U2 Lecture 3

Question 1

Why are the bones of infants soft in certain regions?

Answer 1

• Fontanels (skull)
• Epiphysis of long bones made of cartilage in infant
• Epiphyseal plates stay as cartilage until adulthood (until end of puberty)

Question 2

What are the “bones” of a fetus (before birth) composed of?

Answer 2

1. Loose connective tissue (mesenchyme)

2. Hyaline cartilage

Question 3

Define ossification

Answer 3

the replacement of other connective tissues by bone

• begins during 2nd month of development
• continues many years after birth and into adulthood

Question 4

Types of ossification

Answer 4

1. Intramembranous ossification

2. Endochondral ossification

Question 5

Define intramembranous ossification

Answer 5

“within membrane”

mesenchyme –> bone

Question 6

Bones formed by intramembranous ossification

Answer 6

• flat bones of the skull ( frontal, parietal)
• most facial bones (mandible etc)
• sternum
• clavicles (medial parts)
• heterotropic bones (abnormal stresses can stimulate bone formation in places where bone is not normally found… ex: sesamoid bones)

Question 7

Steps of Intramembranous ossification

Answer 7

1. Development of ossification center - Mesenchymal cells differentiate into osteogenic cells, then into osteoblasts; osteoblasts then secrete bone matrix
2. Calcification - osteoblasts deposit calcium in the matrix; osteoblasts differentiate into osteocytes; extracellular matrix calcifies (hardens)
3. Formation of trabeculae (spongy bone) - exracellular matrix forms trabeculae which forms around a network of blood vessels to make spongy bone
4. Development of periosteum - spongy bone remodels to compact bone

Question 8

Calcification vs. Ossification

Answer 8

Calcification = the deposition of calcium

Ossification = replacement of other connective tissues by bone

• Calcification is an essential part of ossification

Question 9

Endochondral ossification

Answer 9

• most of bones in the body are formed by this process

- growth in length at the epiphyseal plate also by endochondral ossification

Question 10

Steps of endochondral ossification

Answer 10

1. Formation of a cartilage model
2. Cartilage model grows in length and width: model undergoes calcification, blood vessels penetrate model and stimulate differentiation of chondroblasts into osteoblasts, osteoblasts secrete bone matrix, calcification occurs
3. Primary ossification center forms (spongy bone)
4. Osteoclasts invade the newly formed bone and carve a marrow cavity: spongy bone is remodeled into compact bone in the diaphysis, epiphyses remain as cartilage (for now), around time of birth blood vessels penetrate the epiphyses
5. Secondary ossification centers form
6. Spongy bone replaces most of the cartilage (except the epiphyseal plates & articular cartilage

Question 11

Endochondral ossification types

Answer 11

Primary ossification and secondary ossification

Question 12

Define primary ossification

Answer 12

• Marrow cavity formed
• replaces all cartilage with bone
• located in diaphysis
• occurs before birth

Question 13

Define secondary ossification

Answer 13

• no marrow cavity
• some cartilage is left for epiphyseal plates (growth) and articular cartilage
• located in epiphyses
• occurs around the time of birth