Bone

Question 1

What are the two types of bone formation?

Answer 1

Intramembranous ossification and endochondral ossification

Question 2

Where does intramembranous ossification occur?

Answer 2

In the skull and clavicle

Question 3

What is intramembranous ossification?

Answer 3

This is the formation of bone over fibrous membrane mesenchymal membrane without a cartilage framework.

Question 4

Steps in Intramembranous ossification

Answer 4

1. Ossification center: Mesenchymal stem cells will begin to aggregate and differentiate into osteoblasts forming an ossification center as these cells will begin to lay done bone forming materials
2. Osteoid laydown: Osteoblasts will begin to secrete osteoid which is unmineralized bone with all components of the bone matrix (hydroxyapatite, calcium phosphate, calcium carbonate, collagen)
3. Trapped & differentiate: osteoblasts that are trapped in the middle of the ossification center as the osteoid hards will differentiate into osteocytes.
4. Trabeculae formation: As more osteoid is laid down and calcified it is done randomly because it is forming around embryonic vessels.
5. Periosteum and endosteum: develop from unossified mesenchyme membrane on the outer layer.
6. Compact bone outside: compact bone will replace the woven bone on the outer edge as osteogenic cells differentiate and deposit bone
7. Spongy bone inside: spongy bone on the inside will remain to allow space for blood vessels to pass and red bone marrow.

Question 5

What is endochondral ossification?

Answer 5

This is ossification that occurs in all other bones in the body besides flat bones.

It uses a hyaline cartilage framework as a guide for bone deposition

Question 6

Steps in Endochondral ossification

Answer 6

Hyaline cartilage is surrounded by perichondrium which is vascularised and provides nutrients to the cells in the hyaline tissue and developing cells in chondrogenic layer

The blood vessels will penetrate the cartilage and begin to provide blood to a zone of reserved cartilage which will stimulate the mesenchymal cells to differentiate into osteoblasts

Newly formed osteoblasts will go to the inner edge of the perichondrium and produce a bony collar to support the bone as the inner cartilage is removed.

The chondrocytes which are internal to the bony collar are going to begin multiplying in the zone of proliferation

Zone of hypertrophy the chondrocytes are going to enlarge and th surrounding matric is going to calcify cutting off supply for oxygen and nutrients and the cells begin to die.

Periosteal buds will then grow in between the cavity bringing with it mesenchymal cells which will differentiate into osteoblasts and lay down osteoid on remaining cartilage and hematopoietic cells will begin producing red bone marrow.

After a region has ossified, osteoclasts move in to remodel the bone creating a marrow cavity

Question 7

How does longitudinal growth occur?

Answer 7

When secondary ossification centers form at the ends of the bone or epiphysis as they receive blood supply it creates two sites of cartilage: one at the very top edge of the bone forming articular cartilage and one separating epiphysial bone with diaphysis by forming a plate of cartilage called epiphyseal cartilage.

The cartilage in the epiphysela plate is constantly making new cartilage while the part facing the diaphysial bone is constantly ossifying upwards giving the appearance of longitudinal growth