Histology Of PULMONARY ALVEOLI:
These open directly into alveolar sacs or alveolar ducts or respiratory bronchioles and are lined by two types of squamous epithelial cells (pneumocytes).
I. Type-l cells:
Squamous alveolar cells are most numerous (97%) and form a very thin but complete lining of all alveoli. These cells possess a low flat nucleus and minimum cytoplasm. These possess microvilli at their alveolar surface.
These have thin basement membrane, which fuses with basement membrane of endothelium of blood capillaries. These cannot divide. Main role of these cells is to provide a barrier of minimal thickness that is readily permeable to gases as well as these contain pinocytotic vesicles suggesting a probable role in surfactant turnover.
II. Type-ll Cells/Great Alveolar Cells/Secretory Cells/ Septal Cells (3%):
These are cuboldal In shape and occur in the form of groups present at corners of alveolar walls. Luminal surfaces of these cells bear microvilli. Their cytoplasm gives a vacuolated appearance due to presence of lamellar bodies of 1-2 nm in diameter (cytosomes), which contain phospholipids, proteins and glycosaminoglycans. These substances are secreted into alveoli as surfactant.
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