Bone Development & Clinical Application

formation of the skeleton

all features start as fibrous connective tissue or hyaline cartilage

what parts of the skeleton goes through intramembranous ossification ?

flat bone of the skull, parts of the clavicle and the mandible

steps of intramembranous ossification

1. mesenchymal cells cluster around vessel and become osteoblast

2. osteoblast secretes osteoid and creates ossification center

3. osteoblasts grow and continue to create osteoid that surrounds osteoblast

4. becomes osteocyte

5. osteoid continues to form immature spongy bone

6. mesenchyme adheres to external surface and becomes periosteum

7. immature spongy bone is remodeled and replaced with compact bone

8. Central immature spongy bone matures (starts getting its calcium and salts to harden)

what parts of the skeleton goes through endochondral ossification ?

every other bone

steps of endochondral ossification

1. Mesenchymal cells specialize into osteoblasts, perichondrium becomes periosteum 

2. Osteoblasts secrete osteoid but specifically in central portion of model to create bony collar 

3. Chondrocytes left within bony collar, hypertrophy ( increase) and become primary ossification center

4. Chondrocytes from primary oc ossify surrounding cartilage 

5. Chondrocytes die and matrix begins to dissolve 

6. Cavities are now formed within the model ( hollow space) 

7. Remaining cartilage ( ends) are healthy, growing rapidly to elongate the model 

8. Cavity invaded by periosteal bud ( primary nutrient artery to nerve) 

9. Osteoclasts partially erode cartilage matrix, creating matrix in center 

10. Osteoprogenitor cells become osteoblasts (spongy bone formation) 

11. Osteoclasts break down newly formed spongy bone and continue to expand cavity 

12. Epiphysis stays as cartilage ends through birth 

13. After birth, secondary ossification centers develop at epiphysis (short bones only have primary, irregular bones have multiple ossification centers)

14. Secondary ossification centers mimic that of primary except spongy bone remains and no cavity is formed 

15. Space between diaphysis and epiphysis is the epiphyseal plate and is hyaline cartilage until you’re done growing 

postnatal growth in length in order

1. Proliferation

2. Hypertrophic

3. Calcification

4. Ossification

what happens at the resting zone ?

Healthy cartilage, hangs out in epiphyseal plate 

what happens at the proliferation zone ?

• Growth zone 

• Chondrocytes begin rapidly dividing 

what happens at the hypertrophic zone?

• Older chondrocytes are pushed into this zone 

• They begin to enlarge and outgrow lacunae , those begin to erode = empty spaces 

what happens at calcification zone ?

• Matrix calcifies = chondrocytes die 

what happens at ossification zone?

• Bone marrow elements are able to come up and invade zone, bring osteoclast = break down old cartilage and osteoblasts = build new bone 

postnatal growth width

• completed through appositional growth

• Osteoblasts in periosteum secrete bone matrix on external surface 

• Osteoclasts remove bone from endosteal surface 

• Typically more osteoblasts activity compared to osteoclast 

remodeling bone

• takes about 10 years for bone to turn over

what role do hormones play in remodeling bone?

determine if and when bones are remodeled

what role does mechanical stress play in remodeling bone?

determines where bones are remodeled

role of calcium in the body

Homeostasis for maintaining resting membrane potentials

what hormones play a role in influencing calcium during bone development?

1. parathyroid hormone

2. calcitriol

3. calcitonin

hypocalcemia

•  not enough calcium

what happens when there is not enough calcium?

release of parathyroid hormone, this hormone tells osteoclasts to kick up their activity = more bone resorption - more calcium in blood 

hypercalcemia

too much calcium

what happens when there’s too much calcium?

• calcitonin is secreted, reduces osteoclast activity = less bone resorption , osteoblasts increase activity = more bone built = pulls calcium out of blood and returns it to normal 

parathyroid hormone

• secreted by parathyroid gland

• stimulates osteoclasts to promote bone resorption

calcitriol

1. helps with absorption by digestive tract

2. reabsorption by kidneys

3. deposition by osteoblasts

calcitonin

• From the bone in and out of the blood 

what are the sex hormones?

estrogen and testosterone

growth hormone

Promotes growth at epiphyseal plates, but influence all aspects of bone

metabolism

Wolff’s Law

the body adapts to the forces placed upon it 

nondisplaced fracture

• bones are still connected

• incomplete = didn’t go all the way through

• closed

displaced fracture

• bones are not touching anymore

• complete = went all the through the width of the bone

• open

transverse fracture

oblique fracture

spiral fracture

comminuated fracture

avulsion fracture

impacted fracture

fissure fracture

greenstick fracture

Fracture repair

1. Hematoma formation - first 24-48 hours 

2. Soft callus formation - fibroblasts lay down collagen , fibrocartilage 

3. Hard callus formation - osteoblasts lay down temporary bony collar 

4. Bone remodeling - osteoclasts reshape and slowly recreate the normal shape of bone, lasts 3-4 months 

osteoporosis

• Group of diseases and conditions -(osteoclasts) bone resorption out spaces bone deposit (osteoblasts) 

osteoporosis risk factors

low calcium, low Vitamin D, low activity, parathyroid hormone disorder, low estrogen, genetic predisposition 

osteomalacia

• Bones are poorly mineralized 

• Weak and soft bone, calcium salts are not sufficient

osteitis deformans

• bone deposit and resorption

• Abnormally high ratio of spongy

  bone