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Human Anatomy & Physiology

Chang-Sub UhmDepartment of Anatomy

Korea University College of Medicine

Skeletal System

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Introduction

ComponentsBonesLigamentsTendonsCartilages

Bone Classification by ShapesA. Long bones consist of a shaft with

two ends.Examples: femur, humerus

B. Short bones are cube-like.Examples: carpals, tarsals

C. Flat bones are thin and usually curved.

Examples: most skull bones, sternum, scapulae, ribs

D. Irregular bones are not long, short, or flat.

Examples: vertebrae, auditory ossicles

E. Sesamoid bones develop within a tendon.

Example: patellaF. Wormian bones (or sutural bones)

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Bone Classification by Shapes

A. Long bones B. Short bonesC. Flat bonesD. Irregular bonesE. Sesamoid bonesF. Wormian bones (or

sutural bones) are tiny bones within the skull that lie between major skull bones

Structure of Long Bones Diaphysis = shaft.

consists of a central medullary cavity (filled with yellow marrow) surrounded by a thick collar of compact bone.

Epiphyses (pl) = expanded ends.consist mainly of spongy bone surrounded by a thin layer of compact bone.

Epiphyseal line = remnant of epiphysealdisk/plate.

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

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

"shock absorber".

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Structure of Long Bones Periosteum = outer, fibrous, protective covering of diaphysis.

richly supplied with blood & lymph vessels, nerves (nutrition):

Nutrient Foramen = perforating canal allowing blood vessels to enter and leave bone.

Osteogenic layer contains osteoblasts(bone-forming cells) and osteoclasts(bone-destroying cells); serves as insertion for tendons and ligaments.

Endosteum = inner lining of medullarycavity.

contains layer of osteoblasts & osteoclasts.

Structure of Flat Bones

covered by periosteum-covered compact bone; surrounding endosteum-covered spongy bone. In a flat bone, the arrangement looks like a sandwich:

spongy bone (meat), sandwiched between two layers of compact bone (bread).

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

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General Architecture of the Bone

Chemical Components of the Bone

Organic components(35%)

Cellsosteoprogenitor cellsosteoblastsosteocytesosteoclasts

OsteoidInorganic component (65%)

Osteoprogenitor cellsderived from mesenchymecan undergo mitosis and become osteoblasts

Osteoblastsform bone matrix by secreting collagencannot undergo mitosis.

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Chemical Components of the Bone

osteocytesmature bone cells derived from osteoblastsprinciple bone cellcannot undergo mitosismaintain daily cellular activities (i.e. exchange of nutrients & wastes with blood).

osteoclastsfunctions in bone resorption (i.e. destruction of bone matrix),important in development, growth, maintenance & repair of bone.

Osteon and Osteocytes

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Osteon (Haversian System) and Osteocytes

Structural unit of compact bone (Solid, dense, and smooth)

elongated cylinders cemented together to form the long axis of a boneComponents

Concentric lamellae (layers) Central Haversian canal with blood vessels and nervesOsteocytes in "lacunae“

Lay matrix of collagen and calcium Communicating canals within compact bone

Canaliculi connect the lacunae of osteocytesVolkmann's canals connect the blood & nerve supply of adjacent Haversian systems together run at right angles to and connects adjacent Haversian canals

Osteoclasts

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Chemical Components of the Bone

Organic components (35%)Cells

osteoprogenitor cellsosteoblastsosteocytesosteoclasts

Osteoidprimarily collagen high tensile strengthAlso glycolipids and glycoproteins

Inorganic component (65%)Hydroxyapatite (mineral salts)

calcium phosphate [Ca3(PO4) 2.(OH) 2]gives bone its hardness or rigidity

Structure of Spongy (Cancellous) Bone

Poorly organized trabeculae (small needle-like pieces of bone) A lot of open space between trabeculaeNourished by diffusion from nearby Haversian canals

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Development of BoneThe "skeleton" of an embryo

Composed of fibrous CT membranes (formed from mesenchyme and hyaline cartilage) that are loosely shaped like bonesFunction: supporting structures for ossification to beginOssification: At about 6-7 weeks gestation throughout adulthood.

Two Types of Ossification

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Ossification

Intramembranous OssificationWhen a bone forms on or within a fibrous CT membrane. Flat bones (i.e. skull bones, clavicles)

Endochondral OssificationWhen a bone is formed from a hyaline cartilage model.Most bones of the skeleton Primary Ossification center - fetus and infantSecondary Ossification centers – child, during adolescence and early adulthood

Two Types of Ossification

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4 Zones of the Epiphyseal Plate or Disc

Zone of resting cartilagenear epiphysissmall, scattered chondrocytesanchor plate to epiphysis

Zone of proliferating cartilagelarger chondrocytes that resemble a stack of coinsChondrocytes divide to replace those that die at the diaphyseal surface of the epiphysis.

