AssessmentsIdiopathic pulmonary fibrosis
Content Reviewers:Rishi Desai, MD, MPH
Idiopathic pulmonary fibrosis can be broken down into idiopathic which means a disease without a known cause or mechanism, pulmonary which refers to the lungs, and fibrosis which refers to excess collagen in connective tissue, or interstitial tissue between cells, usually after tissue damage.
So idiopathic pulmonary fibrosis is the ongoing repair process of having excess collagen or scar tissue in the interstitial tissue of the lung.
What triggers the repair process is unknown, but it’s a chronic process that leads to a progressive loss of lung tissue.
Normally, gas exchange happens between the alveoli which carry air and capillaries which carry blood.
The alveoli are lined by type I and type II alveolar epithelial cells, also called type I and type II pneumocytes.
Type I pneumocytes make up the majority of cells - they’re simple squamous cells that form a nearly continuous barrier between the air and underlying connective tissue.
Type II pneumocytes are studded throughout the type I - they’re shaped like cubes, have microvilli to sweep away invading particles, and secrete surfactant, an oily mixture of proteins, phospholipids, and neutral lipids which prevent the alveoli from collapsing during exhalation.
Type II pneumocytes also have the ability to divide to make more type II pneumocytes and can also divide and become type I pneumocytes as well.
Now, between the type I and type II pneumocytes and the capillaries is interstitial tissue of the lung, which has macrophages and fibroblasts.
When the alveolar lining is damaged, type I pneumocytes release transforming growth factor beta1, which gets the type II pneumocytes to stimulate fibroblasts to proliferate and develop into myofibroblasts, which are fibroblasts with some smooth muscle cell properties.
The myofibroblasts secrete reticular fibers, a type of collagen which provides structural strength, as well as elastic fibers, which provide the rubber-band like elasticity of the lungs.
The myofibroblasts then undergo apoptosis, or programmed cell death.
The tissue injury that triggers idiopathic pulmonary fibrosis is unknown, but there are some known risk factors including old age, being male, and tobacco smoking.
Once the process is triggered, however that happens, the fundamental problem is that type II pneumocytes over-proliferate during the repair process and it leads to too many myofibroblasts and too much collagen.
To make matters worse, the myofibroblasts don’t undergo apoptosis normally, and instead continue to make even more collagen.