The primary bronchioles arise from the tertiary bronchi. They are histologically distinct from the tertiary bronchi in that their walls do not have hyaline cartilage and they have Clara cells in their epithelial lining. The epithelium starts as a simple ciliated columnar epithelium and changes to simple ciliated cuboidal epithelium as the primary bronchiole decreases in size. The diameter of the primary bronchioles is often said to be less than 1 mm, though this value can actually range from 5 mm to 0.3 mm. As stated, these bronchioles do not have hyaline cartilage to maintain their patency. Instead, they rely on elastic fibers attached to the surrounding lung tissue for support. The inner lining (lamina propria) of these bronchioles is thin with no glands present, and is surrounded by a layer of smooth muscle. As the primary bronchioles get smaller they divide into terminal bronchioles.
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