In the developing growth plate, prehypertrophic chondrocytes secrete Indian Hedgehog (IHH). Which sequence correctly describes the feedback loop by which IHH regulates the pace of endochondral ossification?

• A IHH suppresses the secondary centre of ossification until birth
• B IHH directly promotes terminal hypertrophy of columnar chondrocytes
• C IHH inhibits PTHrP release, accelerating vascular invasion of the cartilage
• D IHH stimulates periarticular PTHrP release, which delays chondrocyte hypertrophy ✓

Explanation

IHH diffuses from prehypertrophic chondrocytes to the perichondrium and periarticular region, where it induces parathyroid hormone related peptide (PTHrP). PTHrP signals back to keep proliferating chondrocytes in the proliferative state and thereby delays their hypertrophy, slowing the advance of the ossification front. The key distractor is option B, because IHH does the opposite of promoting hypertrophy: it holds it off, and only when the signal falls behind does hypertrophy proceed.

Reference: Larsen's Human Embryology, 5th ed.

High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP

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