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Bones and Skeletal Tissues
P A R T A
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Skeletal Cartilage
Contains no blood vessels or nerves Surrounded by the perichondrium
(dense irregular connective tissue) that resists outward expansion
Three types – hyaline, elastic, and fibrocartilage
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Hyaline Cartilage Provides support, flexibility, and
resilience Is the most abundant skeletal cartilage Is present in these cartilages:
Articular – covers the ends of long bones
Costal – connects the ribs to the sternum
Respiratory – makes up larynx, reinforces air passages
Nasal – supports the nose
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Elastic Cartilage
Similar to hyaline cartilage, but contains elastic fibers
Found in the external ear and the epiglottis
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Fibrocartilage
Highly compressed with great tensile strength
Contains collagen fibers Found in menisci of the knee and in
intervertebral discs
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Growth of Cartilage
Appositional – cells in the perichondrium secrete matrix against the external face of existing cartilage
Interstitial – lacunae-bound chondrocytes inside the cartilage divide and secrete new matrix, expanding the cartilage from within
Calcification of cartilage occursDuring normal bone growthDuring old age
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Bones and Cartilages of the Human Body
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Classification of Bones
Axial skeleton – bones of the skull, vertebral column, and rib cage
Appendicular skeleton – bones of the upper and lower limbs, shoulder, and hip
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Classification of Bones: By Shape
• Long bones – longer than they are wide (e.g., humerus)
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Classification of Bones: By Shape
• Short bones– Cube-shaped
bones of the wrist and ankle
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Classification of Bones: By Shape
• Flat bones – thin, flattened, and a bit curved (e.g., sternum, and most skull bones)
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Classification of Bones: By Shape
• Irregular bones – bones with complicated shapes (e.g., vertebrae and hip bones)
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Classification of bones by shape
Sesamoid bonesShort bones located in a tendon
Patella
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Function of Bones
Support – form the framework that supports the body and cradles soft organs
Protection – provide a protective case for the brain, spinal cord, and vital organs
Movement – provide levers for muscles
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Function of Bones
Mineral storage – reservoir for minerals, especially calcium and phosphorus
Blood cell formation – hematopoiesis occurs within the marrow cavities of bones
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Bone Markings
Bulges, depressions, and holes that serve as: Sites of attachment for muscles,
ligaments, and tendonsJoint surfacesConduits for blood vessels and
nerves
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Projections – Sites of Muscle and Ligament Attachment
Tubercle – small rounded projection Tuberosity – rounded projection Trochanter – large, blunt, irregular
surface Crest – narrow, prominent ridge of bone Line – narrow ridge of bone
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Epicondyle – raised area above a condyle
Spine – sharp, slender projection Process – any bony prominence
Projections – Sites of Muscle and Ligament Attachment
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Projections That Help to Form Joints
Head – bony expansion carried on a narrow neck
Facet – smooth, nearly flat articular surface
Condyle – rounded articular projection Ramus – armlike bar of bone
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Bone Markings: Depressions and Openings
Meatus – canal-like passageway Sinus – cavity within a bone Fossa – shallow, basin-like
depression Groove – furrow Fissure – narrow, slit-like opening Foramen – round or oval opening
through a bone
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Gross Anatomy of Bones: Bone Textures
Compact bone – dense outer layer Spongy bone – honeycomb of
trabeculae filled with yellow bone marrow
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Structure of Long Bone
DiaphysisTubular shaft that forms the axis
of long bonesComposed of compact bone that
surrounds the medullary cavityYellow bone marrow (fat) is
contained in the medullary cavity
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Structure of Long Bone Epiphyses
Expanded ends of long bones: distal and proximal
Exterior is compact bone, and the interior is spongy bone
Joint surface is covered with articular (hyaline) cartilage
Epiphyseal line separates the diaphysis from the epiphyses (metaphisis)
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Structure of Long Bone
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Structure of Long Bone
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Structure of Long Bone
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Structure of Long Bone
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Bone Membranes Periosteum – double-layered
