Chapter 6 Bones and Skeletal Tissues



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Chapter 6

Bones and Skeletal Tissues




Cartilage: Basic Structure, Types and Locations

Skeletal Cartilages

  • All contain chondrocytes in lacunae and extracellular matrix

  • Three types

  • Hyaline cartilage

  • Provides support, flexibility, and resilience

  • Collagen fibers only; most abundant type

  • Articular, costal, respiratory, nasal cartilage

  • Elastic cartilage

  • Similar to hyaline cartilage, but contains elastic fibers

  • External ear and epiglottis

  • Fibrocartilage

  • Thick collagen fibers—has great tensile strength

  • Menisci of knee; vertebral discs

Growth of Cartilage

  • Appositional growth

  • Cells secrete matrix against external face of existing cartilage

  • Interstitial growth

  • Chondrocytes divide and secrete new matrix, expanding cartilage from within

  • Calcification of cartilage

  • Occurs during normal bone growth

  • Youth and old age

  • Hardens, but cacified cartilage is not bone

Classification of Bones

  • 206 named bones in skeleton

  • Divided into two groups

  • Axial skeleton

  • Long axis of body

  • Skull, vertebral column, rib cage

  • Appendicular skeleton

Classification of Bones by Shape

  • Long bones

  • Short bones

  • Flat bones

  • Irregular bones

Classification of Bones by Shape

  • Long bones

  • Longer than they are wide

  • Limb, wrist, ankle bones

  • Short bones

  • Cube-shaped bones (in wrist and ankle)

  • Sesamoid bones (within tendons, e.g., Patella)

  • Vary in size and number in different individuals

  • Flat bones

  • Thin, flat, slightly curved

  • Sternum, scapulae, ribs, most skull bones

  • Irregular bones

  • Complicated shapes

  • Vertebrae, coxal bones

Functions of Bones

  • Seven important functions

  • Support

  • Protection

  • Movement

  • Mineral and growth factor storage

  • Blood cell formation

  • Triglyceride (fat) storage

  • Hormone production

Functions of Bones

  • Support

  • Protection

  • For brain, spinal cord, and vital organs

  • Movement

  • Levers for muscle action

Functions of Bones

  • Mineral and growth factor storage

  • Calcium and phosphorus, and growth factors reservoir

  • Blood cell formation (hematopoiesis) in red marrow cavities of certain bones

  • Triglyceride (fat) storage in bone cavities

  • Energy source

  • Hormone production

  • Osteocalcin

  • Regulates bone formation

  • Protects against obesity, glucose intolerance, diabetes mellitus

Bones

  • Are organs

  • Contain different types of tissues

  • Bone (osseous) tissue, nervous tissue, cartilage, fibrous connective tissue, muscle and epithelial cells in its blood vessels

  • Three levels of structure

  • Gross anatomy

  • Microscopic

  • Chemical

Gross Anatomy

  • Bone textures

  • Compact and spongy bone

  • Compact

  • Dense outer layer; smooth and solid

  • Spongy (cancellous or trabecular)

  • Honeycomb of flat pieces of bone deep to compact called trabeculae

Structure of Short, Irregular, and Flat Bones

  • Thin plates of spongy bone covered by compact bone

  • Plates sandwiched between connective tissue membranes

  • Periosteum (outer layer) and endosteum

  • No shaft or epiphyses

  • Bone marrow throughout spongy bone; no marrow cavity

  • Hyaline cartilage covers articular surfaces

Structure of Typical Long Bone

  • Diaphysis

  • Tubular shaft forms long axis

  • Compact bone surrounding medullary cavity

  • Epiphyses

  • Bone ends

  • External compact bone; internal spongy bone

  • Articular cartilage covers articular surfaces

  • Between is epiphyseal line

  • Remnant of childhood bone growth at epiphyseal plate

Membranes: Periosteum

  • White, double-layered membrane

  • Covers external surfaces except joint surfaces

  • Outer fibrous layer of dense irregular connective tissue

  • Sharpey's fibers secure to bone matrix

  • Osteogenic layer abuts bone

  • Contains primitive stem cells – osteogenic cells

  • Many nerve fibers and blood vessels

  • Anchoring points for tendons and ligaments

Membranes: Endosteum

  • Delicate connective tissue membrane covering internal bone surface

  • Covers trabeculae of spongy bone

  • Lines canals that pass through compact bone

  • Contains osteogenic cells that can differentiate into other bone cells

Hematopoietic Tissue in Bones

  • Red marrow

  • Found within trabecular cavities of spongy bone and diploë of flat bones (e.g., sternum)

