Lecture 4-Bone Development

Question 1

Before six weeks of development the skeleton is a ?

Answer 1

Cartilage

Question 2

Process of osteogenesis (bone formation)

Answer 2

• Mesenchyme or chondrocytes are replaced by bone cells (process is called ossification)
• Calcium salts are deposited (Calcification) as part of osteoid secreted by osteoblasts.

Question 3

Instramembranous ossification

Answer 3

-Begins at the week 8 of development
-Occurs in the deeper dermis of the skin (dominated by the mesenchyme and fibrous connective tissue)
-Mesenchymal cells differentiate into osteoblasts
-Bones often called dermal bones or membrane bones
Ex: bones of skull and face, mandible, clavicle, sesamoid bones (patella)

Question 4

Steps of instramembranous ossification

Answer 4

• Step 1: Formation ossification center
• Step 2: Formation of bony spicules
• Step 3: Entrapment of blood vessels
• Step 4: Formation of spongy bone

Question 5

Two types of ossification

Answer 5

-instramembranous ossification
• Replacement of mesenchyme and/ or fibrous connective tissue by bone cells
• Forms the clavicle, mandible, flat bones of face and skull, and others that don’t bear weight.

-Endochondral ossification
•Replacement of existing chondrocytes by bone cells
•Occurs in bones that bear weight (vertebral column, femur, etc.)

Question 6

Step 1 of instramembranous ossification

Answer 6

-Formation of ossification center
•Mesenchymal cells divide, aggregate into clusters, and differentiate into osteoblasts
•Osteoblasts secrete osteoid (organic components of matrix)
•Osteoid matrix becomes mineralized (calcification)
•These steps form one or more ossification centers

Question 7

Step 2 of instramembranous ossification

Answer 7

-Formation of bony spicules

-Bone grows out in small struts (spicules) from each ossification center
•Osteoblasts become trapped by calcified osteoid and mature into osteocytes

• Mesenchymal cells continue to produce more Osteoblasts

Question 8

Step 3 of instramembranous ossification

Answer 8

-Entrapment of blood vessels
•Blood vessels in the tissue branch and grow in the region
•As spicules grow, the surround (entrap) blood vessels

Question 9

Step 4 instramembranous ossification

Answer 9

-Formation of spongy bone

-Continued growth of spicules and calcification of osteoid creates a bony plate of spongy bone interwoven with blood vessels
•Outer boundary cells forms periosteum

Question 10

Endochondral Ossification

Answer 10

-Early “Skeleton” of embryo formed of hyaline Cartilage
-Cartilage gradually replaced by bone through endochondral ossification
•Uses Cartilage as small model
•Bone grows in diameter and length
-Diameter growth involves appositional bone
deposition
-Length growth involves interstitial growth

Question 11

Steps of endochondral ossification

Answer 11

• Step 1: Programmed death of chondrocytes
• Step 2: Formation of bone collar
• Step 3: Formation of primary ossification center
• Step 4: Formation of medullary cavity
• Step 5: Formation of secondary ossification centers
• Step 6: Epiphyseal growth
• Step 7: Mature bone

Question 12

Step 1 of endochondral ossification

Answer 12

• Programmed death of chondrocytes
• Chondrocytes in the center of the Cartilage model enlarge begin to calcify the matrix around them.
• These enlarged chondrocytes then die (Programmed cell death, also called apoptosis)
• This process leaves behind cavities inside the model

Question 13

Step 2 of endochondral ossification

Answer 13

• Formation of bone collar
• Blood vessels grow around perimeter of model
• Mesenchymal cells on outside differentiate into osteoblasts
• Perichondrium differentiates into periosteum
• Osteogenic layer of periosteum forms a bone collar

Question 14

Step 3 of endochondral ossification

Answer 14

-Formation of primary ossification center
•Blood vessels penetrate into the partially calcified Cartilage (no inhibition)
•Osteoblasts (and Osteoclasts) migrate with blood vessels and occupy empty spaces
•Osteoblasts replace partially calcified matrix with spongy bone
•These processes form a single ossification center

Question 15

Step 4 endochondral ossification

Answer 15

-Formation of medullary cavity
•Spongy bone in the shaft (diaphysis) eroded by Osteoblasts, forming a medullary cavity
•Outer ring of compact bone forms
•The diameter of the diaphysis enlarges via appositional growth
•Elongation occurs at metaphysis via interstitial growth

Question 16

Step 5 of endochondral ossification

Answer 16

-Formation of secondary ossification centers
•The processes that created the primary ossification center are duplicated on the ends of the hyaline model
*Apoptosis of chondrocytes, bone collar formation, invasion by blood vessels and bone cells

Question 17

Step 6 of endochondral ossification

Answer 17

-Epiphyseal growth
•Elongation of the bone occurs at the Epiphyseal Cartilage (at the metaphysis)
•Chondrocytes continue to divide and enlarge on Epiphyseal side
•Chondrocytes undergo apoptosis on diaphyseal side and are replaced by Osteoblasts
•Degenerating Cartilage on diaphyseal side replaced by spongy bone

Question 18

Step 7 of endochondral ossification

Answer 18

-Mature bone
•Rate of Epiphyseal Cartilage growth slows
•Rate of Osteoblast activity increases
•Endochondral plate completely converted to bone
•Elongation stops
•Remnant of endochondral plate called Epiphyseal line
•Last remnant of hyaline model is the articular Cartilage on the articular ends of the bone