STEVE GSCHMEISSNER / SCIENCE PHOTO LIBRARY STEVE GSCHMEISSNER / SCIENCE PHOTO LIBRARY
Fetal lung, light micrograph of a section from a human fetal lung at 34 weeks. At low magnification, normal fetal lung appears much more cellular than the adult lung. The alveoli (purple) have not developed completely and there are developing bronchi (yellow) present with surrounding blue cartilage. In immature lung, the air space walls are thicker than those in mature lung due to the taller epithelium and abundant loose connective tissue. When the lung expands with the first breath at birth, the alveolar lining cells assume their characteristic flattened (squamous) shape. The interstitium contains a loose mesenchymal connective tissue. Magnification: x 120 when printed at 10 centimetres wide.
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