Bone formation and growth

Question 1

Terminology

Answer 1

• Human bones grow until about age 25
• Osteogenesis = Bone formation
• Ossification = the process of replacing other tissues w/ bone

Two main forms of ossification:

1. Endochondral ossification (inside cartilage ossification)
2. Intramembranous ossification (b/w membranes ossification)

• Calcification = the process of depositing calcium salts
• Occurs during bone ossification + in other tissues

Question 2

Endochondral Ossification (general)

Answer 2

• Ossifies bones that originate as hyaline cartilage
• Most bones originate as hyaline cartilage (in the foetus/baby)
• There are six main steps in endochondral ossification

Question 3

Endochondral Ossification (steps)

Answer 3

1. Chondrocytes secrete enzymes (alkaline phosphatase) to begin the process of mineralisation
2. Bone collar formation: blood vessels infiltrate + osteoblasts secrete osteoid against the cartilage
3. Internally, chondrocytes promote blood vessel invasion and bringing osteoprogenitor cells, osteoblasts, osteoclasts to the primary ossification centre
4. medullary cavity formation
5. same process occurs in secondary ossification sites at the epiphysis (but these sites do not have a medulla)
6. epiphysis fills with spongy bone and becomes separated from the diaphysis by epiphyseal cartilage “physis”

Question 4

Growth and maturation

Answer 4

• Newly formed bone gradually thickens as more cartilage matrix is laid down, + strengthens the long bone w/ layers of circumferential lamellae
• This process is known as appositional growth

As long bone matures

• Osteoclasts enlarge medullary (marrow) cavity
• Osteons form around blood vessels in compact bone

When long bones stop growing, after puberty:

• Epiphyseal cartilage/physis disappears
• The growth plate (epiphyseal line) on a long bone is visible on X-rays

Question 5

Intramembranous Ossification

Answer 5

Produces FLAT Bones such as mandible (lower jaw) scapula + clavicle (collarbone) (yes, we still refer to it as long!)

Question 6

Blood Supply to mature long bones

Answer 6

1. Nutrient artery + vein
   - A single pair of large blood vessels
   - Enter the diaphysis through the nutrient foramen
   - Femur has more than one pair
2. Metaphyseal vessels
   - Supply the epiphyseal cartilage – bone growth
3. Periosteal vessels
   - Blood to superficial lamellae
   - Secondary ossification centres
   - (also has lymphatic vessels + sensory nerves)