Clara cell

Clara cells are dome-shaped cells with short microvilli found in the small airways (bronchioles) of the lungs. Clara cells are found in the ciliated simple epithelium. These cells may secrete glycosaminoglycans to protect the bronchiole lining.

They are also known as "bronchiolar exocrine cells".

Name
"Clara cells" were originally described by their namesake, Max Clara in 1937. Clara was born in South Tyrol in 1899 and died in 1966. He was a Nazi doctor who used tissue from executed victims of the Third Reich for his research at Leipzig, including the work that led to his discovery of Clara cells. Some scholars believe that the eponymous name of these cells should be changed because of the ethical controversy surrounding the discovery of the cells but other scholars disagree and think that the name should remain because it is a testament to a time when medicine crossed an ethical line.

Function
One of the main functions of Clara cells is to protect the bronchiolar epithelium. They do this by secreting a small variety of products, including Clara cell secretory protein (CCSP) and a solution similar to the component of the lung surfactant. They are also responsible for detoxifying harmful substances inhaled into the lungs. Clara cells accomplish this with cytochrome P450 enzymes found in their smooth endoplasmic reticulum. Clara cells also act as a stem cell and multiply and differentiate into ciliated cells to regenerate the bronchiolar epithelium.

Mechanism
The respiratory bronchioles represent the transition from the conducting portion to the respiratory portion of the respiratory system. The narrow channels are usually less than 2 mm in diameter and they are lined by a simple cuboidal epithelium, consisting of ciliated cells and non-ciliated Clara cells, which are unique to bronchioles. In addition to being structurally diverse, Clara cells are also functionally variable. One major function they carry out is the synthesis and secretion of the material lining the bronchiolar lumen. This material includes glycosaminoglycans, proteins such as lysozymes, and conjugation of the secretory portion of IgA antibodies. These play an important defensive role, and they also contribute to the degradation of the mucus produced by the upper airways. The heterogeneous nature of the dense granules within the Clara cell's cytoplasm suggests that they may not all have a secretory function. Some of them may contain lysosomal enzymes, which carry out a digestive role, either in defense: Clara cells engulf airborne toxins and break them down via their cytochrome P-450 enzymes (particularly CYP4B1, which is only present in the clara cells) present in their smooth endoplasmic reticulum; or in the recycling of secretory products. Clara cells are mitotically active cells. They divide and differentiate to form both ciliated and non-ciliated epithelial cells.

Role in disease
Clara cells contain Tryptase clara, which is believed to be responsible for cleaving the hemagglutinin surface protein of influenza A virus, thereby activating it and causing the symptoms of flu. When the l7Rn6 protein is disrupted in mice, these mice display severe emphysema at birth as a result of disorganization of the Golgi apparatus and formation of aberrant vesicular structures within clara cells.