Once bone is formed, it grows by:

 # Once bone is formed, it grows by:

 A. Interstitial growth only

B. Appositional growth only

C. Both appositional and interstitial growth

D. Degenerative changes into bony structures

The correct answer is B. Appositional growth only.

Which is growth by the addition of new layers on those previously formed. Bone formation begins in the embryo where mesenchymal cells differentiate into either fibrous membrane or cartilage. 

This leads to two paths of bone development:

1. Intramembranous ossification is so called because it takes place in membranes of connective tissue. Osteoprogenitor cells in the membrane differentiate into osteoblasts: a collagen matrix is formed which undergoes ossification. Note: The maxilla and mandible as well as the cranial vault are forrmed this way.

2. Endochondral ossification is how the remainder of the skeleton forms and takes place within a hyaline cartilage model. Cartilage cells are replaced by bone cells (osteocytes replace chondrocytes), organic matrix is laid down and calcium and phosphate are deposited. This type of ossification is principally responsible for the formation of short and long bones. Note: The ethmoid, sphenoid, and occipital bones (bones of the cranial base) form this way.