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The Skeletal System

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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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Functions of Bones

• 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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Bones of the Human Body• 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

Figure 5.2b

Spongybone

Compactbone

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Classification of Bones on the Basis of Shape

Figure 5.1

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

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Classification of Bones on the Basis of Shape

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Classification of Bones• 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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Classification of Bones• 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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Gross Anatomy of a Long Bone

• Diaphysis – shaft of bone– Compact bone– Covered by connective

tissue called periosteum• Epiphysis – ends of

bone– Compact bone

surrounding spongy bone center

Figure 5.2a

Distalepiphysis

Diaphysis

Proximalepiphysis

Articularcartilage

Spongy boneEpiphyseallinePeriosteumCompact boneMedullarycavity (linedby endosteum)

(a

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Structures of a Long Bone• Periosteum – connective

tissue covering diaphysis

• Sharpey’s fibers (perforating fibers)- hold periosteum to the bone

• Arteries – provide nutrients to bone cells and remove wastes

Figure 5.2c

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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

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.) Figure 5.2aFigure 5.2a

Distalepiphysis

Diaphysis

Proximalepiphysis

Articularcartilage

Spongy boneEpiphyseallinePeriosteumCompact boneMedullarycavity (linedby endosteum)

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• Epiphyseal plate– Flat plate of

hyaline cartilage seen in young, growing bone

• Epiphyseal line– Remnant of the

epiphyseal plate– Seen in adult

bones

Anatomy of a Long Bone

Distalepiphysis

Diaphysis

Proximalepiphysis

Articularcartilage

Spongy boneEpiphyseallinePeriosteumCompact boneMedullarycavity (linedby endosteum)

(a)

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Changes in the Human Skeleton

• 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

Figure 5.4a

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Bone Growth (Ossification)• 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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Long Bone Formation and Growth

Figure 5.4bBone Remodeling Activity

http://www.youtube.com/watch?v=yFJ4iswRiu4&safe=active

Bone Markings• 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”

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Microscopic Anatomy of Bone

Figure 5.3

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Microscopic Anatomy of Compact Bone

• 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 a central (Haversian) canal•Canaliculi – tiny canals

off of the central canal to the lacunae, provides each osteocyte with nutrient supplies

Figure 5.4c

Osteon

Lacuna

Centralcanal

Interstitiallamellae

(c)

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Types of Bone Cells

• Osteocytes– Mature bone cells

• Osteoblasts– Bone-forming cells

• 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

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