bone resorption

Definition

Target type: biologicalprocess

The process in which specialized cells known as osteoclasts degrade the organic and inorganic portions of bone, and endocytose and transport the degradation products. [GOC:mah, PMID:10968780]

Bone resorption is a complex biological process that involves the breakdown and removal of bone tissue. It is a normal and essential physiological process that occurs throughout life, playing a crucial role in bone remodeling, repair, and calcium homeostasis. Here's a detailed breakdown of the process:

**1. Osteoclast Activation and Differentiation:**

* The process begins with the activation of osteoclasts, specialized bone-resorbing cells derived from hematopoietic stem cells.
* These cells are stimulated by various factors, including RANKL (receptor activator of nuclear factor kappa-B ligand) expressed by osteoblasts and activated T cells.
* RANKL binds to its receptor RANK on osteoclast precursors, triggering their differentiation into mature, multinucleated osteoclasts.

**2. Osteoclast Attachment and Sealing Zone Formation:**

* Mature osteoclasts attach to the bone surface, forming a tight seal called the "sealing zone."
* This seal isolates a microenvironment where bone resorption occurs, preventing the leakage of enzymes and acids.

**3. Bone Degradation:**

* Within the sealing zone, osteoclasts release potent enzymes and acids.
* **Hydrochloric acid (HCl):** Osteoclasts secrete HCl, which lowers the pH in the resorption lacuna, dissolving the inorganic mineral component of bone (hydroxyapatite).
* **Cathepsin K and Matrix Metalloproteinases (MMPs):** These enzymes break down the organic matrix of bone, primarily collagen.

**4. Resorption Pit Formation:**

* The combined action of HCl and enzymes creates a "resorption pit" on the bone surface, where bone fragments are dissolved and removed.

**5. Degradation Product Removal:**

* Degraded bone fragments, including calcium and other minerals, are transported across the osteoclast membrane and released into the bloodstream.

**6. Osteoclast Apoptosis:**

* Once bone resorption is complete, osteoclasts undergo apoptosis (programmed cell death) to prevent excessive bone loss.

**7. Osteoblast Activity:**

* Bone resorption is tightly coupled with bone formation. Following resorption, osteoblasts, the bone-forming cells, are recruited to the site to rebuild new bone, ensuring balanced bone remodeling.

**Factors Influencing Bone Resorption:**

* **Hormones:** Parathyroid hormone (PTH) stimulates bone resorption, while calcitonin inhibits it.
* **Cytokines:** Inflammatory cytokines, such as TNF-alpha and IL-1, can increase bone resorption.
* **Mechanical Stress:** Weight-bearing exercise promotes bone formation and reduces resorption.
* **Age:** Bone resorption generally exceeds bone formation with aging, leading to bone loss.
* **Diet:** Calcium deficiency and vitamin D deficiency can contribute to increased bone resorption.
* **Diseases:** Conditions such as osteoporosis, rheumatoid arthritis, and cancer can disrupt the balance between bone resorption and formation, leading to bone loss and fractures.'
"

Proteins (11)

Compounds (494)