essentials

RESPIRATORY SYSTEM

Essentials of Histology

    The respiratory system is thought of as having a conducting portion and a respiratory portion. The conducting portion is composed by the pharynx, larynx, trachea, bronchi and bronchioles. The respiratory portion is composed by the respiratory bronchioles, alveolar ducts and alveoli.
    The nasal cavity is overlaid internally by a mucosa, which alternate with the region in study. In the initial portion, that makes direct contact with the exterior, the mucosa is composed by a stratified squamous epithelium, supported by a lamina propria of dense connective tissue. Is this epithelium, as happens in the skin, it is possible to find hairs and glands, that help in the air filtration process. The most part of the nasal cavity is recovered by ciliated pseudoestratified columnar epithelium with goblet cells, epithelium that is found in some other regions of the respiratory system. This portion of the nasal cavity is known by respiratory portion, and besides the epithelium, presents a fibrous lamina propria with mucous glands. A third region in the nasal cavity presents a specialized olfactory epithelium. This epithelium is also of pseudoestratified columnar type, where it is possible to observe supporting, basal and olfactory cells. This latter cells are indeed bipolar neurons, that present non motile sensorial cilia in their apical portion.
   The epithelium described in the respiratory portion of the nasal cavity, as shown before, is common to several other regions of the respiratory tract. This epithelium, as a function of its widespread distribution is called respiratory epithelium and is composed by at least 5 different types of cells: - ciliated columnar cells; - basal cells, which are responsible to the continue renovation of the epithelium; the goblet cells, mucous producing cells; brush cells, that received this denomination in function of the presence of microvilli in the luminal border; and the bronchial Kuchitsky cells, presumed to have neurendocrine function.
     The pharynx, that follows the nasal cavity presents respiratory epithelium and in regions where it makes contacts with food, in the oropharynxs, presents a . The larynx is a hollow organ interposed between the pharynx and the trachea, and have as a most remarkable aspect, folds of the mucosa, being the second fold, the true vocal cords. As happens with the pharynx, the larynx presents regions with respiratory epithelium and others subject to air friction, where this epithelium is replaced by stratified squamous epithelium. The larynx is sustained by several cartilaginous plates, the bigger ones of hyaline type and the smaller ones of elastic type.
   The trachea is a tube sustained by several (16-20) incomplete rings of hyaline cartilage. The trachea is internally overlaid by the respiratory epithelium, and presents a lamina propria of loose connective tissue. The mucosa of the trachea presents beyond the goblet cells, several mucous glands. The cartilaginous rings are opened posteriorly and connected by smooth muscle fibers. They are recovered externally by loose connective tissue, that constitute the layer called adventitia.
    The trachea branch out in 2 bronchi, which after a short route penetrate in the lung, where successive divisions happens. When the bronchi reach a small dimension, so that it is not possible to recognize in a transverse section any cartilaginous plates, only smooth muscle, it become a bronchiole. THe diminution in the caliber of these conducts, is also followed by successive modification in their histological characteristics. In larger bronchial branches, the epithelium is the same as the one observed in the most part of the respiratory tract, pseudoestratified columnar type, but is slowing be replaced by a simple columnar type, still with goblet and ciliate cells. The lamina propria, as the one in the trachea, is reach in elastic fibers, and is externally surrounded by a connective tissue with cartilaginous plates and smooth muscle. The decrease in caliber of these ducts is followed by a diminution in the dimension of the cartilaginous plates, and a increase in the amount of smooth muscle, relatively to the size of the duct. In the bronchioles this modifications go on further. The epithelium, which are in larger bronchioles of simple columnar, become simple cuboidal, with a decrease in the number of ciliate and goblet cells. As stated before, it is not found any more cartilage surround the tube, only smooth muscle. As the tube decrease in size, an even greater diminution is observed in the surrounding layer of connective tissue. The terminal bronchiole present a very thin wall and a cuboidal epithelium, with absence of goblet cells. In replacement of this latter type of cells, it is possible to observe others called Clara cells.
    THe respiratory bronchioles is consider to be as a part of respiratory portion of the system. They present an epithelium that is similar to the one studied in the terminal bronchioles, but now with the presence of alveolar sacs opening in their wall. Following the respiratory bronchioles, are the alveolar ducts. The alveolar ducts is a thin-walled tube which a succession of alveolar openings. Only in the septum between two alveolar sacs, it is possible to find traces of the duct wall, with cuboidal epithelium, and rare elastic and reticular fibers and smooth muscle as a lamina propria. The final portion of the respiratory system is composed by alveolar sacs and alveolus. The alveolus present a thin wall as a request for easy air passage. The epithelium that recovers the alveolus is of simple squamous type. Two types of cells are observed in this epithelium: the pneumatocyte I and the pneumatocyte II. The pneumatocyte I are highly flattened cells that cover the alveolar surface. The pneumatocyte II are round cells that are responsible for the production of a surfactant. These cells are found near the septa between the alveolar sacs. The space between 2 alveolar sacs, frequently is only interplace by blood capillaries, in a such way that the blood-air barrier consist only of the epithelium and endothelium cells and their respective basal lamina. In the portion of the septum, where no air exchange happens, it is possible to find reticular and elastic fibers, besides the fibroblasts, that are responsible for their production. Alveolar macrophages are observed in the interalveolar septa and alveolar lumina. these cells are responsible for the cleaning and protection of this ducts.