protective membraneOuter fibrous layer is dense regular
connective tissueInner osteogenic layer is composed
of osteoblasts and osteoclastsRichly supplied with nerve fibers,
blood, and lymphatic vessels, which enter the bone via nutrient foramina
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Bone Membranes
Secured to underlying bone by Sharpey’s fibers
Endosteum – delicate membrane covering internal surfaces of boneIt contains osteoclasts and
osteoblasts
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Structure of Short, Irregular, and Flat Bones
Thin plates of periosteum-covered compact bone on the outside with endosteum-covered spongy bone (diploë) on the inside
Have no diaphysis or epiphyses Contain bone marrow between the
trabeculae
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Structure of a Flat Bone
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Location of Hematopoietic Tissue (Red Marrow)
In infantsFound in the medullary cavity and all
areas of spongy bone In adults
Found in the diploë of flat bones, and the head of the femur and humerus
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Microscopic Structure of Bone: Compact Bone
Haversian system, or osteon – the structural unit of compact boneLamella – weight-bearing, column-like
matrix tubes composed mainly of collagen. Concentric, circumferential, interstitial
Haversian, or central canal – central channel containing blood vessels and nerves
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Microscopic Structure of Bone: Compact Bone
Volkmann’s canals – channels lying at right angles to the central canal, connecting blood and nerve supply of the periosteum to that of the Haversian canal
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Microscopic Structure of Bone: Compact Bone
Osteocytes – mature bone cells Lacunae – small cavities in bone that
contain osteocytes Canaliculi – hairlike canals that connect
lacunae to each other and the central canal
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Microscopic Structure of Bone: Compact Bone
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Microscopic Structure of Bone: Compact Bone
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Microscopic Structure of Bone: Compact Bone
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Microscopic Structure of Bone: Compact Bone
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Chemical Composition of Bone: Organic
Osteoblasts – bone-forming cells Osteocytes – mature bone cells Osteoclasts – large cells that
resorb or break down bone matrix Osteoid – unmineralized bone
matrix composed of proteoglycans, glycoproteins, and collagen
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Chemical Composition of Bone: Inorganic
Hydroxyapatites, or mineral saltsSixty-five percent of bone by
massMainly calcium phosphatesResponsible for bone hardness
and its resistance to compression
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Bone Development
Osteogenesis (ossification) – the process of bone tissue formationThe formation of the bony
skeleton in embryosBone growth until early adulthoodBone thickness, remodeling, and
repair Calcification
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Formation of the Bony Skeleton
Begins at week 8 of embryo development
Intramembranous ossification – bone develops from a fibrous membrane
Endochondral ossification – bone forms by replacing hyaline cartilage
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Intramembranous Ossification
Formation of most of the flat bones of the skull and the clavicles
Fibrous connective tissue membranes are formed by mesenchymal cells
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Stages of Intramembranous Ossification
An ossification center appears in the fibrous connective tissue membrane
Bone matrix is secreted within the fibrous membrane
Woven bone and periosteum form Bone collar of compact bone forms,
and red marrow appears
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Stages of Intramembranous Ossification
Figure 6.7.1
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Stages of Intramembranous Ossification
Figure 6.7.2
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Stages of Intramembranous Ossification
Figure 6.7.3
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Stages of Intramembranous Ossification
Figure 6.7.4
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Bones and Skeletal Tissues
P A R T B
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Endochondral Ossification
Begins in the second month of development
Uses hyaline cartilage “bones” as models for bone construction
Requires breakdown of hyaline cartilage prior to ossification
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Stages of Endochondral Ossification
Figure 6.8
Osteoblasts on the periosteum form abone collararound the hyalinecartilage model.
Hyalinecartilage
As the cartilage in the center of the diaphysis calcifies, the chondrocytes die, and the center disintegratesforming cavities.
Invasion ofinternal cavitiesby the periostealbud and spongybone formation.
Osteoclasts erode the central spongy boneforming the medullary cavity. Also appearance of secondary ossification centers in the epiphyses in preparation for stage 5.
Ossification of theepiphyses; whencompleted, hyalinecartilage remains onlyin the epiphyseal platesand articular cartilages.
