An osteoblast is a young cell of mesenchymal origin which secretes the bone matrix formed from type I collagen, proteoglycans and glycoproteins (osteopontin and osteocalcin). This matrix which is not yet calcified is known as osteoid. Osteoids are cells with a large cell body from which thin extensions leave and meet the extensions of neighbouring cells in communicating junctions (gap junctions).
Osteoblasts are involved in osteogenesis and also in osteolysis.
The osteoblasts have a dual role: constructing bone and activating the osteoclast.