CHAPTER 6: BONES & SKELETAL TISSUE

Water lends resiliency

Contains no blood vessels or nerves

All contain chondrocytes in lacunae and extracellular matrix

Skeletal Cartilage

What are the three types of skeletal cartilages?

Hyaline, elastic, and fibrocartilage

Provides support, flexibility, and resilience

Collagen fibers only; most abundant type

Everywhere two bones meet

Articular, costal, respiratory, nasal cartilage

Hyaline cartilage

Similar to hyaline cartilage, but contains elastic fibers

External ear and epiglottis

Elastic cartilage

Surrounds

Dense connective tissue girdle

Contains blood vessels for nutrient delivery

Resists outward expansion

Perichondrium

Thick collage-fibers — has great tensile strength

Menisci of knee; vertebral discs, pubic synthesis

Fibrocartilage

Cells secrete matrix against external face of existing cartilage

Appositional growth

Chondrocytes divide and secrete new matrix, expanding cartilage from within

Interstital growth

When cartilage dies

Occurs during normal bone growth (youth and old age)

Hardens, but calcified cartilage is not bone

Calcification of cartilage

How many bones are in the skeleton?

206

Long axis of body

Skull, vertebral column, rib cage

Axial skeleton

Bones of upper and lower limbs

Girdles attaching limbs to axial skeleton

Appendicular skeleton

What are the functions of the bones?

Support, protection, movement

Blood cell formation

hematopoiesis

Regulates bone formation

Protects against obesity, glucose intolerance, diabetes mellitus

Osteocalcin

What are the three levels of bone structure?

Gross anatomy, microscopic, chemical

Dense outer layer; smooth and solid

Compact bone

Honeycomb of flat pieces of bone deep to compact called trabeculae

Spongy bone

Tubular shaft forms long axis

Compact bone surrounding medullary cavity

Diaphysis

Bone ends

External compact bone; internal spongy bone

Articular cartilage covers articular surfaces

Between is epiphyseal line

Epiphyses

Remnant of childhood bone growth at…..

Epiphyseal plate

White, double-layered membrane

Covers external surfaces except joint surfaces

Outer fibrous layer of dense irregular connective tissue

Osteogenic layer abuts bone

Many nerve fibers and blood vessels

Anchoring points for tendons and ligaments

Periosteum

Secure to bone matrix

Sharpey’s fibers

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

Endosteum

Found withing trabecular cavities of spongy bone and diploe of flat bones

In medullary cavities and spongy bone of newborns

Adult long bones have little (heads of femur and humerus only)

Red marrow in diploe and some irregular bones is most active

Yellow marrow can convert to red, if necessary

Red marrow

What are the five major cell types of bone tissue?

osteogenic cells, osteoblasts, osteocytes, osteoclasts, bone lining cells

Also called osteoprogenitor cells

Mitotically active stem cells in periosteum and endosteum

When stimulated differentiate into osteoblasts

Some persist

Osteogenic cells

Bone-forming cells

Secrete unmineralized bone matrix or osteoid

Includes collagen (90% of bone protein) and calcium-binding proteins

Osteoblasts

Mature bone cells in lacunae

Monitor and maintain bone matrix

Act as stress or strain sensors

Osteocytes

Derive from hematopoietic stem cells that become macrophages

Giant, multinucleate cells for bone resorption

When active, display a ruffled border

Osteoclasts

Structural unit of compact bone

Elongated cylinder parallel to long axis of bone

Hollow tubes of bone matrix

Osteon/haversian system

Runs through core of osteon

Contains blood vessels and nerve fibers

Central (haversian) canal

Small cavities that contain osteocytes

Lacunae

Hairlike canals that connect lacunae to each other and central canal

Canaliculi

65% of bone by mass

Mainly tiny calcium phosphate crystals in and around collagen fibers

Responsible for hardness and resistance to compression

Hydroxyapatites

Horizontal lines on bones

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

Growth arrest lines

Process of bone tissue formation

Formation of bony skeleton (begins in 2nd month of development)

Postnatal bone growth (until early adulthood)

Bone remodeling and repair (lifelong)

Ossification

Bone forms by replacing hyaline cartilage

Bones called cartilage bones

Forms most of skeleton

Endochondral ossification

Bone develops from fibrous membrane

Bones called membrane bones

Forms flat bones, clavicles, and cranial bones

Intramembranous ossification

What are the five zones within cartilage (interstitial growth)?

