1. Type I cells. Also called type I alveolar cells, type I pneumocytes, and squamous alveolar cells, these are squamous epithelial cells that make up 97% of the alveolar surfaces.
They are specialized to serve as very thin (often only 25 nm in width) gas-permeable components of the blood-air barrier. Their organelles leg, Golgi complex, endoplasmic reticulum, mitochondria) cluster around the nucleus.
Much of the cytoplasm is thus unobstructed by organelles, except for the abundant small pinocytotic vesicles that are involved in the turn over of pulmonary surfactant and the removal of small particles from the alveolar surfaces. They attach to neighboring epithelial cells by desmosomes and occluding junctions.
The latter reduce pleural effusion--leakage of tissue fluid into the alveolar lumen. Type I cells can be distinguished from the nearby capillary endothelial cells by their position bordering the alveolar lumen and by their slightly more rounded nuclei.
2. Type II cells. These cells, which are also called type II alveolar cells, type II pneumocytes, great alveolar cells, and alveolar septal cells, cover the remaining 3% of the alveolar surface. They are interspersed among the type I cells, to which they attach by desmosomes and occluding junctions.
Type II cells are roughly cuboidal with round nuclei; they occur most often in small groups at the angles where alveolar septal walls converge. At the electron microscope level, they contain many mitochondria and a well-developed Golgi complex, but they are mainly characterized by the presence of large (0.2-um), membrane-limited lamellar (mutlilamellar) bodies. These structures, which exhibit many closely apposed concentric or parallel membranes (lamellae), contain phospholipids, glycosaminoglycans, and proteins.
Type II cells are secretory cells. Their secretory product, pulmonary surfactant, is assembled and stored in the lamellar bodies, which also carry it to the apical cytoplasm. There, the bodies fuse with the apical plasma membrane and release surfactant onto the alveolar surface. 3. Alveolar marcrophages. Known also as dust cells, these large monocytc-derived repre sentatives of the mononuclear phagocyte system are found both on the surface of alveolar septa and in the interstitium. Macrophages are important in removing any debris that escapes the mucus and cilia in the conducting portion of the system.
They also phagocytose blood cells that enter the alveoli as a result of heart failure. These alveolar macrophagcs, which stain positively for iron pigment (hemosiderin), are thus designated heart failure cells.
Alveolar Cell Types:
BRONCHIAL TREE - 2
D. Bronchioles: These are branches of the smallest bronchi. The largest bronchioles differ from the smallest bronchi only by the absence of cartilage and glands in their walls. Large bronchioles are lined by typical respiratory epithelium; as they branch further, the epithelial height and complexity decrease to simple ciliated columnar or cuboidal. Each bronchiole gives rise to 5-7 terminal bronchioles.
E. Terminal Bronchioles: The smallest components of the conducting portion of the respiratory system, these are lined by ciliated cuboidal or columnar epithelium and have few or no goblet cells. The lining here also includes dome-shaped cilia-free Clara cells, whose cytoplasm contains glycogen granules, lateral and apical Golgi complexes, elongated mitochondria, and a few secretory granules. The function of these cells is unclear. Each terminal bronchiole branches to form 2 or more respira tory bronchioles.
F. Respiratory Bronchioles: These are the first part of the respiratory portion, with a cuboidal epithelial lining which resembles that of the terminal bronchioles but which is interrupted by thin-walled saccular evaginations called alveoli. The number of alveoli increases as the respiratory bronchioles proceed distally. As the alveoli increase in number, the cilia decrease until they disappear. Goblet cells are absent.
G. Alveolar Ducts: These are simply the distal extensions of the respiratory bronchioles where the alveoli are so dense that the wall consists almost entirely of these sacs, and the lining has been reduced to small knobs of smooth muscle covered by cilia-free simple cuboidal cells.
H. Atria and Alveolar Sacs: Atria are the distal terminations of alveolar ducts. The arrangement is comparable to a long hallway (alveolar duct) leading to a rounded foyer (atrium). The foyer has small doorways leading to some small rooms (alveoli), but also has 2 or more larger doorways leading into short, dead-end hallways (alveolar sacs). The short hallways are also lined by small rooms (alveoli). Put simply, the difference between atria and alveolar sacs is that the atria open into alveolar ducts, alveoli, and alveolar sacs, while the alveolar sacs open only into alveoli and atria.