Bones and Bone structure

What does the skeletal system have?

– Bones of the skeleton

– Cartilages, ligaments, & other connective tissues

Primary functions of the skeletal system

– Support

– Storage of minerals and lipids

– Blood cell production (RBC/WBC/Platelets)

– Protection

– Leverage

What classifies bones?

shape & structure

Bone shapes

– Sutural

– Irregular

– Short

– Flat

– Long

– Sesamoid

Sutural bones

(Wormian bones)

– Small, flat, irregularly shaped bones

– Between flat bones of the skull

pieces of jigsaw puzzle*

(# varies among individuals)

Irregular bones

– Have complex shapes w/ short, flat, notched, ridged surfaces

– Examples: spinal vertebrae, pelvic bones, many bones in skull

Short bones

– Boxy

– ex: carpal bones and tarsal bones

Nemonic:

carpals: “so long to pinky here comes the thumb” OR “some lovers try positions that they can’t handle”

tarsals: “tiger cubs need milc”

Flat bones

– Thin w/ parallel surfaces

– ex: bones of skull roof, sternum, ribs, & scapulae

(protect soft underlying tissues; think about the cortex/diple/cortex sandwich)

Long bones

– Long & slender

– Found in arms, legs, palms, soles, fingers, toes

Sesamoid bones

– Usually small, round, and flat

– within tendons near joints of knees, hands, & feet

– Location & number vary between people

– ex: patella/patelle

Bone markings

(surface features)

– Projections

• Where muscles, tendons, & ligaments attach

• were articulations w/ other bones take place

– Openings and depressions

• For passage of blood vessels & nerves

Long bone structure

– Epiphysis (wide part at each end)

• Mostly spongy bone/trabecular bone

– Metaphysis

• Where diaphysis and epiphysis meet

–Diaphysis (shaft)

• Wall of compact bone

• Central space inside is medullary cavity (marrow cavity)

Flat bone structure

– Made of spongy bone between two layers of compact bone; cortex

– Layer of spongy bone is called the diploë

cortex→ diploë→cortex (bone sandwhich)

ex: parietal bones of skull

Bone tissue

– Dense, supportive connective tissue

– has specialized cells (vascularized

– Solid extracellular matrix w collagen fibers (perforating fibers in the periosteum!!!)

Characteristics of bone

Dense matrix: calcium salts make bone hard/dense

Osteocytes: bone cells located in lacunae, around blood vessels (vascularized=growth)

Canaliculi: tiny passageways that allow nutrients, wastes, & gases to be exchanged (w osteocytes & blood cells)

Periosteum: covers the outer surface of bone (except at joints)

Outer layer: fibrous; hardness and protection

Inner layer: cellular; growth

Bone matrix physiology

• Calcium phosphate makes up 2/3 of bone mass (main substance) (other 1/3=collagen fibers)

• Combines with calcium hydroxide → forms hydroxyapatite crystals (makes stronger bones!!)

• no calcified matrix, bone looks normal but becomes very flexible

*constant remodelation

Bone cells

– 2% of bone mass

• Osteogenic cells

• Osteoblasts

• Osteocytes

• Osteoclasts

Osteogenic cells

(osteoprogenitor cells)

– stem cells that divide to produce osteoblasts

– Located in inner (where made) cellular layer of periosteum & in endosteum

– helps in fracture repair

Osteoblasts

– Immature cells that make new bone matrix during osteogenesis (ossification; make cells)

– Osteoid: matrix made by osteoblasts that hasn’t been calcified yet (w/ calcium salts so not strong)

(after mineralization w/ calcium salts= bone matrix)

– Osteoblasts surrounded by bone matrix become osteocytes

basically make their own matrix, to become osteocytes

Osteocytes

– Mature bone cells that don’t divide

– Live in lacunae

–cell extensions pass through canaliculi (passageways)

Two major functions

• Maintain protein & mineral content of matrix; maintain the bone

• Help repair damaged bone

Osteoclasts

– break down osteocytes

– Secrete acids & protein-digesting enzymes to break down bone

• break down bone matrix & release minerals

• Osteolysis: breakdown of bone; to regulate calcium important in homeostasis

Lacunae

a small, bubble-like space or cavity located within the hard bone matrix; cuna for the osteocytes/home; around blood vessels that branch through the bone matrix

What is an osteon, and what is found inside it?

