Anatomy & PhysiologyChapter 7: Skeletal System

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Introduction to Skeletal System

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• Human skeleton is initially cartilage and fibrous membranes

• By age 25 the skeleton is completely hardened

• 206 bones make up the adult skeleton (20% of body mass)• 80 bones of the axial skeleton• 126 bones of the appendicular skeleton

•The organs of the skeletal system include the bones and structures that connect bones to other structures including ligaments, tendons, and cartilages.

Bone Classification

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Bone Classification:

• Long bonesex. femur

• Short bonesex. tarsals

•Flat bonesex. skull

• Irregular bonesex. vertebrae

• Sesamoid bonesex. patella

(a) (e)

(b)

(c)

(d)

Parts of a Long Bone

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• Epiphysis• Distal• Proximal

• Diaphysis• Metaphysis• Compact bone• Spongy bone• Articular cartilage• Periosteum• Endosteum• Medullary cavity• Trabeculae• Bone marrow

• Red marrow and yellow marrowFemur

Periosteum

Yellow marrow

Medullary cavity

Space containingred marrow

Spongy bone

Compact bone

Articular cartilage

Epiphyseal plates

Proximalepiphysis

Distalepiphysis

Diaphysis

Endosteum

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Parts of a Long BoneDiaphysis = shaft

a. consists of central medullary cavity

b. surrounded by a thick collar of compact bone

Epiphyses = expanded ends

a. consist mainly of spongy bone

b. surrounded by a thin layer of compact bone

c. proximal epiphysis vs. distal epiphysis

Epiphyseal line = remnant of epiphyseal disc/plate

a. cartilage at the junction of the diaphysis and epiphyses (growth plate)

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Parts of a Long BonePeriosteum = outer protective covering of diaphysis

a. supplied w/ blood, lymph vessels & nerves (nutrition)

b. osteogenic layer contains osteoblasts (bone-forming cells) and osteoclasts (bone-destroying cells)

c. serves as insertion for tendons and ligaments

Endosteum = inner lining of medullary cavity

a. contains layer of osteoblasts/osteoclasts

Articular cartilage = pad of hyaline cartilage on the epiphyses where long bones articulate or join

a. “shock absorber”

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Parts of a Flat BoneFlat bones

1. covered by periosteum – covered compact bone

2. surrounding endosteum – covered spongy bone

3. In a flat bone the arrangement looks like a sandwich:

a. spongy bone (meat) sandwiched between

b. two layers of compact bone (bread)

Hematopoetic tissue (red marrow) is located in the spongy bone within the epiphyses of long bones and flat bones

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Microscopic Structure:Chemical Composition of Bone

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Organic components (approx. 35%)Cells:osteoprogenitor cells

1. can undergo mitosis and become osteoblastsosteoblasts

1. form bone matrix by secreting collagen2. cannot undergo mitosis

osteocytes1. mature bone cells derived from osteoblasts2. principle bone cell3. cannot undergo mitosis4. maintain daily cellular activities (ie. exchange of nutrients & wastes with blood)

Microscopic Structure:Chemical Composition of Bone

Organic components…cont.

Cells:

Osteoid

1. primarily collagen (90% of bone protein) which gives bone its high tensile strength

2. other bone proteins include osteocalcin, osteonectin, and osteopontin

3. also contains glycolipids and glycoproteins Inorganic components

Hydroxyapatite (mineral salts) which is primarily

a. calcium phosphate [Ca3(PO4)2(OH)2]

b. gives bone its hardness or rigidity9

Microscopic Structure:Compact Bone

Compact bone is solid, dense, and smooth

Structural unit = Haversian system or osteon

a. elongated cylinders cemented together to form the long axis of a bone

b. components of Haversian system

osteocytes (spider shaped bone cells in “lacunaea”

that have laid down a…

matrix of collagen and calcium salts in…

concentric lamellae (layers) around a…

central Haversian canal containing…

blood vessels and nerves.

CONTINUED NEXT SLIDE10

Microscopic Structure: Compact Bonec. Communicating canals with compact bone

-canaliculi connect the lacunae of osteocytes

-Perforating (Volkmann’s) canal connect the

blood & nerve supply of adjacent Haversian

systems together.

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Compact Bone

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• Osteon• Haversian System

• Central canal• Perforating canal

• Volkmann’s canal• Osteocytes• Lamellae• Lacunae• Bone matrix• Canaliculi

Nerve

Osteon

Nerve

Nerve

Canaliculus

Osteocyte

Periosteum

Endosteum

Trabeculae

Pores

Bone matrix

Bloodvessels

Spongybone

Compac

t

bone

Compactbone

Lacuna(space)

Bloodvessels

Perforatingcanal

Central canalcontaining bloodvessels and nerves

Centralcanal

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Microscopic Structure: Spongy Bone

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Consists of poorly organized trabeculae ( small needle-like pieces of bone)

with a lot of open space between them

nourished by diffusion from nearby Haversian canals

Spongy Bone

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• Spongy bone is aka cancellous bone

(a)

(c)

Spongy bone Compact bone

(b)

Spongybone

Compactbone

Remnant ofepiphyseal plate

Spongybone

Compactbone

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a: © Ed Reschke; b,c: Courtesy of John W. Hole, Jr.