  • In medullary cavities and spongy bone of newborns

  • Adult long bones have little red marrow

  • Heads of femur and humerus only

  • Red marrow in diploë and some irregular bones is most active

  • Yellow marrow can convert to red, if necessary

Bone Markings

  • Sites of muscle, ligament, and tendon attachment on external surfaces

  • Joint surfaces

  • Conduits for blood vessels and nerves

  • Projections

  • Depressions

  • Openings

Bone Markings

  • Projections

  • Most indicate stresses created by muscle pull or joint modifications

  • Depressions and openings

  • Usually allow nerves and blood vessels to pass

Microscopic Anatomy of Bone: Cells of Bone Tissue

  • Five major cell types

  • Each specialized form of same basic cell type

  • Osteogenic cells

  • Osteoblasts

  • Osteocytes

  • Bone lining cells

  • Osteoclasts

Osteogenic Cells

  • Also called osteoprogenitor cells

  • Mitotically active stem cells in periosteum and endosteum

  • When stimulated differentiate into osteoblasts or bone lining cells

  • Some persist as osteogenic cells

Osteoblasts

  • Bone-forming cells

  • Secrete unmineralized bone matrix or osteoid

  • Includes collagen and calcium-binding proteins

  • Collagen = 90% of bone protein

  • Actively mitotic

Osteocytes

  • Mature bone cells in lacunae

  • Monitor and maintain bone matrix

  • Act as stress or strain sensors

Bone Lining Cells

  • Flat cells on bone surfaces believed to help maintain matrix

  • On external bone surface called periosteal cells

  • Lining internal surfaces called endosteal cells

Osteoclasts

  • Derived from hematopoietic stem cells that become macrophages

  • Giant, multinucleate cells for bone resorption

  • When active rest in resorption bay and have ruffled border

  • Ruffled border increases surface area for enzyme degradation of bone and seals off area from surrounding matrix

Microscopic Anatomy of Bone:
Compact Bone

  • Also called lamellar bone

  • Osteon or Haversian system

  • Structural unit of compact bone

  • Elongated cylinder parallel to long axis of bone

  • Hollow tubes of bone matrix called lamellae

  • Collagen fibers in adjacent rings run in different directions

  • Withstands stress – resist twisting

Microscopic Anatomy of Bone: Compact Bone

  • Central (Haversian) canal runs through core of osteon

  • Contains blood vessels and nerve fibers

  • Perforating (Volkmann's) canals

  • Canals lined with endosteum at right angles to central canal

  • Connect blood vessels and nerves of periosteum, medullary cavity, and central canal

  • Lacunae—small cavities that contain osteocytes

  • Canaliculi—hairlike canals that connect lacunae to each other and central canal

Canaliculi Formation

  • Osteoblasts secreting bone matrix maintain contact with each other and osteocytes via cell projections with gap junctions

  • When matrix hardens and cells are trapped the canaliculi form

  • Allow communication

  • Permit nutrients and wastes to be relayed from one osteocyte to another throughout osteon

Lamellae

  • Interstitial lamellae

  • Circumferential lamellae

  • Just deep to periosteum

  • Superficial to endosteum

  • Extend around entire surface of diaphysis

  • Resist twisting of long bone

Microscopic Anatomy of Bone:
Spongy Bone

  • Appears poorly organized

  • Trabeculae

  • Align along lines of stress to help resist it

  • No osteons

  • Contain irregularly arranged lamellae and osteocytes interconnected by canaliculi

  • Capillaries in endosteum supply nutrients

Chemical Composition of Bone: Organic Components

  • Includes cells and osteoid

  • Osteogenic cells, osteoblasts, osteocytes, bone- lining cells, and osteoclasts

  • Osteoid—1/3 of organic bone matrix secreted by osteoblasts

  • Made of ground substance (proteoglycans and glycoproteins)

  • Collagen fibers

  • Contributes to structure; provides tensile strength and flexibility

  • Resilience of bone due to sacrificial bonds in or between collagen molecules

  • Stretch and break easily on impact to dissipate energy and prevent fracture

  • If no addition trauma, bonds re-form

Chemical Composition of Bone: Inorganic Components

  • Hydroxyapatites (mineral salts)

  • 65% of bone by mass

  • Mainly of tiny calcium phosphate crystals in and around collagen fibers

  • Responsible for hardness and resistance to compression

Bone

  • Half as strong as steel in resisting compression

  • As strong as steel in resisting tension

  • Last long after death because of mineral composition

  • Reveal information about ancient people

  • Can display growth arrest lines

  • Horizontal lines on bones

  • Proof of illness - when bones stop growing so nutrients can help fight disease



Bone Development

Two Types of Ossification

  • Endochondral ossification

  • Bone forms by replacing hyaline cartilage

  • Bones called cartilage (endochondral) bones

  • Forms most of skeleton

  • Intramembranous ossification

  • Bone develops from fibrous membrane

  • Bones called membrane bones

  • Forms flat bones, e.g. clavicles and cranial bones


Appositional Growth: Growth in Width

  • Allows lengthening bone to widen

  • Occurs throughout life

  • Osteoblasts beneath periosteum secrete bone matrix on external bone

  • Osteoclasts remove bone on endosteal surface

  • Usually more building up than breaking down


Hormonal Regulation of Bone Growth

  • Growth hormone

  • Most important in stimulating epiphyseal plate activity in infancy and childhood

