Download Compact bone

Ch. 5 – The Skeletal System
• 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
Functions of Skeletal System
• 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)
Osseous (bony) tissue
• 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
Parts of a typical
long bone
Metaphysis
Epiphysis
Internal bone cavities –
contain
• red marrow
– Blood cell production
• yellow marrow
Diaphysis
– Fat storage
Epiphysis
Articular cartilage
Defining Bone as a Tissue
• Bone is alive and dynamic
• Extremely well vascularized
• Bone is a constant
“remodeling project”
Astronauts lose 1-2%
of mass each month (NASA.gov)
Bone is living tissue
• Basic unit is the
osteon
• Osteocytes
Canaliculi
in a lacuna
Lamellae
– Mature Bone cells
– Maintain bone
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
Bone cells
Formation of the Human Skeleton
• 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
Bone formation (ossification)
• Endochondral ossification
– Hyaline cartilage model
– Bone replaces cartilage through
the action of osteoblasts
Endochondral ossification
(Articular
cartilage)
(Epiphyseal
plate)
Epiphyseal plate vs line
• Epiphyseal plate
– Flat plate of hyaline cartilage
seen in young, growing bone
• Epiphyseal line
– Remnant of the epiphyseal
plate
– Seen in adult bones
Bone growth
• 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
Bone remodeling
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
Fracture repair
Aging and bones
• 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
Aging and bones
• 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)