Zone of Hypertrophic cartilageextremely large chondrocytes that are arranged in columnsmaturing cells

Zone of calcified cartilageonly a few cells thickdead cells because the matrix around them became calcifiedThis calcified matrix is destroyed by osteoclastsand is then invaded by osteoblasts and capillaries from the diaphysis.The osteoblasts lay down bone on the calcified cartilage that persists.As a result, the diaphyseal border of the plate is firmly cemented to the bone of the diaphysis.

Growth at the Epiphyseal Plate

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Bone GrowthGrowth in length at the epiphyseal plates (called longitudinal growth)

Cartilage cells are produced by mitosis on the epiphyseal side of the plateThen destroyed and replaced by bone on the diaphyseal side of the plateThe thickness of the plate remains almost constant, while the bone on the diaphyseal side increases in length

Growth in width: appositional growth

Bone Growth

Growth in lengthGrowth in width: appositional growth

Along with increasing in length, bones increase in thickness or diameter.Occurs in osteogenic layer of periosteumOsteoblasts lay down matrix (compact bone) on outer surfaceAccompanied by osteoclasts destroying the bone matrix at the endosteal surface

Control of bone growth rate

human Growth Hormone (hGH) from the pituitarysex hormones from the gonads

Ossification of most bonesCompleted by age 25

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

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Bone FunctionsSupport

The bones in legs and pelvis support the trunk.The atlas (1st vertebra) supports the skull, etc.

Protection of underlying organs

The skull protects the brainThe rib cage protects the heart and lungs, etc.

Body MovementSkeletal muscles attached to bones by tendons. Serve as levers to move bones

HematopoiesisDefinition = Blood Cell FormationIn the red marrow of certain bones.

Inorganic Salt StorageBone stores many minerals: calcium, phosphorus, others.Also a means of calcium homeostasis

Energy StorageYellow marrow in the shaft of long bones Serve as an important chemical energy reserve

Bone as Levers

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Movement of Skeleton

Bone Remodeling and Repair

Once a bone has been formedContinuously being remodeled throughout life.

Rate of Remodeling Varies:Distal femur is replaced every four months.Diaphysis may not be fully replaced during one's lifetime.

Mechanism involves:The action of osteoblasts and osteoclastsTwo hormones (calcitonin & parathyroid hormone) affects blood calcium homeostasis

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Bone Remodeling and Repair

OsteoclastsLarge multinucleated cells responsible for bone resorptionSecretes lysosomal enzymes that digest the organic matrixSecrete acids that decompose calcium salts into Ca++ and PO4

- ions, which can then enter blood

Hormones Involved in Bone Remodeling and Repair

Parathyroid hormone (PTH)Secreted by the parathyroid glands when blood calcium levels are lowStimulates osteoclast activity (resorption of bone occurs) releases Ca++ into the bloodCauses kidney tubules to reabsorb Ca++ back into the bloodCauses intestinal mucosa to increase dietary absorption of Ca++

Causes an increase in blood calcium levels (back to normal)Calcitonin

Secreted by the thyroid gland when blood calcium levels are highInhibits bone resorption, increases osteoblast activity (i.e. causes a deposition of bone matrix)Causes the kidney tubules to secrete excess Ca++ into the urine Results in a decrease in blood calcium levels (back to normal)

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Regulation of blood calcium level

(calcium homeostasis)

Factors Affecting Bone Development, Growth and Repair

Minerals needed for bone remodeling:Calcium (component of hydroxyapatite matrix)Phosphorus (component of hydroxyapatite)Magnesium (needed for normal osteoblast activity)Boron (inhibits calcium loss)Manganese (needed for new matrix)

Vitamins needed for bone growth, remodeling, repair Vitamin D : greatly increases intestinal absorption of dietary calcium & retards its urine loss.

Deficiency causes rickets in children and osteomalacia in adults.Vitamin C : helps maintain bone matrix (collagen synthesis)

Deficiency causes scurvyVitamin A is required for bone resorption, controls the activity, distribution and coordination of osteoblasts & osteoclasts during developmentVitamin B12 may play a role in osteoblast activity

HormonesExercise

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Factors Affecting Bone Development, Growth and Repair

MineralsVitaminsHormones needed for bone growth & remodeling

Human Growth Hormone (hGH)Secreted by pituitaryResponsible for the general growth of all tissuesStimulates reproduction of cartilage cells at epiphyseal plate

Sex hormonesEstrogens & androgens (testosterone)Aid osteoblast activity (i.e. promote new bone growth)Also degenerate cartilage cells in epiphyseal plate (i.e. close epiphyseal plate)

Estrogen effect is greater than androgen effectThyroid hormones (T3 and T4)

T3 = TriiodothyronineT4 = ThyroxineStimulates replacement of cartilage by bone in epiphyseal plate

PTH & Calcitonin (discussed previously) Exercise increases bone growth