Bone II

Question 1

How is bone develops?

Answer 1

Bone develops in one of two ways:
-Intramembranous bone formation involves development within layer of condensed mesenchyme

-Endochondrial bone formation occurs via a cartilage model that is replaced by bone

Immature bone forms first in either bone formation process and is later replaced by mature bone

Remodeling continues throughout life, although it is slower in mature bone

Question 2

Describe intra membranous bone formation

Answer 2

• Begins when mesenchymal cells condense to form a primary ossification center, from which osteoblasts differentiate and begin secreting osteoid
• Osteoblasts become trapped in their own mix
• ossification centers expand into spicules as more osteoblasts aggregate
• Bone trabeculae(fused spicules) is the name given to the bone developing at these sites
• Spongy bone develops as bony Trabeculae join together
• Blood vessels invade the area at the same time that undifferentiated mesenchymal cells give rise to bone marrow cells
• Periosteum forms from surrounding mesenchymal cells
• numerous ossification centers fuse together forming bone
• Examples of bond formed in this fashion include most of the flat bones of the skull

Question 3

Describe endochondrial ossification

Answer 3

Primary bone is initially produced. It is later replaced by mature secondary bone

As the spicules continue to grow, they fuse with adjacent spicules and this results in the formation of trabeculae

As growth continues, trabeculae become interconnected and woven bone is formed

Question 4

What are the steps in endochondrial bone formation?

Answer 4

1. Outline of bone in hyaline cartilage
2. Formation of periosteum and sub periosteal bone collar for support in diaphyseal region
3. Cartilage matrix is calcified
4. Blood vessels erode into calcified cartilage
5. Formation of periosteal bud consisting of osteogenic cells and blood vessels (primary ossification center)
6. The sub periosteal collar becomes thicker and bone forms on the calcified cartilage complexes
7. secondary ossification centers (in a smaller manner) form in the epiphyses.
8. Epiphyseal cartilage (epiphyseal growth plate) is formed between the primary and secondary ossification centers
9. Epiphyseal plate disappearance occurs at different times
10. Fusion of diaphyseal and epiphyseal marrow cavities

Question 5

Describe endochondrial bone formation at the epiphyseal plate

Answer 5

• Epiphyseal plate continues to grow by adding new cartilage at the epiphyseal end while it is being replaced by bone at the diaphyseal end
• Contains five histologically distinctive zones
• Diaphyseal bone become continuous with epiphyseal bone(connecting the two marrow cavities) at age about 20

Question 6

What are the zones of the Epiphyseal plate?

Answer 6

Zone of reserve cartilage

Zone of cell proliferation

Zone of hypertrophy

Zone of calcified cartilage

Zone of resorption

Question 7

What is the function of zone of reserve cartilage?

Answer 7

Cartilage with small, randomly arranged inactive Chondrocytes. Served as stem cell line

Question 8

What is the function of zone of cell proliferation?

Answer 8

Rapid mitotic divisions give rise to rows of cartilage cells

Question 9

What is the function of zone of hypertrophy?

Answer 9

Chondrocytes are greatly enlarged and the cartilage matrix between neighboring cells becomes thin

Question 10

What is the function of zone if calcified cartilage?

Answer 10

Lacunae coalesce and the interlacunar matrices become calcified, causing apoptosis of Chondrocytes

Question 11

What is the function of zone of resorption?

Answer 11

Bone in beginning to be elaborated upon the calcified cartilage, and osteolytic activity begins to resort the calcified bone-cartilage complex

Question 12

What happens when bone spicule undergoes endochondrial ossification?

Answer 12

A small portion of bone spicule showing endochondrial ossification.
-Remnants of calcified cartilage matrix appear covered by dark blue stained bone tissue

• The newly formed bone is surrounded by osteoblasts
• Some osteoblasts that were captured by the pseudostratified matrix become osteocytes

Question 13

What are the steps of osteoblasts development?