Deterioratingcartilagematrix
Epiphysealblood vessel
Spongyboneformation
Epiphysealplatecartilage
Secondaryossificatoncenter
Bloodvessel ofperiostealbud
Medullarycavity
Articularcartilage
Spongybone
Primaryossificationcenter
Bone collar
1
2
34
5
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Postnatal Bone Growth in Length The epiphyseal plate consist of hyaline
cartilage in the middle, with a transitional zone on each side
MetaphysisThe transitional zone facing the
marrow cavity
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Functional Zones in Long Bone Growth Resting zone
Facing the epiphysisZone of inactive cartilage
Proliferation zoneChondrocytes multiplying and
lining up in rowsEpiphysis is pushed away from
the diaphysis
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Functional Zones in Long Bone Growth Hypertrophic zone
Cessation of mitosisEnlargement of chondrocytesEnlargement of lacunae
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Functional Zones in Long Bone Growth Calcification zone
Calcification of the matrixDeath and deterioration of
chondrocytes Osteogenic zone
Bone deposition by osteoblastsFormation of the trabeculae of the
spongy bone
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Growth in Length of Long Bone
Figure 6.9
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Long Bone Growth and Remodeling
Growth in length During growth the epiphyseal
plate maintain a constant thickness
Growth on the epiphysis side is balanced by bone deposition on the diaphysis side
Interstitial growth
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Long Bone Growth and Remodeling
Figure 6.10
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During infancy and childhood, epiphyseal plate activity is stimulated by growth hormone
Hormonal Regulation of Bone Growth During Youth
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Hormonal Regulation of Bone Growth During Youth
During puberty, testosterone and estrogens: Initially promote adolescent
growth spurtsCause masculinization and
feminization of specific parts of the skeleton
Later induce epiphyseal plate closure, ending longitudinal bone growth
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Bone Growth in Width
It is by appositional growth Osteoblasts in the inner layer of the
periosteum deposit osteoid Osteoid calcifies Osteoblasts become osteocytes Circumferential lamellae is formed Osteoclasts of the endosteal layer
resorb bone tissue to keep its proportion
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Bone Remodeling
Also by appositional growth Remodeling units – adjacent
osteoblasts deposit bone at periosteal surfaces and osteoclasts resorb bone at endosteal side
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Bone Remodeling
Deposition Occurs also where bone is injured or
added strength is needed Requires a diet rich in protein,
vitamins C, D, and A, calcium, phosphorus, magnesium, and manganese
Alkaline phosphatase is essential for mineralization of bone
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Bone Remodeling
Sites of new matrix deposition are revealed by the:Osteoid seam – unmineralized
band of bone matrixCalcification front – abrupt
transition zone between the osteoid seam and the older mineralized bone
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Bone Remodeling
Resorption Accomplished by osteoclasts
Lysosomal enzymes digest organic matrix
Hydrochloric acid convert calcium salts into soluble forms
Resorption bays – grooves formed by osteoclasts as they break down bone matrix
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Bone Remodeling
Dissolved matrix is transcytosed across the osteoclast’s cell where it is secreted into the interstitial fluid and then into the blood
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Importance of Ionic Calcium in the Body
Calcium is necessary for:Transmission of nerve impulsesMuscle contractionBlood coagulationSecretion by glands and nerve
cellsCell division
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Control of Remodeling
Two control loops regulate bone remodelingHormonal mechanism maintains
calcium homeostasis in the bloodMechanical and gravitational
forces acting on the skeleton
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Response to Mechanical Stress
Wolff’s law – a bone grows or remodels in response to the forces or demands placed upon it
Observations supporting Wolff’s law includeLong bones are thickest midway
along the shaft (where bending stress is greatest)
Curved bones are thickest where they are most likely to buckle
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Response to Mechanical Stress
Trabeculae form along lines of stress Large, bony projections occur where
heavy, active muscles attach
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Response to Mechanical Stress
Figure 6.12
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Bone Fractures (Breaks)
Bone fractures are classified by:The position of the bone ends
after fractureThe completeness of the breakThe orientation of the bone to the
