During the process of bone remodelling, which cellular action is primarily responsible for the resorption of bone tissue?

The cellular action primarily responsible for the resorption of bone tissue during the process of bone remodeling is carried out by osteoclasts. Here are the steps:

1. Activation: The process begins when osteoclast precursors are stimulated to differentiate into mature osteoclasts. This is often triggered by the release of certain signaling molecules, such as RANKL (Receptor Activator for Nuclear Factor κ B Ligand), from osteoblasts.

2. Resorption: The mature osteoclasts then attach to the bone surface and begin to resorb bone tissue. They do this by secreting hydrogen ions to dissolve the mineral component of the bone, and enzymes to break down the organic matrix (mostly a protein called collagen). This creates a small pit in the bone known as a resorption lacuna.

3. Reversal: Once the osteoclasts have completed their work, they undergo apoptosis (programmed cell death) and are removed from the bone surface. This marks the end of the resorption phase and the beginning of the formation phase.

4. Formation: Osteoblasts (bone-forming cells) then move in to fill the resorption lacuna with new bone tissue. This is done by secreting collagen to form the organic matrix of the bone, and then mineralizing this matrix with calcium and phosphate ions.

5. Resting: Once the resorption lacuna has been completely filled in, the osteoblasts become dormant and the bone enters a resting phase until the next cycle of remodeling begins.

So, in summary, the cellular action primarily responsible for the resorption of bone tissue during bone remodeling is the activity of osteoclasts.