Histological section of a human fetal lung at the stage of 14 weeks of gestation. Staining with an anti CD81 antibody (member of the tetraspanin family), in brown, and with hematoxylin (for nuclear DNA), en blue. Magnification: 150x. At this stage the lung is immature and the future airways (e.g. bronchi) look like glands. The parenchyma and the forming cells (visualized by their nuclei in blue) are in monolayer, without differentiation The epithelium (colored in brown with an anti CD81 (specific) antibody is in monolayer, not stratified and undifferentiated since there is no distinct cell types such as mucous, brush, etc. Some of the cells are in mallow a color resulting from the overlay of the nuclei which are in blue with the cytoplasm (at the sites of antibody signal).