Resting, proliferation, hypertrophic, calcification, ossification

Cartilage on epiphyseal side of epiphyseal plate

Relatively inactive

Resting zone

Cartilage on diaphysis side of epiphysis plate

Rapidly divide pushing epiphysis away from diaphysis → lengthening

Proliferation zone

Older chondrocytes closer to diaphysis and their lacunae enlarge and erode → interconnecting spaces

Hypertrophic zone

Surrounding cartilage matrix calcifies, chondrocytes die and deteriorate

Calcification zone

Chondrocyte deterioration leaves long spicules of calcified cartilage at epiphysis-diaphysis junction

Spicules eroded by osteoclasts

Covered with new bone by osteoblasts

Ultimately replaced with spongy bone

Ossification zone

Bone lengthening ceases (requires presence of cartilage)

Bone of epiphysis and diaphysis fuses

Females - about 18 years

Males - about 21 years

Epiphyseal plate closure

Most important in stimulating epiphyseal plate activity in infancy and childhood

Growth hormone

Modulates activity of growth hormone

Ensures proper proportions

Thyroid hormone

Promote adolescent growth spurts

End growth by inducing epiphyseal plate closure

Testosterone/estrogen

Produced by parathyroid glands

Removes calcium from bone regardless of bone integrity

Parathyroid Hormone (PTH)

Produced by parafollicular cells of thyroid gland

In high doses lowers blood calcium levels temporarily

Calcitonin

What occurs when calcium levels in the blood are too low

Hyperexcitability

What occurs when calcium levels in the blood are too high

Nonresponsiveness

Hormone released by adipose tissue

Role in bone density regulation

Inhibits osteoblasts in animals

Leptin

Neurotransmitters regulating mood and sleep

Most made in gut

Secreted into blood after eating

Serotonin

Bones grow or remodel in response to demands placed on it

Wolff’s Law

Ends of bones retain normal position

Nondisplaced

End of bones out of normal alignment

Displaced

Bones broken all the way through

Complete

Bones not broken all the way through

Incomplete

When the skin is penetrated by bone

Open (compound)

When the skin is not penetrated by bone

Closed (simple)

Bone fragments into three or more pieces

Particularly common in the aged, whose bones are more brittle

Comminuted fracture

Bone is crushed

Common in porous bones (osteoporotic bones) subjected to extreme trauma, as in a fall

Compression Fracture

Ragged break occurs when excessive twisting forces are applied to a bone

Common sports fracture

Spiral Fracture

Epiphysis separates from the diaphysis along the epiphyseal plate

Tends to occur where cartilage cells are dying and calcification of the matrix is occurring

Epiphyseal Fracture

Bones poorly mineralized

Calcium salts not adequate

Soft, weak bones

Pain upon bearing weight

Osteomalacia

Bowed legs and other bone deformities

Bones ends enlarged and abnormally long

Cause: vitamin D deficiency or insufficient dietary calcium

Rickets

Group of diseases

Bone resorption outpaces deposit

Spongy bone of spine and neck of femur most susceptible

Vertebral and hip fractures are common

Osteoporosis

Decrease osteoclast activity and number

Partially reverse in spine

Bisphosphonates

Mimic estrogen without targeting breast and uterus

Selective estrogen receptor modulators

Though for lowering cholesterol, also increases bone mineral density

Statins

Monoclonal antibodies

Reduces fractures in men with prostate cancer

Improves bone density in elderly

Denosumab

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

Causes unknown → possibly viral

Treatments include calcitonin and bisphosphonates

Paget’s Disease