Chapter 6: Bones and Skeletal Tissue

Attachment Point

Skeletal muscle attaches to bone via tendons

Blood cell formation

Hematopoiesis

• Formation of blood cells in red bone marrow

Hormone Production

Osteocalcin

• Regulates insulin release, glucose homeostasis, and energy expenditure

Cartilage Components

Strength and Resilience

• Can be compressed and return to original shape

High Water Context

• Contributes to flexibility

No Nerve supply and is avascular

Surrounded by fibrous connective tissue called perichondrium

• Contains blood vessels

Hyaline Cartilage

• Most abundant type

• Contains collagen fibers

• Ex:

  • Articular cartilage, costal cartilage, respiratory cartilage, nasal cartilage

Elastic Cartilage

• Contains more elastic fibers

• Ex

  • External ear, epiglottis

Fibrocartilage

• Contain rows of chondrocytes alternating with thick collagen bands

• Most compressible and great tensile strength

  • Holds up majority of our body weight

• Ex:

  • Vertebral discs, knee, pubis symphysis

Appositional Cartilage Growth

• Laying down new cartilage on old cartilage

• Occurs at the surface of cartilage tissue

• Causes cartilage to grow in thickness

Interstitial Cartilage Growth

• Cells divide and secrete matrix within pre existing cartilage

• Occurs deeper in cartilage tissue

• Causes grow in length

Axial Skeleton

• Long axis of body

• Skull, vertebral column, and ribs

Appendicular Skeleton

Limbs (appendages) and the pectoral and pelvic girdles

Long Bones

Longer than they are wide

• Ex: almost all arm and leg bones

Short Bones

Cube Shaped

• Ex: Bones in wrist and ankles (carpals and tarsals)

Sesamoid Bones

• Bone that forms a tendon

• Ex: Patella

Flat Bones

Thin, flat, curved bones

• Ex: Sternum, scapulae, ribs, most cranial bones

Irregular Bones

Anything that does not fit in an above category

• Ex: Vertebrae, os coxa (hip bones)

Spongy vs Compact Layer of bone

Outer Layer

• Compact (lamellar) bone

• Looks smooth and solid

Inner Layer

• Spongy (trabecular) bone

• Has open spaces with needle-like pieces of bone called trabeculae

  • Open space is filled with red or yellow Marrow

  • Trabeculae found in greatest concentration along lines of stress

Diaphysis vs Epiphysis

Diaphysis: Bone shaft

• Composed of compact bone collar with internal medullary cavity

  • Cavity contains bone marrow

• Nutrient Artery and nutrient vein serve diaphysis

  • Large blood cells

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Epiphysis: Bone End

• Composed of compact bone externally and spongy bone internally

• Covered with articular cartilages

• Epiphyseal artery and ephyseal vein serve each epiphyses

Periosteum Fibrous Membrane

Perio = outside or external membrane

Covers external bone surface except at epiphysis

• This is because of the presence of hyaline cartilage

• very well vascularized and innervated

Endosteum Fibrous Membrane

Endo = internal or inside

Covers internal bone surfaces —> trabeculae in spongy bone, cavities in compact bone

Lamellar (Compact) Bone Anatomy: Osteon

Structural unit of compact bone, allows bone to withstand pressure/stress

• A single osteon is composed of several layers (called lamella)

  • Collagen fibers run in one direction for a single lamella (always opposite directions in adjacent lamella)

Lamellar (Compact) Bone Anatomy: Central Canals

Run through center of each osteon

• Contains nerve and blood vessels

Perforating canals extending from central canal connect neighboring osteons and medullary cavity

Lamellar (Compact) Bone Anatomy: Interstitial Lamellae

Incomplete lamellae found in between complete osteons

• They fill gaps between osteons

• Prevents bone from becoming less solid and more susceptible to breaks

Lamellar (Compact) Bone Anatomy: Circumferential Lamellae

Found just deep to periosteum

Extends completely around circumference of diaphysis

• Resists twisting of long bone from multiple layers

• Makes outer bone look hard and solid in appearance

Hematopoietic Tissue: Red Bone Marrow

Produces blood cells

• In adults —> skull, ribs, hips. sternum, clavicles, scapula, vertebrae, heads of femur and humerus

Hematopoietic Tissue: Yellow Bone Marrow

In adults —> medullary cavity of long bones

• Contains more fat and less blood supply than red marrow

• Can be converted back to red marrow if body needs desperate nutrients from yellow marrow

