embryology - bone development: ossification

Question 1

intramembranous ossification

Answer 1

relies on mesenchymal stem cells present within developing bone.

Mesenchymal stem cells can differentiate straight into osteoblasts, allowing intramembranous ossification to occur without needing a cartilage step.

• flat bones of skull and face

Question 2

condensation of mesenchymal stem cells

Answer 2

The process of intramembranous ossification begins with condensation of the mesenchymal stem cells.

They undergo proliferation and undergo morphological changes and differentiate into osteoprogenitor cells which will in turn develop into osteoblasts.

Question 3

osteogeneic cells

Answer 3

start to deposit bone matrix which are arranged in bony spicules

The differentiating osteoblasts arrange themselves along the spicules and begin to secrete more bone matrix.

As more matrix gets laid down, the spicules increase in size and will fuse together.

As these grow, they will fuse with more and more spicules and this results in the formation of trabeculae.

Question 4

osteoclasts

Answer 4

• large multinucleated cells, ruffled borders
• resorb bone matrix and are important for remodelling, growth and repair of bone
• derived from blood monocytes/macrophages
• secretory, have prominent golgi apparatus and vesicles
• secrete enzymes [carbonic anhydrase- acidifies matrix and causes decacify and hydrolyses, breaks down the matrix]

Question 5

osteoblasts

Answer 5

• synthesis the bone matrix, responsible for mineralisation
• derived from osteoprogenitor cells- mesenchymal stem cell line

Question 6

osteocytes

Answer 6

• inactive osteoblats that have become trapped within bone they have formed