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The Skeletal System
• Parts of the skeletal system
– Bones
– Joints
– Ligaments
– Cartilage
• Divided into two divisions
– Axial – longitudinal axis
– Appendicular – limbs and girdles
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• Support of the body
• Protection of soft organs
– Skull and vertebrae for brain and spinal cord
– Rib cage for thoracic cavity organs
• Movement due to attached skeletal muscles
• Storage of minerals (Ca+ and P) and fats
• Blood cell formation (hematopoiesis)
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• The adult skeleton has 206 bones
• Two basic types of bone tissue
– Compact Bone – dense, looks smooth and homogenous
- Spongy Bone – needlelike pieces of bone and open space
Spongy bone
Compact bone

• Bones are classified as:
– Long
– Short
– Flat
– Irregular

Classification of Bones on the Basis of Shape
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• Long bones – longer than they are wide
– Usually shaft w/head at ends
– Limb bones except wrist and ankle
– Mostly compound bone
– Example: Humerus
• Short bones – cube shaped
– Mostly spongy bone
– Wrist and ankle
– Sesamoid bones are a type of short bone which form within tendons (patella)
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• Flat bones – Thin, flat and usually curved
– 2 thin layers of flat compound bone sandwiching spongy bone
– Ex. Skull, ribs and sternum
• Irregular bones – miscellaneous
– Vertebrae, hip bones
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• Diaphysis – shaft of bone
– Compact bone
– Covered by connective tissue called periosteum
• Epiphysis – ends of bone
– Compact bone surrounding spongy bone center
Proximal epiphysis
Diaphysis
Distal epiphysis
(a
Articular cartilage
Spongy bone
Epiphyseal line
Periosteum
Compact bone
Medullary cavity (lined by endosteum)

• Periosteum – connective tissue covering diaphysis
• Sharpey’s fibers
(perforating fibers)hold periosteum to the bone
• Arteries – provide nutrients to bone cells and remove wastes

Structures of a Long Bone
• Articular cartilage – covers the epiphyses
– made of hyaline cartilage
– Reduces friction at joints
• Medullary cavity
– (yellow marrow)
– Adults – a storage area
Proximal epiphysis
Diaphysis for adipose
– Infants have red bone marrow here
** Red bone marrow in adults is in spongy bone of flat bones and epiphyses of some long bones (pelvis, femur, etc.)
Distal epiphysis
Articular cartilage
Spongy bone
Epiphyseal line
Periosteum
Compact bone
Medullary cavity (lined by endosteum)

• Epiphyseal plate
– Flat plate of hyaline cartilage seen in young, growing bone
• Epiphyseal line
– Remnant of the epiphyseal plate
– Seen in adult bones
Proximal epiphysis
Diaphysis
Distal epiphysis
(a)
Articular cartilage
Spongy bone
Epiphyseal line
Periosteum
Compact bone
Medullary cavity (lined by endosteum)
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• In embryos, the skeleton is primarily hyaline cartilage
– In a fetus the cartilage is covered over with bone and the cartilage is digested, forming the medullary cavity
• During development, much of this cartilage is replaced by bone ossification
• Cartilage remains in isolated areas
– Bridge of the nose
– Parts of ribs
– Joints
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Long Bone Formation and Growth

• Epiphyseal plates allow for growth of long bone during childhood
– New cartilage is continuously formed
– Older cartilage becomes ossified
• Cartilage is broken down
• Bone replaces cartilage
• Bones are remodeled and lengthened until growth stops
– Bones change shape somewhat
– Bones grow in width
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http://www.youtube.com/watch?v=yFJ4iswRiu
4&safe=active
Bone Remodeling Activity

• Surface features of bones
– Sites of attachments for muscles, tendons, and ligaments
– Passages for nerves and blood vessels
• Categories of bone markings
– Projections or processes—grow out from the bone surface
• Terms often begin with “T”
– Depressions or cavities—indentations
• Terms often begin with “F”

• Osteocyte – mature bone cells found in lacunae
• Osteon (Haversian System)
– A unit of bone
• Central (Haversian) canal
– Opening in the center of an osteon
– Carries blood vessels and nerves
• Perforating (Volkman’s) canal
– Canal perpendicular to the central canal
– Carries blood vessels and nerves
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Microscopic Anatomy of Bone
• Lacunae – cavity containing osteocytes
(bone cells)
• Lamellae – circles of lacunae arranged around off of the central canal to the lacunae, provides each osteocyte with nutrient supplies
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Figure 5.4c

• Osteocytes
– Mature bone cells
• Osteoblasts
– Bone-forming cells
Rickets – disease where bones fail to calcify, legs bow out
-Caused by a lack of vitamin D
-Bones can also atrophy in bed ridden people
• Osteoclasts— giant bone-destroying cells
– Break down bone matrix for remodeling and release of calcium in response to parathyroid hormone
• Bone remodeling is a process by both osteoblasts and osteoclasts
• Bones are remodeled in response to Ca+ levels in the blood and the pull of gravity and muscles
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