Cellular Composition of Bone: Osteoblasts

Immature bone forming cells

Immature cell that is responsible for laying down bone matrix (bone forming)

• Secretes osteoid that forms bone tissue

• These bone tissue do have calcium phosphate so it is soft tissue —> salt turns it hard

Cellular Composition of Bone: Osteocytes

Mature Bone Cell

• Monitor and maintain bone matrix

Respond to

• Mechanical stress on bone

• Ex: Weightlessness, bone leading

Chemical Signals

• Ex: Calcium levels in bone

• Too much calcium is not good because it breaks more easily

Cellular Composition of Bone: Osteoclasts

Bone degrading cells

• Maintains, repairs, and remodels bones

• Important function in blood calcium homeostasis

Cellular Composition of Bone: Organic vs Inorganic

Organic

• Cells and Osteoid

Inorganic

• Mineral salts —> mostly calcium phosphate packed around collagen fibers

• Collagen fibers become more rigid and more resistant to breaking

Bone Formation: Endochondral Ossification

Formation of ossified bone by replacement of cartilage with bone

• Occurs in most bones below the skull

Hyaline Cartilage used as a blueprint to form ossified bone

Endochondral Ossification: Step 1

Formation of Bone Collar

• Osteoblasts lay down cartilage surface to form a collar

Formation of Primary Ossification Center after bone collar formation

• POC is rigid and tough on outside, cartilage on inside

• Inside is avascular —> cartilage dies

Endochondral Ossification: Step 2

Cavity Forms in Diaphysis Center due to the loss of cartilage inside

Cartilage outside cavity continues to grow —> elongates bone

• The epiphysis can grow to such a large size that it can crush the bone structures, so bone structure keeps epiphysis in place

Endochondral Ossification: Step 3

Formation of initial spongy bone in diaphysis

Periosteal bud invades cavity

• Buds contain nutrient artery/ vein, nerve fibers, red marrow elements, osteoprogenitor cells, osteoclasts

Osteoblasts secrets matrix around calcified cartilage of cavity 

Endochondral Ossification: Step 4

Formation of Medullary Cavity and elongation of diaphysis

• Initial spongy bone is broken down by osteoclasts —> forms medullary cavity

Second ossification center appears inn epiphysis

Endochondral Ossification: Step 5

Secondary Ossification Continues in Epiphysis

• Similar to primary ossification

• Spongy bone is retained in bone ends —> no medullary cavity formed

2 Parts of Skeleton with Cartilage

Bone Growth in Length

Accomplished by interstitial growth which occurs at the Epiphyseal Plate

• New cartilage laid down in epiphyseal plate —> Cartilage cells at center enlarge and cartilage is calcified —> Calcified cartilage is broken down by osteoclasts/blasts and lay down new bone tissue

Bone Growth in Width

Accomplished by appositional growth (stacking bricks)

Occurs at same time as bone lengthening to make sure growth in all directions is balance

• Blasts secrete new matrix at the periosteum

• Clasts break down bone tissue at the endosteum

  • Blasts must exceed clasts to grow

Growth Hormones

Controls activity at the epiphyseal plate

• Released by anterior pituitary in brain

• Binds to cartilage cells, when growth hormone is released epiphyseal cells lay down more new cartilage

Bone Remodeling

Bone deposition and resorption involved

Maintenance of Ca2+ homeostasis

• Ca2+ is essential for excitability of body cells (especially neurons and muscle cells)

  • No Ca2+ = neurons don’t fire + muscle don’t contract

Bone Health

• Mechanical/gravitational forces acting on bone tissue drive remodeling —> strengths bone exactly where it is needed

Control of Deposition and Resorption: Parathyroid Hormone (PTH)

Released in response to decreasing blood Ca2+ levels

Effects of increased PTH release

• Number of clasts at bone increase and become more active in bone tissue

Once blood Ca2+ returns to normal, PTH release decrease

Control of Deposition and Resorption: Mechanical Stress

Wolfs Law

• Bones will remodel to adapt to the mechanical forces that are (or are not) place on them

The more stress is placed on a bone or a certain region of a bone…

• The more trabeculae will be found at the location of that location (more osteons

• The thicket the compact bone will be at the location of loading

Bone Repair

Bone must be reduced for repair to begin

• Take broken edges and make sure they are re-aligned

1. A hematoma forms

   1. Clot/bruise

2. Fibrocartilaginous callus forms

   1. Body forming its own “splint”, holding broken ends of bones together

3. Bony Callus Forms

   1. Osteoblasts/clasts fibrocartilage become ossified

4. Bone remodeling occurs