Overview

Active osteoblasts are cuboidal, basophilic cells (abundant rough ER and a large Golgi apparatus for collagen synthesis) that form a single layer on bone surfaces being built. They secrete osteoid — type I collagen, osteocalcin, osteonectin, bone sialoprotein — towards the bone surface, and release matrix vesicles rich in alkaline phosphatase that nucleate hydroxyapatite. Serum bone-specific ALP and osteocalcin are therefore markers of bone formation.

Osteoblasts have three fates: about 10–20% become osteocytes by burying themselves in matrix; many become flattened, quiescent bone-lining cells; the majority undergo apoptosis. They are a key signalling hub: they carry receptors for PTH and calcitriol, and in response express RANKL (and M-CSF), which stimulates osteoclastogenesis, while secreting osteoprotegerin (OPG), a decoy receptor that inhibits it. Thus hormones that resorb bone act largely through osteoblasts.

Location: Surfaces of forming bone — periosteum (cambium layer), endosteum, trabeculae, growth-plate ossification zone.

Anatomy

Histology: Cuboidal cells with basophilic cytoplasm and a prominent negative Golgi image; a pale layer of unmineralised osteoid (osteoid seam) lies between them and mineralised bone.

Function

Synthesise and secrete osteoid
Initiate mineralisation via matrix vesicles and alkaline phosphatase
Regulate osteoclast formation (RANKL, M-CSF, OPG)
Respond to PTH, calcitriol, oestrogen, GH/IGF-1 and mechanical signals

Regulation: Stimulated by Wnt/β-catenin (inhibited by sclerostin and DKK1), BMPs, IGF-1, intermittent PTH, oestrogen and mechanical loading; inhibited by glucocorticoids.

Clinical significance

Glucocorticoid-induced osteoporosis

Glucocorticoids suppress osteoblast differentiation and promote osteoblast and osteocyte apoptosis — the commonest secondary cause of osteoporosis; fracture risk rises within months of starting therapy.

Raised alkaline phosphatase

High serum ALP of bone origin indicates osteoblast activity — growth in children, healing fractures, Paget disease, osteomalacia, osteoblastic metastases (prostate).

Test yourself

Parathyroid hormone increases osteoclastic bone resorption mainly by which mechanism?

Binding directly to PTH receptors on mature osteoclasts
Increasing osteoprotegerin secretion
Inhibiting calcitriol synthesis
Increasing RANKL expression by osteoblasts

Answer: Increasing RANKL expression by osteoblasts

Osteoclasts lack PTH receptors. PTH acts on osteoblasts (and osteocytes), raising RANKL and lowering OPG; RANKL binds RANK on osteoclast precursors, promoting their differentiation and activity. PTH also stimulates renal 1α-hydroxylase, increasing calcitriol.