Osteoblast Products — MFDS Part 1 MCQ
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Correct answer: B — Tissue-nonspecific alkaline phosphatase
Tissue-nonspecific alkaline phosphatase is expressed by osteoblasts and directly facilitates bone mineralisation. It hydrolyses extracellular inorganic pyrophosphate, an inhibitor of mineralisation, while increasing the local availability of inorganic phosphate for hydroxyapatite crystal growth. Tartrate-resistant acid phosphatase and cathepsin K are characteristic osteoclast products involved in bone resorption. RANKL can be expressed by osteoblast-lineage cells, but it promotes osteoclast differentiation and activation rather than directly mineralising osteoid. Calcitonin is secreted by thyroid parafollicular cells and suppresses osteoclastic bone resorption. The original combined osteocalcin and alkaline-phosphatase option was imprecise because osteocalcin's effects on crystal formation are complex, whereas the mineralising mechanism of alkaline phosphatase is well established.
Reference: Atanasova D et al. Glycoproteomic profile of human tissue-nonspecific alkaline phosphatase expressed in osteoblasts. JBMR Plus. 2024;8(2):ziae006. https://pubmed.ncbi.nlm.nih.gov/38505526/