The transitional system of the respiratory tract is composed of bronchioles, which are microscopic structures that serve as a transition zone between the conducting system (ciliated) and the gas exchange (alveolar) system (see Fig. This trachea was specially fixed to preserve the mucous layer, which consists of an internal, clear, hypophase-fluid layer (not visible here) surrounding microvilli and kinocilia and an external mucous epiphase at the level of the tips of the kinocilia (cut in both transverse and longitudinal section here). B, Normal ciliated epithelium, trachea, cow. Ciliated cells are more abundant in proximal airways, whereas secretory cells are more numerous in distal portions of the conducting and transitional systems. The proportion of ciliated to nonciliated cells varies depending on the level of airways. Nonciliated secretory cells have a dome-shaped surface with abundant microvilli (arrowheads). Ciliated cells have numerous slender cilia (arrows). Mucosa consists of ciliated cells and nonciliated secretory cells. The mucous layer was removed before fixation to expose the external surface of the epithelium. Ultrastructural Morphology of Respiratory Mucosa.Ī, Normal bronchial mucosa, bronchus, rat.
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