Lecture 7: Bone Formation and Growth

Question 1

Osteoblasts (Obs)

Answer 1

Active bone depositing cells that secrete osteoid

Question 2

Osteocytes (Ocs)

Answer 2

Mature osteoblasts that have become entrapped within the bone matrix. Maintain gap junction communication and nutrition via dendritic processes in canaliculi

Question 3

Bone lining cell

Answer 3

Quiescent, inactive osteoblast. Flat and squamous.

Question 4

Osteoprogenitor (Ops)

Answer 4

MSC-derived progenitor cell for osteoblasts, osteocytes, and bone lining cells. Located in endosteum/cellular periosteum.

Question 5

Osteoclast (Ocls)

Answer 5

Bone specific macrophage that arises from fusion of monocytes (HSC derived). Large, multinucleated cell that reabsorbs bone by creating Howship’s lacuna and secreting acid and enzymes.

Question 6

Ruffled border

Answer 6

Active, apical surface of Ocl. Contains folds for more surface area for H+ and enzyme pumps to dissolve bone.

Question 7

Howship’s lacuna

Answer 7

Resorption bay created by Ocls and sealed off through integrins/f-actin

Question 8

Basolateral zone

Answer 8

Area in Ocl opposite the ruffled border; contains nuclei and exocytosis activity.

Question 9

Factors stimulating bone resorption

Answer 9

PTH and RANKL (for RANK receptors) increase Ocl activity, releasing more calcium to the blood

Question 10

Factors stimulating bone formation

Answer 10

Osteoprotegrin (OPG), calcitonin, and estrogen inhibit Ocl activity, reducing calcium released to the blood.

Question 11

Osteoporosis

Answer 11

“Porous bone” caused by an imbalance in Ocl/Ob activity.

Question 12

Osteomalacia

Answer 12

Soft bones and bowed legs due to insufficient Ca + Vit. D; leads to lack of bone mineralization.

Question 13

Osteopetrosis

Answer 13

Overmineralized bones due to defective Ocls; appears dense but results in fragile structure.

Question 14

Bone remodeling unit

Answer 14

Cutting cone formed by front of Ocls followed by a closing cone formed by layers of Ops, Obs. Results in resorption of old osteons and creation of new ones.

Question 15

Types of bone formation

Answer 15

1. Intramembranous
2. Endochondral

Question 16

Intramembranous ossification

Answer 16

Ossifications centers from condensed, vascularized mesenchyme. Interstitial bone growth mineralizes and centers merge into trabecular bone. Further appositional growth then on woven bone. Outer/internal mesenchyme become periosteum and endosteum/marrow. Forms flat bones.

Question 17

Endochondral ossification

Answer 17

Bone replaces a hyaline cartilage precursor structure. Invading blood vessels bring cells + minerals. Starts with periosteal bone collar developing into primary/secondary ossification centers. Growth continues at epiphyseal growth plate with articular cartilage left behind. Forms long bones.

Question 18

Calcification factors for endochondral ossification

Answer 18

Hypertrophic chondrocytes release alkaline phosphatase and osteocalcin to stimulate calcification.

Question 19

Bone fracture and repair stages

Answer 19

1. Inflammatory phase
2. Reparative/regenerative phase
3. Remodeling phase

Question 20

Inflammatory phase

Answer 20

1st stage of bone fracture repair. Torn blood vessels create a hematoma and local Ocs die. Macrophages clear the hematoma, Ocls clear dead Ocs/bone matrix

Question 21

Reparative/regenerative phase

Answer 21

2nd stage of bone fracture repair. New Ops migrate and differentiate into chondroblasts (low O2 areas) and Obs (high O2 areas). Periosteum is repaired and cartilaginous procallus is deposited, then replaced by spongy bone.

Question 22

Remodeling phase

Answer 22

3rd phase of fracture repair. Procallus is replaced with woven bone through intramembranous and endochondral ossification. Woven bone is remodeled into compact bone.

Question 23

Epiphyseal growth plate zones

Answer 23

1. Resting
2. Proliferation
3. Maturation + hypertrophy
4. Degeneration + calcification
5. Resorption + ossification

Question 24

Resting zone

Answer 24

Chondrocytes fill a matrix of typical hyaline cartilage

Question 25

Proliferation zone

Answer 25

Chondrocytes undergo mitotic division, creating “life-saver stacks” of lacunae

Question 26

Maturation + hypertrophy zone

Answer 26

Chondrocytes expand their lacunae and compress the cartilage

Question 27

Degeneration + calcification zone

Answer 27

Chondrocytes become apoptotic with pyknotic nuclei; matrix calcifies.

Question 28

Resorption + ossification zone

Answer 28

Invading vasculature supplies Ops and mineral components; chondrocyte lacunae/matrix is replaced by bone, which rapidly mineralizes

Question 29

Appositional growth

Answer 29

Addition of bone to inner/outer bone surfaces. Driven by peri/endosteal cells, creating thin lamellae on surfaces.

Question 30

Haversian remodelling

Answer 30

Remodeling of the inner parts of bones. Forms osteons (Haversian systems). Created by Ocl cutting cone and Ob closing cone.

Question 31

Sesamoid bones

Answer 31

Bones forming within tendons, e.g. patella

Question 32

Fibrodysplasia ossificans progressiva

Answer 32

Extra skeletal bones forming from damaged soft tissue. Caused by mutation in Bone Morphogenic Protein receptor.