Psio Exam 2 (Lecture 12)

Front 1

Differentiate between the processes of intramembranous and endochondral ossification.

Back 1

• Intramembranous ossification
o “within membrane”
o Mesenchyme→ bone
• Endochondral ossification
o “Inside cartilage”
o Mesenchyme→cartilage→bone

Front 2

Be able to determine the order of events involved in intramembranous and endochondral ossification.

Back 2

• Steps of intramembranous ossification:
1. Development of ossification center
a. Mesenchymal cells differentiate into osteogenic cells, then into osteoblasts
b. Osteoblasts secrete bone matrix
2. Calcification
a. Osteoblasts deposit calcium into the matrix
b. Osteoblasts differentiate into osteocytes
3. Formation of trabecullae (spongy bone)
4. Development of periosteum
a. Remodeling of spongy bone to compact bone
• Steps to endochondral ossification:
1. Cartilage model forms from mesenchyme
2. Growth of cartilage model in length and width and begins to calcify
3. Blood vessels penetrate model and stimulate differentiation of chrondroblasts into osteoblasts
4. Osteoblasts form bone on the outer surface of the model (osteoblasts secrete bony matrix, calcification occurs)
5. Osteoblasts create primary ossification center (spongy bone) (bone replaces cartilage)
6. Osteoclasts invade newly formed bone and create a marrow cavity
7. Spongy bone remodeled to compact bone at diaphysis (initially epiphyses remain as cartilage)
8. Around birth, blood vessels penetrate the epiphyses and a secondary ossification center forms
9. Spongy bone replaces cartilage at the epiphyses with the exception of epiphyseal plates and articular cartilage

Front 3

List the bones formed by intramembranous and endochondral ossification.

Back 3

• Intramembranous Ossification forms: cranial bones, mandible, sternum, clavicle, heterotrophic bone formation (seasmoid bones- stress that causes formation of bones in areas where bone is not normally found…like base of thumb and big toe)

• Endochondral Ossification forms: most bones in the body; growth in length at the epiphyseal plate

Front 4

Describe how the presence of an epiphyseal plate facilitates the lengthening of bones.

Back 4

• Interstitual Growth (Endochrondral Ossification)
o Resting cartilage
o Proliferating cartilage→cell division, chondroblasts, producing cellular matrix , new chondrocytes are formed, looks like stacked coins
o Hypertrophic cartilage→ grow get so large and then…
o Calcified cartilage→ …they start to die and calcify, old chondrocytes are replaced by bone (new diaphysis)
o Diaphysis (bone)
o Diaphyseal osteoclasts break down calcified cartilage
o Osteoblasts lay down spongy bone
o Chondroblasts will continue to form cartilage matrix at the proliferating cartilage region
o Growth in length stops when the osteoclasts/osteoblasts work faster than the chondroblasts → epiphyseal plate closes and becomes the epiphyseal line
o When growing in length we need to make the osteons longer
o Remodeling happens at the same time of appositional and interstitial growth
Plate does not change in size as bone grows

Front 5

Describe the process of appositional growth.

Back 5

• Appositional Growth
o Periosteal osteoblasts build bone on outer surface of bone (build more than endiosteum osteoclasts break down which creates cortical width)
o Endosteal osteoclasts increase the diameter of the marrow cavity by breaking down bone
o Bone diameter and cortical width (thickness of wall) increase (when bone grows in width we need to add osteons by periosteum as periosteum becomes endostieum of new osteon)
o Osteons are added to periosteal side of bone:
1. Ridges in periosteum create groove for periosteal blood vessel
2. Periosteal ridges fuse, forming an endosteum-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.
Other important info: 2 year old is still growing epiphyses (epiphyseal plate is made of hyaline cartilage)