• Elements of the skeletal system :
– Bones of the skeleton (~206 for an adult, 270 at birth)
– Cartilages, ligaments, and other CTs that stabilize and connect the bones

• Physical/structural support
• Movement (provides levers for muscles to pull on)
• Protection (of soft parts)
• Storage of minerals (calcium and phosphorus) and fat/energy
(yellow marrow)
• Blood cell production (red marrow)

• Compact bone :
– Makes up external layer of all bones and shaft of long bones
– For dense protection and support
• Spongy bone :
– Makes up middle of most bones and ends of long bones
– For lighter-weight support and red marrow storage

• periosteum
– CT that surrounds bone
– for bone growth, repair, and blood/nerve supply
Internal bone cavities – contain
• red marrow
– Blood cell production
• yellow marrow
– Fat storage
Metaphysis
Epiphysis
Diaphysis
Epiphysis
Articular cartilage

• Bone is alive and dynamic
• Extremely well vascularized
• Bone is a constant
“ remodeling project ”
Astronauts lose 1-2% of mass each month (NASA.gov)

• Basic unit is the osteon
• Osteocytes
– Mature Bone cells
– Maintain bone
Canaliculi
Lamellae in a lacuna
Perforating canal

• Osteoprogenitor cells – stem cells divide/differentiate into osteoblasts (progenitor = “ gives rise to ” )
• Osteoblasts – build/deposit new bone (blast = “ germ ” or “ bud ” )
– Wall themselves in lacunae and differentiate into osteocytes
• Osteocytes – mature bone cells
– Maintain local area of bone
• Osteoclasts – break down bone to release minerals (clast =
“ to break ” )
– Release:
• enzymes – break down collagen
• acids – dissolve mineralized matrix

• In embryos, the skeleton is primarily hyaline cartilage
• During development, much of this cartilage is replaced by bone
• Cartilage remains in isolated areas
– Bridge of the nose
– Parts of ribs
– Joints

• Endochondral ossification
– Hyaline cartilage model
– Bone replaces cartilage through the action of osteoblasts

(Articular cartilage)
(Epiphyseal plate)

• Epiphyseal plate
– Flat plate of hyaline cartilage seen in young, growing bone
• Epiphyseal line
– Remnant of the epiphyseal plate
– Seen in adult bones

• A bone can grow in 2 ways:
1) length – epiphyseal cartilage expands increasing length of bone
• Can only occur before cartilage epiphyseal plates
“ close ” (completely ossify)
2) thickness –
• Can occur anytime throughout life
• Growth regulated by hormones such as:
–Growth hormone
–Thyroid hormone (thyroxine)
–Sex hormones (testosterone and estrogens) – especially at puberty

Is a balance between bone deposition (via osteoblasts) and reabsorption (via osteoclasts)
•Continuous process that occurs throughout lifetime
•Responsive to demands/stresses
•Non-stressed bone actually loses density
•Plays a role in regulating calcium levels
Functions?
•Renews bone tissue before it deteriorates
•Provides way to repair fractures

• Osteopenia – Decrease in bone mass
– Decreased osteoblast activity
(osteoclasts continue at same level)
– Begins age 30-40
– Women: ↓ 8% bone mass per decade
– Men: ↓ 3% bone mass per decade

• Osteoporosis – osteopenia that is severe enough to cause decrease in function: bones thin, break
– Some contributing factors:
• Malnutrition (↓ calcium, protein, vitamin
C, vitamin D)
• ↓ intestinal calcium absorption
• ↓ protein synthesis
• ↓ sex hormones (e.g. menopause)
• Lack of use (↓ mechanical stress)