  • Thyroid hormone

  • Modulates activity of growth hormone

  • Ensures proper proportions

  • Testosterone (males) and estrogens (females) at puberty

  • Promote adolescent growth spurts

  • End growth by inducing epiphyseal plate closure

  • Excesses or deficits of any cause abnormal skeletal growth


Bone Deposit

  • Evidence of new matrix deposit by osteoblasts

Bone Resorption

  • Is function of osteoclasts

Control of Remodeling

  • Occurs continuously but regulated by genetic factors and two control loops

  • Negative feedback hormonal loop for Ca2+ homeostasis

  • Responses to mechanical and gravitational forces


Importance of Calcium

  • Functions in

  • Nerve impulse transmission

  • Muscle contraction

  • Blood coagulation

  • Secretion by glands and nerve cells

  • Cell division

  • 1200 – 1400 grams of calcium in body

  • 99% as bone minerals

  • Amount in blood tightly regulated (9-11 mg/dl)

  • Intestinal absorption requires Vitamin D metabolites

Dietary intake required
Hormonal Control of Blood Ca2+

  • Parathyroid hormone (PTH)

  • Produced by parathyroid glands

  • Removes calcium from bone regardless of bone integrity

  • Calcitonin may be involved


Fracture Classification

Refer to Table6.2


Homeostatic Imbalances

  • Osteomalacia

  • Bones poorly mineralized

  • Calcium salts not adequate

  • Soft, weak bones

  • Pain upon bearing weight

  • Rickets (osteomalacia of children)

  • Bowed legs and other bone deformities

  • Bones ends enlarged and abnormally long

  • Cause: Vitamin D deficiency or insufficient dietary calcium

  • Osteoporosis

  • Group of diseases

  • Bone resorption outpaces deposit

  • Spongy bone of spine and neck of femur most susceptible

  • Vertebral and hip fractures common


Risk Factors for Osteoporosis

  • Risk factors

  • 30% 60 – 70 years of age; 70% by age 80

  • 30% Caucasian women will fracture bone because of it

  • Men to lesser degree

  • Sex hormones maintain normal bone health and density

  • As secretion wanes with age osteoporosis can develop


Additional Risk Factors for Osteoporosis

  • Petite body form

  • Insufficient exercise to stress bones

  • Diet poor in calcium and protein

  • Smoking

  • Hormone-related conditions

  • Hyperthyroidism

  • Low blood levels of thyroid-stimulating hormone

  • Diabetes mellitus

  • Immobility

  • Males with prostate cancer taking androgen-suppressing drugs


Treating Osteoporosis

  • Traditional treatments

  • Calcium

  • Vitamin D supplements

  • Weight-bearing exercise

  • Hormone replacement therapy

  • Slows bone loss but does not reverse it

  • Controversial due to increased risk of heart attack, stroke, and breast cancer

  • Some take estrogenic compounds in soy as substitute


New Drugs for Osteoporosis Treatment

  • Bisphosphonates

  • Decrease osteoclast activity and number

  • Partially reverse in spine

  • Selective estrogen receptor modulators

  • Mimic estrogen without targeting breast and uterus

  • Statins

  • Though for lowering cholesterol also increase bone mineral density

  • Denosumab

  • Monoclonal antibody

  • Reduces fractures in men with prostate cancer

  • Improves bone density in elderly


Preventing Osteoporosis

  • Leaches minerals from bone so decreases bone density

  • Plenty of weight-bearing exercise

  • Increases bone mass above normal for buffer against age-related bone loss


Paget's Disease

  • Excessive and haphazard bone deposit and resorption

  • Bone made fast and poorly – called Pagetic bone

  • Very high ratio of spongy to compact bone and reduced mineralization

  • Usually in spine, pelvis, femur, and skull

  • Rarely occurs before age 40

  • Cause unknown - possibly viral

  • Treatment includes calcitonin and biphosphonates

Developmental Aspects of Bones

  • Embryonic skeleton ossifies predictably so fetal age easily determined from X rays or sonograms

  • Most long bones begin ossifying by 8 weeks

  • Primary ossification centers by 12 weeks

  • At birth, most long bones well ossified (except epiphyses)

  • At age 25 ~ all bones completely ossified and skeletal growth ceases


Age-related Changes in Bone

  • Children and adolescents

  • Bone formation exceeds resorption

  • Young adults

  • Bone density changes over lifetime largely determined by genetics

  • Gene for Vitamin D's cellular docking determines mass early in life and osteoporosis risk as age

  • Bone mass, mineralization, and healing ability decrease with age beginning in 4th decade

  • Except bones of skull

  • Bone loss greater in whites and in females

  • Electrical stimulation; Daily ultrasound treatments hasten repair




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