Osteon = functional unit of compact bone (Haversian system)

What found inside:

Central canal → has the blood vessels

Lamellae → layers of bone matrix

Perforating canals

What are the 3 types of lamellae and what do they do?

layers of bone matrix around central canal

Concentric lamellae → surround the central canal

Interstitial lamellae → fill spaces between osteons

Circumferential lamellae → found at outer & inner surfaces of bone

What are perforating canals?

• run perpendicular to the osteons to connect them

• carry blood vessels deep into bone & marrow

Spongy bone

Doesn’t have osteons

• matrix made of open network of trabeculae

• Red bone marrow

-Fills spaces between trabeculae; makes blood cells

-Has blood vessels (vasuclarize) that provide nutrients to osteocytes by diffusion=(spread out; not quick)

• Yellow bone marrow

-Found in other areas of spongy bone; stores fat

-Energy reserves; triglycerides: 3 fatty acids w/ 1 glycerol backbone

weight-bearing bones

• Trabeculae found in the epiphysis (end) of the femur

• Transfers forces from the pelvis → compact bone of the femur shaft

Medial side of the shaft = compressed

Lateral side of the shaft = tension/stretching

compact + spongy working tgt

-OPPOSITE FORCES

lateral impact→ medial force

Periosteum

membrane that covers outside of bones; except within joint cavities

Outer = fibrous layer (protection & strength)

Inner = cellular layer (growth & maintenance)

– Fibers connect periosteum to tendons

–Perforating fibers extend into (within) the bone → strengthen tendon-to-bone attachments

Functions of periosteum

• isolates bone from surrounding tissues (bc around)

• pathway for blood vessels & nerves

• Helps w/ bone growth & repair (bone cells )

Endosteum

Endosteum = thin, incomplete layer of cells that lines the inside of bone; medullary cavity

• Covers trabeculae of spongy bone

• Lines central canals of compact bone

• Made of thin layer of osteogenic cells

• Active in bone growth, repair, & remodeling

What is bone development & ossification?

• Ossification (osteogenesis) = formation of bone

• Calcification = buildup of calcium salts in bone

-Happens during ossification

2 types of ossification:

-Endochondral ossification

-Intramembranous ossification

Some bones continue growing until about age 25

Bone remodeling

• happens throughout life

• Maintains bones by recycling & renewing bone tissue

Involves Osteocytes, Osteoblasts, & Osteoclasts

• bone breakdown & replacement are balanced (influx/eflux)

More breakdown than replacement → bones weaken

More bone building than breakdown → bones strengthen

Effects of exercise on bone

• Bones adapt to stress bc of mineral recycling

• More stress → bones become thicker & stronger

• Weight-bearing exercise stimulates osteoblasts (bone-tissue building cells)

What happens to bones during inactivity?

• Bones can degenerate quickly

• Lack of stress = bone loss

• Up to 1/3 of bone mass can be lost in few weeks of inactivity

What minerals & vitamins are important for healthy bones?

• Calcium + phosphorus → required in the diet

• Small amounts of magnesium, fluoride, iron, & manganese are also needed

• Vitamin D3 (cholecalciferol) → used to make calcitriol

• Calcitriol → made in the Kidneys

-Helps digestive tract (stomach) absorb calcium & phosphate

-Essential for normal bone health

How do vitamins affect bone health?

• Vitamin C

-Needed to make collagen

-Helps osteoblasts develop (division)

• Vitamin A → stimulates osteoblast activity

• Vitamins K & B12 → needed to make bone proteins

How do hormones affect bone growth and calcium levels?

• Growth hormone + thyroxine → stimulate bone growth

• Estrogen + testosterone (sex hormones) → stimulate osteoblasts (build bone)

• Parathyroid hormone + calcitonin → help maintain calcium ion balance in the blood (homeostasis)

How do bones act as a calcium reserve?

