Surfactant in newborn lung

A section through part of an air sac (alveolus) in the lung of a new-born lamb viewed with a transmission electron microscope (TEM). At the bottom left is one of the cells which line the alveolus. The cell contains sacs of material which will go on to form surfactant, a substance that helps to expand the air spaces in the lungs to allow breathing, here seen top right.