Answer 13

1. Ridges in periosteum create groove for periosteal blood vessel
2. Periosteal ridges fuse, forming an endisteum-lined tunnel
3. Osteoblasts in endosteum build new concentric lamellae inward toward center of tunnel, forming a new osteon
4. Bone grows outward as osteoblasts in periosteum build new circumferential lamellae. Osteon formation repeats as new periosteal ridges fold over blood vessels

Question 14

Summarize development of osteons

Answer 14

• Longitudinal ridges from along the bone and osteogenic cells in the periosteum transform into osteoblasts
• Osteoblast start producing bone matrix which form ridges that close off periosteal capillaries as they meet
• Periosteum lining the newly formed canal becomes the endosteum and starts forming concentric lamellae to form the osteon

Question 15

Summarize bone repair

Answer 15

Fractured bone results in damage to bone matrix, bone cells in the region, as well as blood vessels supplying the area

• Hemorrhaging is followed by blood clotting, and. Macrophages remove much of the debris via phagocytosis
• Fibroblasts proliferate in the periosteum and endosteum and surround the area internally and externally to isolate it

Question 16

Explain callus formation

Answer 16

A fibrocartilaginous callus is formed both internally and externally

Bone is elaborated in the fracture zone via intramemnranous bone formation by osteoblasts derived from osteogenic layer of the periosteum & endosteum in the vicinity of the fracture

• Chondrocytes also differentiate from this connective tissue and elaborate cartilage, which will be replaced by bone via endochondrial bone formation, thus forming a bony callus composed of primary bone
• Bony callus is eventually resorbed and replaced with secondary bone as the repair process continues

Question 17

Give the histophysiology of bone

Answer 17

• supports the body and provides sites of attachment for muscles
• Protects the CNS and vital organs
• Dynamic tissues that constantly undergoes changed in shape and form due to the stresses placed upon it
• Growth is directed toward to applied stresses

Question 18

Give the histophysiology of the calcium reserve

Answer 18

Bone contains about 99% of the body’s calcium, an element essential for muscle contraction, enzymatic activities, transmission of nerve impulses, cell adhesion , and blood coagulation

De calcification of bone results from diet inadequate in calcium

During the remodeling of spongy bone, calcium is transferred from the bone into the blood stream

Question 19

What is the effect of low protein diets on bones?

Answer 19

Diets low in protein result in deficiency of amino acids essential for collagen synthesis by osteoblasts

Question 20

What is the effect of low calcium diet on bone?

Answer 20

Can be due to either low intake or inadequate absorption by small intestine (due to lack of vitamin D), results in poorly calcified bone, which leads to rickets in children and osteomalacia

Question 21

What is the rickets and osteomalacia cause?

Answer 21

Histologically, these appear as wide osteoid seams with delay in the mineralization rate.

With rickets, the growth plates become irregular, and widened, with delayed apoptosis of hypertonic Chondrocytes

Question 22

What is vitamin D for?

Answer 22

Necessary for proper ossification (excess May actually cause bone resorption)

Question 23

What does vitamin A deficiency lead to?

Answer 23

Inhibits proper bone formation and growth (excess accelerates osteoclast activity and makes bones fragile)

Question 24

What is the vitamin C for?

Answer 24

Excess aryl fir collagen formation- deficiency results in scurvy, characterized by poor wound healing and bone repair after fractures

Question 25

What is the effect of parathyroid hormone?

Answer 25

• Indirectly stimulates osteoclasts to resort and release calcium, thus elevating blood calcium levels
• May activate osteocytes to initiate osteolysis, whereby they liberate calcium from the walls of their lacunae, thus elevating blood calcium levels
• In excess renders bone more susceptible to fracture and subsequent deposition of calcium in arterial walls and certain organs such as the kidney

Question 26

What is the function of calcitonin?

Answer 26

Produced by parafollicular cells of the thyroid gland

• inhibits matrix resorption by osteoclast and thus prevents the release of calcium
  
  • lowers blood calcium lowers

Question 27

What is the function of the pituitary growth hormone (GH)?

Answer 27

Stimulates epiphyseal cartilage growth, so that an excess in childhood produces a giant, while a lack produces a dwarf

• In adulthood excess GH results in acromegaly

Question 28

What is osteoporosis ?

Answer 28

Means ‘porous bone’ and result from progressive loss bone density leading to increased risk of fracture

Question 29

Describe primary type 1 osteoporosis

Answer 29

In post-menopausal women estrogen levels decrease with menopause.

Estrogen normally limits activity of osteoclasts

1/3 post menopausal women affected (hormone therapy, selective estrogen receptor modulators therapy available)

Question 30

When is osteoporosis type 2 common?

Answer 30

In elderly in their 70-80s

Question 31

What is secondary osteoporosis?

Answer 31

Develops as a consequence of drug therapy (corticosteroids) or other disease process (malnutrition, weightlessness, metastatic cancer, radiation)