Osteoblast

Osteoblasts (from the Greek words for "bone" and "germ" or embryonic) are mononucleate cells that are responsible for bone formation; in essence, osteoblasts are specialized fibroblasts that in addition to fibroblastic products, express bone sialoprotein and osteocalcin.

Osteoblasts produce a matrix of osteoid, which is composed mainly of Type I collagen. Osteoblasts are also responsible for mineralization of this matrix. Zinc, copper and sodium are some of the minerals required in this process. Bone is a dynamic tissue that is constantly being reshaped by osteoblasts, in charge of production of matrix and mineral, and osteoclasts, which remodel the tissue. Osteoblast cells tend to decrease with age, affecting the balance of formation and resorption in the bone tissue.

Osteogenesis
Osteoblasts arise from osteoprogenitor cells located in the deeper layer of periosteum and the bone marrow. Osteoprogenitors are immature progenitor cells that express the master regulatory transcription factor Cbfa1/Runx2.

Osteoprogenitors are induced to differentiate under the influence of growth factors, in particular the bone morphogenetic proteins (BMPs). Aside from BMPs, other growth factors including fibroblast growth factor (FGF), platelet-derived growth factor (PDGF) and transforming growth factor beta (TGF-β) may promote the division of osteoprogenitors and potentially increase osteogenesis.

Once osteoprogenitors start to differentiate into osteoblasts, they begin to express a range of genetic markers including Osterix, Col1, BSP, M-CSF, ALP, osteocalcin, osteopontin, and osteonectin. Although the term osteoblast implies an immature cell type, osteoblasts are in fact the mature bone cells entirely responsible for generating bone tissue in animals and humans.

Morphology and histological staining


Hematoxylin and eosin staining reveals that the cytoplasm of osteoblasts is basophilic due to the presence of a large amount of rough endoplasmic reticulum. A large Golgi apparatus is also present in the centre. The nucleus is spherical and large. Active osteoblasts synthesize, and stain positively for, Type-I collagen and alkaline phosphatase.

Osteoblasts and osteocytes
Osteoblasts that become trapped in the bone matrix and remain isolated in lacunae become osteocytes. They cease to generate osteoid and mineralized matrix, and instead act in a paracrine manner on active osteoblasts. They are believed to respond to mechanosensory stimuli.