Skeletal System

I. Introduction A. Framework of bones & cartilage B. Composed of various tissues 1. Bone 2. Cartilage 3. Epithelium 4. Fat 5. Neurons

C. Functions 1. Support 2. Protection 3. Movement 4. Mineral storage 5. Blood cell production 6. Fat storage

II. Bone Tissue A. Introduction 1. A kind of connective tissue 2. Crystals of inorganic calcium salts make it more rigid than other connective tissues 3. Network of collagen fibers provides flexibility & strength

B. Compact or dense bone tissue 1. Contains few spaces 2. Forms a layer over spongy bone 3. Provides protection, support & strength 4. Has a concentric ring structure

C. Spongy or cancellous bone tissue 1. Contains many large spaces 2. Makes up most of the bone tissue of: i. Skull bones and vertebrae ii. Epiphyses or tips of long bones

3. Latticeworks of thin bone plates are called trabeculae 4. In some bones the spaces are filled with red marrow which produces blood cells 5. Spongy bone provides the greatest strength with the least weight

III. Bone Anatomy Parts of a typical long bone: A. Diaphysis B. Epiphyses C. Metaphysis 1. Where the diaphysis joins the epiphysis 2. Contains the epiphyseal plate were bone growth occurs

D. Medullary or marrow cavity 1. Space within the diaphysis 2. Contains fatty yellow marrow in adults E. Endosteum Layer of osteoblasts lining the marrow cavity & scattered osteoclasts F. Articular cartilage Thin layer of cartilage covering the epiphysis at a joint with another bone

G. Periosteum 1. Fibrous covering around the bone (where it is not covered by articular cartilage) 2. Consists of: i. Connective tissue containing blood vessels ii. Lymphatic vessels iii. Nerves iv. Cells that can become osteoblasts

IV. Microscopic Anatomy A. Osteon or Haversian system 1. Structural unit of bone 2. Elongated cylinder parallel to bone’s long axis 3. Function as weight bearing pillars 4. Osteon is a hollow tube of bone i. Like rings on a tree ii. Each ring is called a lamella 5. Collagen fibers in adjacent lamella run in opposite directions

B. Central or Haversian canal 1. Canal that runs through the center of the osteon 2. Contains small blood vessels 3. Nerve fibers

C. Perforating or Volmann's canals 1. Lie at right angles to the central canal 2. Connect nerves & blood vessels in central canals with rest of the body

D. Osteocytes 1. Mature bone cells 2. Found in lacunae at the junctions of the lamellae 3. Canaliculi connect lacunae

E. Circumferential lamellae 1. Located on the outside or surface of the shaft 2. Surround the osteons inside the bone 3. Like bark on a tree

V. Bone formation & growth A. Ossification – bone formation 1. Embryo begins with a 'skeleton' of cartilage & fibrous membranes shaped like bones 2. Ossification begins week 6 - 7 after conception & continues throughout adulthood

4. Once a framework is made, calcification occurs 3. Embryonic connective tissue cells multiply & enlarge & form cartilage & bone 4. Once a framework is made, calcification occurs Calcium is deposited making the bone hard

6. Bones also increase in diameter during growth 5. In long bones: i. Shaft ossifies first ii. Epiphyses ossify secondarily iii. Epiphyseal plate between the shaft & ends remains cartilage Permits growth of the bone shaft iv. When lengthening stops the epiphyseal plate becomes the epiphyseal line 6. Bones also increase in diameter during growth

Allows bones & blood to exchange calcium B. Bones are continuously remodeled 1. Bone tissue is reabsorbed & replaced Allows bones & blood to exchange calcium 2. Controlled by remodeling units i. Osteoblasts ii. Osteoclasts 3. Diet must contain enough: i. Calcium ii. Phosphorus iii. Vitamin D

C. Bone regulation hormones 1. Growth hormone i Pituitary gland in the brain ii. Regulates general body growth

2. Calcitonin i. Thyroid gland in the throat ii. Promotes calcium deposition into bones iii. Osteoblasts 3. Parathyroid hormone i. Parathyroid glands in the throat ii. Promotes removal of calcium from the bones iii. Osteoclasts

4. Serotonin i. Made in the intestine ii. A neurotransmitter iii. Inhibits osteoblast production iv. Shifts balance to the osteoclasts v. Bone degradation

D. Hormonal control of remodeling 1. When blood concentration of Ca2+ is too high i. Rising blood levels of stimulate calcitonin release ii. Calcitonin stimulates the osteoblasts to deposit Ca2+ into bones iii. Ca2+ concentration of blood returns to the homeostatic level

2. When blood concentration of Ca2+ is too low i. Falling Ca2+ levels stimulate parathyroid hormone release ii. Hormone stimulates osteoclasts to degrade the bone iii. Ca 2+ released into the blood iv. Ca2+ levels in blood increase to the homeostatic level

E. Remodeling & mechanical factors 1. Functions to strengthen parts of the skeleton that are under stress 2. How this occurs is still uncertain But may involve locally produced growth factors that stimulate the osteoblasts

3. Aging shifts the balance between organic matter & calcium in bones 4. More inorganic salts & less organic matter 5. Causes less flexible & more brittle bones

VI. What looking at bones can tell you A. Sex 1. Female pelvic opening i. Wide & oval-shaped ii. Wide to permit the passage of babies head

2. Males i. Narrow pelvic opening & heart-shaped ii. Thicker skulls iii. More prominent browridge

B. Age 1. Different bones have specific times that they stop growing 2. Teeth erupt at specific ages 3. Tooth wear

C. Other kinds of information 1. Based on minerals incorporated into bones: i Diet ii. Location person grew up based on the proportions of isotopes in the bones 2. Shapes & sizes of bones i. Size of muscles ii. Activities of a person