From Nicotine to Squamous Lung Cancer: How Cigarettes Destroy Our Lungs

As long as we are alive, our respiratory tract is constantly exchanging gases with the outside world. Each day we inhale about 10,000 liters of air, which contains the oxygen our bodies need as well as a great many harmful substances. But over the long course of evolution our respiratory system has developed a complete protective system that effectively helps us fend off external harmful substances.

In this protective system, the pseudostratified ciliated columnar epithelium lining our respiratory tract deserves the greatest credit. The pseudostratified ciliated columnar epithelium is composed of columnar cells, goblet cells and pyramidal cells; the free surface of the columnar epithelial cells bears cilia that beat in a directed manner, and the goblet cells secrete mucus. When harmful substances in the air enter our respiratory tract, the mucus secreted by goblet cells surrounds them, and the cilia on the free surface of the columnar cells beat directionally to clear these harmful substances out of our body. The sputum we cough up is produced in this way: mucus enveloping the harmful components.

In smokers, the pseudostratified ciliated columnar epithelium of the respiratory tract is damaged by nicotine and other harmful substances; the cilia gradually disappear, and the goblet cells can no longer function. The body has the ability to adapt automatically to environmental changes: when these harmful substances damage the respiratory tract and the pseudostratified ciliated columnar epithelium can no longer block them, the undifferentiated cells at the base of the epithelium differentiate into a new protective stratified squamous epithelium.

Stratified squamous epithelial cells are shaped like fish scales, hence also called squamous cells; they are the most wear-resistant of all the cells in our epithelial tissues, because their surface has structures called desmosomes. Normally this type of epithelium appears only on the luminal surface of our skin, esophagus, oral cavity, and the end of the rectum. Because these luminal surfaces may frequently rub against harder materials, this wear-resistant epithelium is needed for protection.

Over hundreds of millions of years of evolution in nature, which type of cell is distributed in each part of the body has become basically fixed. Once one type of cell in an organ or tissue is replaced by another, it means a mutation has occurred there. Mutation leads to pathological hyperplasia, and this hyperplastic tissue is what we call a tumor; it has broken away from the body's natural growth rules, so its growth is uncontrolled. As it continually spreads to the surroundings, it forms metastatic tumors. Ultimately these tumors destroy the function of other tissues and organs, which can no longer work normally, leading to death.

This process is how a smoker's lungs turn cancerous. In pathology, the process in which the lung's pseudostratified ciliated columnar epithelial cells disappear and the original epithelium is replaced by stratified squamous epithelium is called metaplasia. Because its essence is the appearance of stratified squamous epithelial cells nicknamed "squamous cells," the lung cancer induced by it is called squamous-cell lung carcinoma. Most smokers eventually develop squamous-cell lung carcinoma, and this cancer is entirely preventable. As long as smoking is banned, our probability of developing this cancer becomes much smaller.

Smoking not only causes lung cancer; by destroying the respiratory tract's pseudostratified ciliated columnar epithelium it also weakens the respiratory tract's "self-purification" ability, inducing a series of chronic respiratory diseases such as chronic bronchitis and emphysema.