What are osteoblasts?

Osteoblasts are immature, differentiated bone cells that are derived from undifferentiated osteogenic stem cells of the bone and are primarily responsible for bone formation. 

When a bone grows or changes, the resultant chemical reactions or hormones activate osteoblasts, which generate and secrete proteins such as collagen, mixed with calcium phosphate, and other minerals. Together these make up the bone matrix, which is deposited in spaces on a bone that needs to grow or be repaired or strengthened. On completing their task of generating and laying down new bone tissue, some osteoblasts differentiate into osteocytes while those that are no longer needed undergo apoptosis.