long axisWhether or not the bones ends
penetrate the skin
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Types of Bone Fractures
Nondisplaced – bone ends retain their normal position
Displaced – bone ends are out of normal alignment
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Types of Bone Fractures
Complete – bone is broken all the way through
Incomplete – bone is not broken all the way through
Linear – the fracture is parallel to the long axis of the bone
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Types of Bone Fractures
Transverse – the fracture is perpendicular to the long axis of the bone
Compound (open) – bone ends penetrate the skin
Simple (closed) – bone ends do not penetrate the skin
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Common Types of Fractures
Comminuted – bone fragments into three or more pieces; common in the elderly
Spiral – ragged break when bone is excessively twisted; common sports injury
Depressed – broken bone portion pressed inward; typical skull fracture
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Common Types of Fractures
Compression – bone is crushed; common in porous bones
Epiphyseal – epiphysis separates from diaphysis along epiphyseal line; occurs where cartilage cells are dying
Greenstick – incomplete fracture where one side of the bone breaks and the other side bends; common in children
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Common Types of Fractures
Table 6.2.1
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Common Types of Fractures
Table 6.2.2
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Common Types of Fractures
Table 6.2.3
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Stages in the Healing of a Bone Fracture
• Hematoma formation– A mass of clotted blood
(hematoma) forms at the fracture site
– Granulation tissue formation • Blood vessels grow• Macrophages,
osteoclasts, etc invade the fracture
Figure 6.13.1
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Stages in the Healing of a Bone Fracture
• Fibrocartilaginous callus formation
• Fibroblasts deposit collagen fibers in the granulation tissue
• Osteogenic cells become chondroblasts and produce fibrocartilage that connect the broken ends Figure 6.13.2
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Stages in the Healing of a Bone Fracture
• Bony callus formation– Other osteogenic
cells differentiate into osteoblasts
– Formation of a bony hard callus of spongy bone around the fracture
– Bone callus begins 3-4 weeks after injury, and continues until firm union is formed 2-3 months later
Figure 6.13.3
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Stages in the Healing of a Bone Fracture
• Bone remodeling– Excess material on the
bone shaft exterior and in the medullary canal is removed
– Compact bone is laid down to reconstruct shaft walls• In a manner similar
to intramembranous ossification
Figure 6.13.4
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Homeostatic Imbalances
OsteomalaciaBones are inadequately
mineralized causing softened, weakened bones
Main symptom is pain when weight is put on the affected bone
Caused by insufficient calcium in the diet, or by vitamin D deficiency
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Homeostatic Imbalances
RicketsBones of children are
inadequately mineralized causing softened, weakened bones
Bowed legs and deformities of the pelvis, skull, and rib cage are common
Caused by insufficient calcium in the diet, or by vitamin D deficiency
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Homeostatic Imbalances
OsteopeniaOsteoblast activity decreases,
osteoclast activity remains the same
Epiphysis, jaw, vertebra
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Homeostatic Imbalances
OsteoporosisOsteopenia beyond expectedSpontaneous fracturesSpongy bone of the spine is most
vulnerableOccurs most often in postmenopausal
women
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Paget’s Disease Characterized by excessive bone
formation and breakdown Pagetic bone, along with reduced
mineralization, causes spotty weakening of bone
Osteoclast activity wanes, but osteoblast activity continues to work
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Fetal Primary Ossification Centers
Figure 6.15
12-week-old fetus
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Developmental Aspects of Bones
Mesoderm gives rise to embryonic mesenchymal cells, which produce membranes and cartilages that form the embryonic skeleton
The embryonic skeleton ossifies in a predictable timetable that allows fetal age to be easily determined from sonograms
At birth, most long bones are well ossified (except for their epiphyses)
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Developmental Aspects of Bones
By age 25, nearly all bones are completely ossified
In old age, bone resorption predominates
A single gene that codes for vitamin D docking determines both the tendency to accumulate bone mass early in life, and the risk for osteoporosis later in life