Other bones, homeostasis, permanent chondrocytes and articular cartilage ANS 123

What is modeling?

Growth-stage process where bone changes shape, size, and proportions

When is modeling active?

Postnatally until sexual maturity

What is an example of modeling?

Appositional growth of the diaphysis

What is remodeling?

Lifelong maintenance process after the skeleton is mature

What happens during remodeling?

Bone is constantly broken down and rebuilt

What does remodeling respond to?

• Stress

• repair micro-damage

• old tissue replacement

• calcium regulation

What bones can form by intramembranous ossification only?

Flat bones of skull and mandible

What bones form by endochondral ossification?

Vertebrae

What factors affect growth termination?

• Genetics

• nutrition

• metabolic/hormonal changes

• mechanical factors

What genetic factor was mentioned for growth?

IGF2

How can castration affect growth?

prolong growth by altering hormones

How can mechanical compression affect growth?

Compressional force can inhibit growth

What is bone homeostasis?

Balance between bone resorption and bone formation

What cell resorbs bone?

osteoclast

What cell forms bone?

osteoblast

What happens if osteoclast activity is greater than osteoblast activity?

bone weakens

What happens if osteoblast activity is greater than osteoclast activity?

bone becomes too dense

How does remodeling help with calcium?

Calcium stored in compact bone can be mobilized when the body needs more than dietary intake provides

What happens to bone homeostasis with aging?

Bone replacement by osteoblasts does not keep up with osteoclast resorption

Why does bone replacement decrease with age?

Osteoblasts lose ability to produce as much osteoid

What happens to central canals/Haversian canals with aging?

appear larger

What mineral is lost during bone aging?

calcium

What can happen to entire osteons with aging?

start to disappear

What may happen to neighboring Haversian canals?

may appear to fuse

What happens to compact bone as porosity increases?

becomes more porous and weaker

What are consequences of osteoporosis?

• Loss of integrity

• brittle bones

• increased fracture risk

Where does spongy bone remodeling occur in this material?

• epiphysis

What triggers remodeling in spongy bone?

• CM instability

• osteocyte apoptosis due to blood exposure

What is resorption in spongy bone remodeling?

Osteoclasts resorb unstable ECM to form a pit/cavity.

what is reversal in spongy bone remodeling?

Osteoblast-lineage cells find clean ECM and mature/differentiate

What is formation in spongy bone remodeling?

Osteoblasts make osteoid to fill the pit/cavity

What happens to some osteoblasts during formation?

They become trapped and mature into osteocytes.

What is resting in spongy bone remodeling?

Bone-lining/stem cells remain ready for future remodeling

Where does compact bone remodeling occur?

Diaphysis

Why is compact bone remodeling different?

Compact bone is dense, so cells must tunnel through it

What structure do osteoclasts form in compact bone remodeling?

A cutting cone

Where do compact bone remodeling cells start?

From the blood supply in the Haversian canal

What is the cutting cone?

A tunnel-like cone-shaped path carved by osteoclasts through compact bone to reach damage

What happens during reversal in compact bone?

Osteoblast precursors are attracted to the cutting cone

What happens during formation in compact bone?

Osteoblasts differentiate and produce osteoid

What happens during resting in compact bone?

New bone-lining cells remain for future remodeling

what are mature anlage chondrocytes/permanent chondrocytes?

Cells that underwent early differentiation to form the interzone and as a result form synovial joints in appendicular skeleton

What are the four connective tissue populations/structures of a synovial joint?

• Articular cartilage

• synovial membrane

• fibrous outer membrane

• bone tissue

What does articular cartilage do?

Prevents bone-on-bone contact

What does the synovial membrane produce?

Synovial fluid

What does synovial fluid do?

Lubricates and acts as an aqueous shock absorber

what do the articular cartilage and the synovial membrane do together?

• allow for frictionless movement

What does the fibrous outer membrane do?

holds the joints together and provides stability

What does bone tissue in the joint come from?

Osteocytes that replaced temporary chondrocytes

What is articular cartilage?

A sheet of hyaline cartilage at the ends of bones forming a joint

What does articular cartilage reduce?

Friction between bone ends to absorb compression pressure

What are the fixed cells of articular cartilage?

chondrocytes

What do articular cartilage chondrocytes make?

ECM

What fibers are found in articular cartilage ECM?

collagen fibers

What ground substance is in articular cartilage ECM?

proteoglycans and water

Compared to synovial fluid, does articular cartilage have more or less proteoglycan?

more proteoglycan

How does articular cartilage get nutrients?

diffusion/osmosis of nutrition from nearby blood vessels in periosteum around the outside of synovial joint  

What are the two major regions of articular cartilage?

• uncalcified zone

• calcified zone

Which zone faces the synovial fluid?

uncalcified zone

Which zone sits against bone?

calcified zone

How many zones are within the uncalcified zone?

three

What does zone 1 do?

Proliferative chondrocytes help cartilage fill laterally

Where is zone 1?

Closest to synovial fluid

What does zone 2 do?

Cushioning/load-bearing; not involved in growth

What does zone 3 do?

Proliferative chondrocytes help cartilage fill vertically

Where is zone 3?

closest to the calcified cartilage