Trachea

The trachea is a short, flexible air tube that transports and conditions inspired air. It extends from the larynx to the middle of the thorax. At T4/T5 level, it divides into the two main primary bronchi. The lumen stays open because of the arrangement of a series of C shaped hyaline cartilaginous rings. Fibroelastic tissue and smooth muscle (trachealis muscle) bridge the gap between the free ends of the C shaped cartilages at the posterior border of the trachea. The trachea wall is developed into the following:

• Mucosa is made of pseudostratified ciliated columnar epithelium and a fiber rich lamina propria. It consists of:
• Ciliated columnar cells are the most numerous. Immediately below the cilia is a dark line formed by basal bodies. Cilia help remove small inhaled particles towards the pharynx.
• Mucous cells or Goblet cells produce mucus secretions
• Brush cells are columnar cells with blunt microvilli. The basal surface is in contact with afferent nerve ends for general sensation. They are considered receptor cells
• Small Granule cells or Kulchitsky cells are part of the diffuse neuroendocrine system (DNES). They occur single in the trachea and represent 3% of the cells. They form polypeptide hormones (stored in the granules) that then act locally most of the time (paracrine hormones). Some DNES cells are in contact with nerve fibers, known as pulmonary neuroendothelial bodies. They monitor for local hypoxia and alert the respiratory center of the brain to increase respiration. It is sensitive to nicotine, and known to be involved in small cell lung cancer.
• Basal cells serve as a reserve cell population. They maintain individual cell replacement in the epithelium.
• Basal lamina is a thick prominent layer, thicker in smokers and asthmatics
• Lamina propria is made of loose connective tissue and numerous lymphocytes, which infiltrate into the epithelium. It has both diffuse and nodular lymphatic tissue, equivalent to bronchus-associated lymphatic tissue (BALT). There are numerous elastic fibers, which form a band of elastic tissue demarcating the junction with submucosa.
• Submucosa is made of connective tissue, mucous glands with serous demilunes (seromucous glands), diffuse lymphatic tissue and nodules, large vessels, and lymphatics. The seromucous glands are mucus-secreting acini with serous demilunes. The ducts are lined with simple cuboidal epithelium. There are numerous in the posterior cartilage free gap. The submucosa ends where its connective tissue fibers blend with perichondrium of the cartilage layer.
• Cartilaginous layer is the C shaped hyaline cartilage. The spaces between the cartilage are filled with connective tissue. It is continuous with the perichondrium. With age it may get ossified.
• Adventitia is the connective tissue that binds the trachea to adjacent structures. It contains the largest blood vessels, nerves, and lymphatics.