Type I pneumocytes are extremely thin epithelial cells lining the alveoli of the respiratory tree. Although there as many type II pneumocytes as type I within each alveolus, type I occupies up to 95% of the surface area due to its arrangement into a squamous layer.
Type I cells, joined to each other by tight junctions, form specialised connections with underlying capillaries at the thin portions of the alveolar-capillary complexes. It is at the complexes that the type I cells are in close enough proximity to the capillaries - 0.5 micrometres - to permit gaseous diffusion which is their key function.
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