• Bones store about 99% of the body’s calcium; & other minerals (39% calcium but osteoclast breaking it down provides 99% of what we get in body) HYDROXYAPPEPTIE

• Calcium is most abundant mineral in the body

• Calcium ions are important for many body processes

How do hormones regulate calcium ion balance?

• Calcium levels in body fluids must be closely regulated

• Parathyroid hormone (PTH) + calcitonin help control calcium levels by affecting:

-Bones → calcium storage (to breakdown)

-Digestive tract → calcium absorption (if needed, if not go to kidney)

-Kidneys → calcium excretion (when enough)

What does parathyroid hormone (PTH) do?

• Made by parathyroid glands in the neck

• Increases blood calcium levels by:

-Stimulating osteoclast activity→ releases calcium from bone

- Increasing calcium absorption in the intestines

• By increasing calcitriol production in the kidneys

-Decreasing calcium excretion by the kidneys

What does calcitonin do?

• Made by C cells in the thyroid

• Decreases blood calcium levels by:

-Preventing osteoclast activity → less calcium released from bone

-Decreasing calcium absorption in the intestines

-Reducing calcitriol secretion by the kidneys

-Increasing calcium excretion by the kidneys

Fractures

– Cracks/breaks in bones bc of physical stress

• Open (compound)

• Closed (simple)

Transverse Fracture

break goes straight across the bone, broke in half along horizontal axis

Displaced Fracture

broken bone pieces are out of normal alignment; into 2 of more pieces

Compression fracture

bone is crushed/compressed, often affecting vertebrae; osteoporosis; bone thinning

Spiral fracture

fracture twists around the bone, usually caused by a twisting force; long bone

Epiphyseal fracture

fracture happens through the epiphysis/growth plate area of a bone (end/head of bone)

Comminuted fracture

shattered bony fragments broken into multiple pieces (3+)

Greenstick fracture

bone partially breaks and bends; not fully broken (most common in kids)

Colles fracture

fracture of the radius near the wrist (distal end), w broken end typically displaced backward

Pott's fracture

fracture involving the lower leg near the ankle (mallelar), typically involving the fibula & often the tibia too

4 steps of fracture repair?

1. Fracture hematoma formation → blood clot forms at the break

2. Callus formation → tissue forms around and stabilizes the break

3. Spongy bone formation

4. Compact bone formation

(not talk about yet)

What happens during fracture hematoma & callus formation?

• Fracture hematoma formation

-A large blood clot forms; makes fibrous network

-Some bone cells near the fracture die

• Callus formation

- endosteum & periosteum cells divide & move to fracture

-Calluses stabilize broken bone

Internal callus → forms inside the medullary cavity

External callus → cartilage & bone form around the break

(not talk about yet)

What happens during the final 2 steps of fracture repair?

• 1. Spongy bone formation

  • Osteoblasts replace the cartilage in the external callus w/ spongy bone

• 2. Compact bone formation

  • Spongy bone is replaced w/ compact bone

  • repaired bone may be slightly thicker & stronger than normal

(not talk about yet)

What is osteopenia, and how does aging affect bone mass

• Osteopenia = reduced bone mass bc of lack of bone formation (ossification)

-Usually begins around ages 30–40

• Bone loss each decade:

Women: about 8%

Men: about 3%

• Most affected: epiphyses, vertebrae, & jaws

-Cause fragile bones, reduced height, & tooth loss

(not talk about yet)

What is osteoporosis?

Osteoporosis = severe loss of bone mass

• bones weak & fragile

• interferes w/ normal bone function

• More common w/ increasing age

• Sex hormones help maintain bone mass

-In women, bone loss increases after menopause

(not talk about yet) more severe

How can cancer cause bone loss?

• cancerous tissues release osteoclast-activating factors

• factors stimulate osteoclasts

• Increased osteoclast activity → more bone breakdown

• leads to bad osteoporosis

(not talk about yet)