Connective Tissues 3

Question 1

1. Describe the two different processes that lead to bone formation. Describe how long bones grow in length and in width.

Answer 1

1) Intramembranous ossification:
- mesenchymal cells come together (condensation) and transform into osteoprogenitors –> osteoblasts –> secrete osteoid.
- Bone calcifies (hydroxyappetite is the actual crysttal) eventually, but first forms trabecular bone so blood vessels can grow in.
- Once bone calcified, it gets remodeled by osteoclasts and new bone deposition laid down by osteoblasts to form some compact lamellar bone. FLAT bones all form this way. Only by apositional growth.

2) Endochondral ossification:
- happens in previously made cartilage tissue;
- some cells within the perichondrium are converted into osteoprogenitors –> allows cartilage to be replaced by bone.
- Chondrocytes secrete matrix made of hyaline cartilage and get encased in lacuna.
- Can then grow by appositional growth (surface) to interstitial growth (within).
- Within perichondrium, a collar of bone is formed and osteoblasts arise –> transforms perichondrium into periosteum.
- Chondrocytes enlarge and cartilage matrix gets calcified.
- Osteoclasts degrade the calcified cartilage matrix so blood vessels can grow into the region (which brings more osteoprogenitors).
- These form the primary ossification center in the diaphysis (secondary ones are at the epiphyseal ends = epiphyseal plate).
- Growth plate maintained till full length of bone is achieved so cartilage is crucial to growth in bone length –> allows for continuous endochondral ossification.
- Chondrocytes add more cartilage and osteoclasts/capillaries/osteoblasts encroach on it from the diaphyseal side to grow it.
- Bone growth stops when proliferation of cartilage stops.
- Articular cartilage remains at the end of the bone.
- Growth in diameter occurs along periosteum on outside via appositional growth.
- Requires proliferation of osteoprogenitors in periosteum and differentiation into osteoblasts to lay down matrix.
- Bone never grows by interstitial growth.

Question 2

2. Describe the sequence of events that occur in bone remodeling.

Answer 2

• Very regulated process –> allows for constant turn-over of bone matrix in mature bones.
• Allows bone resorption in older calcified bone as well as osteoblast secretion.
• Resorptive activity occurs at endosteal surface and is coupled to formation (and formation is coupled to resorption = balance).
• No net gain or loss of bone.
• Osteoporosis = defects in resorption/formation. -Osteopetrosis = increase in bone mass.
• Osteomalacia = increase in uncalcified osteoid.

-Osteoblasts pinch off matrix vesicles that contain calcium, phosphate, and alkaline phosphatase that can generate free phosphate so it can precipitate with Ca = hydroxyapatite.

Question 3

4. Describe how defects in bone remodeling leads to disease.

Answer 3

Osteoporosis = defects in resorption/formation. Osteopetrosis = increase in bone mass. Osteomalacia = increase in uncalcified osteoid.
Osteoblasts pinch off matrix vesicles that contain calcium, phosphate, and alkaline phosphatase that can generate free phosphate so it can precipitate with Ca = hydroxyapatite.

Question 4

5. Describe how calcium is deposited and resorbed from bone matrix, and how regulation of bone cells controls the levels of blood calcium.

Answer 4

PTH causes calcium liberation/bone resorption and osteoclast activity, while calcitonin causes calcium uptake and osteoblast activity.

steroid hormones help with bone growth

Question 5

3. Describe how bone formation and remodeling is regulated.

Answer 5

1) Short range signals. BMPs = bone morphogenetic proteins secreted by cells, bind receptors, and cause intracellular protein phosphorylation that alters gene expression  specific patterns of differentiation. Can stimulate chondrogenesis or osteogenesis. BMPs control bone development, remodeling, and turnover. (FOP disease is missing by chromosome translocation)
2) Long range signals = endocrine glands. Ex: estrogen and Ca regulation hormones. PTH causes calcium liberation/bone resorption and osteoclast activity, while calcitonin causes calcium uptake and osteoblast activity. Vitamin D helps.
3) Mechanical stress like muscular movements and strain can alter bone remodeling.
4) Nervous system can control bone metabolism.