Anatomy and histology of the respiratory tract

The respiratory tract can be divided into upper and lower compartments. The upper airway extends from the sino-nasal area to the larinx. The lower tract extends from trachea to the lungs and is the major focus of respiratory cytology. Cells of the upper airway are occasionally observed in the specimens from the lower tract.

The trachea and bronchi are lined by ciliated columnar cells; they have a basal nucleus with finely granular chromatin and a thick bar with cilia on the luminal surface. Goblet cells are present in the epithelium at a ratio of one per six columnar cells, with the cytoplasm distended by mucus. Basal or reserve cells, small and undifferentiated, are adjacent to the basement membrane; they are the forerunners of the ciliated and goblet cells. Neuroendocrine cells (Kultchitsky cells), argyrophil-positive, are also present but not identifiable with routine stains; ultrastructurally, they have dense-core granules.

The bronchioles are lined by Clara cells, which are non-ciliated cuboidal to columnar cells usually not recognized on cytologic preparations. Type I pneumocytes are flat cells covering the gas exchange portion of the alveolar surface. Type II pneumocytes are plumper, cuboidal cells on the alveolar surface; they secrete pulmonay surfactant, which is seen ultrastructurally as lamellar bodies. On cytologic preparations, type II pneumocytes have round, vacuolated cytoplasm; they may be hard to distinguish from macrophages.
Alveolar macrophages have round to oval nuclei and lacy or foamy cytoplasm, often with phagocytosed cytoplasmic material (such as carbon particles). Alveolar macrophages must be observed on a sputum sample to judge it as adequate.

A few white blood cells (neutrophils, lymphocytes) may be normally present in the alveolar compartment. An increased number of white blood cells indicates an inflammatory condition. 

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