood cell production- RBC, WBC and other blood elements are produced within the RED marrow. d. Protection- Delicate tissues and organs are surrounded by skeletal elements for protection.
e. Leverage- Bones of the skeleton function as levers that change the magnitude and direction of the forces generated by skeletal muscles.
B. Organization of the Skeleton System 1.Osteology is the study of bones. a.Each bone is an organ that plays a part in the total functioning of the skeleton system. b.An adult human has approximately 206 bones.
c. Number of bones differs from person to person depending on age and genetic variations. d. At birth, a baby has about 270 bones. As the bones develop (ossification – hardening of the bones) the number decreases. e. During adolescence, the number decreases, due to gradual fusion of separate bones.
f. Some adults have extra bones within the sutures (joints) of the skull called suture. g. Additional bones may develop in tendons in response to stress as the tendons repeatedly move across a joint.
C. Macroscopic features of bone 1.The bones of the human skeleton have four general shapes, long, short, flat and irregular. (See page 128) 2.Long bones are longer than they are wide whereas short bones are of roughly equal dimensions. 3. Long bones examples: bones in limbs. (Femur or humerus)
4. Short bones include the bones of the wrist (carpal) and ankles (tarsal) 5. Flat bones are thin and relatively broad, such as the parietal bones in the skull, the ribs and the scapulae. 6. Irregular bones have complex shapes that do not fit easily into any other category. Example- vertebrae of the spinal column.
7. A long bone has a central shaft, or diaphysis and an expanded portion at each end or epiphysis. 8. The diaphysis surrounds a central marrow cavity. 9. The ends or epiphyses are covered by cartilage.
10. The two types of bone tissue are compact/dense bone or spongy bone/cancellous. 11. The outer surface of a bone is covered by a periosteum, which consists of a fibrous outer layer and a cellular inner layer.
12. Inside the bone, a cellular endosteum lines the marrow cavity and other surfaces. 13. See figure 6.2 on page 129.
D. The Axial skeleton consists of the bones that form the axis of the body and that support and protect the organs of the head, neck and torso.
1. Skull- has two main sets of the bones: the cranial bones that form the cranium and the facial bones that support the eyes, nose, and jaws. There are 22 bones total in the skull.
2. Auditory ossicles are present in the middle ear chamber of each ear and serve to transmit sound. There are 6 bones total.
3. The hyoid bone is located above the larynx and below the lower jaw, supports the tongue and assists in swallowing. One bone total.
4. Vertebral column (backbone) consists of 26 individual vertebrae separated by cartilaginous intervertebral discs. In the pelvic region several vertebrae are fused to form the sacrum.
5. The rib cage or thoracic cage includes the 12 pairs of ribs, the flattened sternum and the costal cartilage that connects the ribs to the sternum.
E. The appendicular skeleton is composed of bones of the upper and lower extremities and the bony girdles, which anchor the appendages to the axial skeleton.
1. Pectoral girdle is made up of the paired scapulae and the clavicles. The primary function of the pectoral girdle is to provide attachment for the muscles that move the brachium and forearm. Pectoral girdle has 4 bones.
2. Upper extremity- Each upper extremity consists of a proximal humerus, an ulna and radius, carpal bones of the wrist and the metacarpal and phalangeal bones of the hand. Upper extremity has 60 bones.
3. Pelvic girdles are formed by two ossa coxae (hipbones) untied by the symphysis pubis. The pelvic girdle has 2 bones.
4. The lower extremity consists of a femur, tibia, fibula, tarsals bones of the ankle, and the metatarsal and phalangeal bones of the foot. The patella is also included. The lower extremity has 60 bones.
F. Cells in bone 1.Osteocytes, mature bone cells. Osteocytes maintain normal bone structure by recycling the calcium salts in bony matrix around themselves. 2.Osteoclasts, acids and enzymes secreted by osteoclasts dissolve the bony matrix and release the stored minerals through osteolysis or resorption.
3. Osteoblasts- the cell responsible for the production of new bone, a process called ostogenesis
II. Bone development A. Skeletal growth begins about 6 weeks after fertilization, when the embryo is about 0.5 in long. 1. At this time, all skeletal elements are made of cartilage
2. Bone growth continues through adolescence, and portions of the skeleton usually do not stop growing until around age 25. 3. Osteogenesis is bone formation and growth. 4. During development, cartilage or connective tissue is replaced by bone.
5. Replacing tissue with bone is called ossification. 6. There are two major forms of ossification. a.Intramembraneous ossification bone develops within sheets. b.Endochondral ossification bone is replaced with cartilage.
B. Intramembranous ossification 1. Intramembranous ossification begins when osteoblasts start to differentiate into stem cells where they become calcified.
2. The place where ossification first occurs is called an ossification center. 3. An ossification proceeds and new bone branches.
C. Endochondral ossification 1.Most of the bones of the skeleton are formed through the endochondral ossification of existing hyaline cartilage. 2.The cartilages develop first; they are like miniature cartilage models of the future bone.
3. By the time an embryo is 6 weeks old, the cartilage models of the future limb bones begin to be replaced by true bone. a. Step 1- Endochondral ossification starts as chondrocytes within the cartilage model enlarge and the surrounding matrix begins to calcify.
b. Step 2- Bone formation first occurs at the shaft surface. Blood vessels invade the area and osteoblasts begin producing bone matrix. c. Step 3- Blood vessels invade the inner region of the cartilage and newly osteoblasts from spongy bone within the center of the shaft at the primary center of ossification
d. Step 4- As the bone enlarges, osteoclasts break down some of the spongy bone and create a marrow cavity. e. Step 5- The centers of the epiphyses begin to calcify.
4. When sex hormone production increases at puberty, bone growth accelerates dramatically, and osteoblasts begin to produce bone faster than epiphyseal cartilage expands. = growing pains.
D. Bone growth and body portions 1.The timing of epiphyseal closure varies from bone to bone and individual to individual. 2.Toes may complete ossification by age 11. 3.Some of the pelvis or the wrist may continue to enlarge up to 25 years old!
4. The epiphyseal plates in the arms and legs usually close by age 18 in women and 20 in men. 5. Differences in sex hormones account for variations in body size and proportions between men and women.
E. Requirements for normal bone growth 1.Normal bone growth and maintenance cannot occur without a reliable source of mineral, especially calcium. 2.During prenatal development these minerals are absorbed from the mother’s bloodstream.
3. The demands are so great that the maternal skeleton often loses bone mass during pregnancy. 4. From infancy to adulthood, the diet must provide adequate amounts of calcium and phosphate to be able to absorb and transport these minerals to sites of bone formation.
5. Vitamin D plays an important role in normal calcium metabolism. 6. The vitamin is converted in the liver into calcitriol, a hormone that stimulates the absorption of calcium and phosphate ions in the digestive track.
7. Rickets is a condition marked by a softening and bending of bones that occurs in growing children as a result of vitamin D deficiency.
8. Vitamin A and Vitamin C are also important. 9. A deficiency of vitamin C will cause scurvy. This condition causes a reduction in osteoblast activity that leads to weak and brittle bones.
10. Hormones can also play an important role in normal skeletal growth and development – such as growth hormones, thyroid hormones, and sex hormones.
III. Remodeling and Homeostatic Mechanisms A.Support and storage depends on the bones. 1.The turn over rate for bone is quite high.
2. In adults, roughly 18% of the portion and mineral components are removed and replaced each year through the process of remodeling. 3. Each part of every bone may not be affected, but new bone is always being formed.