Bone Development and Growth

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Introduction

The “skeleton” of an embryo is composed of fibrous CT membranes ( formed from mesenchyme and hyaline cart) that is loosely shaped like bone.

This skeleton provides supporting structures for ossification to begin.

At about 6-7 wks gestation ossification begins and continues throughout adulthood.

Bone Development and Growth

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Ossification follows one of two patterns1. Intermembranous Ossification

When bone forms on or within a fibrous CT membraneex. Flat bones are formed in this manner

2. Endochondral ossificationOccurs when a bone is formed from a hyaline cartilage model.

a. most bones of the skeleton are formed this wayb. Primary ossification center hardens as fetus or infantc. Secondary ossification centers develop in child and harden during adolescence and early adulthood

During infancy and childhood long bones lengthen entirely by growth at the epiphyseal plates (longitudinal growth)

Bones grow thicker by “appositional growth”

Endochondral Ossification

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• Hyaline cartilage model• Primary ossification center• Secondary ossification centers

• Epiphyseal plate• Osteoblasts vs. osteoclasts

(b) (c) (d) (e) (f)(a)

Cartilaginousmodel

Calcifiedcartilage

Articularcartilage

Developingperiosteum

Compact bonedeveloping

Primaryossificationcenter

Medullarycavity

Medullarycavity

Medullarycavity

Secondaryossificationcenter

Secondaryossificationcenter

Bloodvessel

Epiphysealplate

Remnant ofepiphysealplate

Remnants ofepiphysealplates

Epiphysealplates

Compactbone

Spongybone

Articularcartilage

Spongybone

Growth at the Epiphyseal Plate The epiphyseal plate allows for bone lengthening until adulthood.

As a child grows…

a. cartilage cells are produced by mitosis on the epiphyseal side of the plate

b. they are then destroyed and replaced by bone on the diaphyseal side of the plate

*therefore the thickness of the plate remains almost constant while the bone on the diaphyseal side increases in length.

c. the cartilage of the epiphyseal plate is replaced by bone forming the epiphyseal line.

d. ossification of most bones is completed by age 25.

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Bone Thickening: Appositional Growth

Along with increasing in length bones increase in thickness or dia.

1. occurs in osteogenic layer of periosteum

2. Osteoblasts lay down matrix (compact bone) on outer surface

3. This is accompanied by osteoclasts destroying the bone matrix

at the endosteal surface.

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Homeostasis of Bone Tissue Once bones are formed, the actions of osteoclasts and osteoblasts

continually remodel them

Bone remodeling occurs throughout life

a. osteoclasts resorb bone

b. osteoblasts replace the bone

c. these opposing processes are highly regulated so that total mass of bone tissue in adult skeleton normally remains constant

even though 3-5% of bone calcium is exchanged each year.

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Homeostasis of Bone Tissue

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• Bone Resorption – action of osteoclasts via stimulation from parathyroid hormone (PTH) • Bone Deposition – action of osteoblasts and via stimulation from calcitonin

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Developingmedullarycavity

Osteoclast

Bone Function

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Supporta. Bones in legs and pelvis support trunkb. The atlas (1st vertebra) supports the skull etc

Protection of underlying organsa. The skull protects the brainb. The rib cage protects the heart and lungs etc.

Body Movementa. Skeletal muscles attached to bones by tendonsb. Serve as levers to move bones.

HematopoiesisBlood cell formationa. All blood cells formed in the red marrow of certain bones

Divisions of the Skeleton

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• Axial Skeleton• Skull • Spine • Rib cage

• Appendicular Skeleton• Upper limbs• Lower limbs• Shoulder girdle• Pelvic girdle

Hyoid

Cranium

Face

Clavicle

Scapula

Sternum

Ribs

Humerus

Ulna

Hipbone

Radius

Femur

Patella

Tibia

Fibula

Tarsals

Metatarsals

Phalanges

Phalanges

Skull

Vertebralcolumn

Vertebralcolumn

Sacrum

Coccyx

Carpals

Metacarpals

(a) (b)

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7.6: Skull

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• Is composed of the cranium (brain case) and the facial bones

7.7: Vertebral Column

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• The vertebral column, or spinal column, consists of many vertebrae separated by cartilaginous intervertebral discs.