(Left) The ducts, some vessels and lung alveoli are shown in three dimensions.

(Right) The same structures are shown in a longitudinal section (see also Figure 10.15).

The terminal bronchiole is lined with a simple epithelium sitting on a continuous layer of smooth muscle fibres. It is continuous with a respiratory bronchiole whose duct is lined with a simple epithelium composed of Clara cells (see Figures 10.16 to 10.19). This duct is perforated by widely separated pores that open into lung alveoli.

The respiratory bronchioles are continuous with branching alveolar ducts. These alveolar ducts are lined with Clara cells sitting on bundles of smooth muscle fibres arranged in the form of a chicken wire net (bottom left). Through the openings of such networks, the lumina of the alveolar ducts communicate with the surrounding lung alveoli.

(Right) The wall of branching alveolar ducts appears as a series of knobs, each composed of a fascicle of smooth muscle cells seen in cross section.

(Left) The alveolar ducts are shown to connect with grapelike clusters of spheroidal lung alveoli.

(Top left) This field shows a respiratory bronchiole (B) connected to branching alveolar ducts (*) of decreasing size.

These ducts are delimited by knobs of smooth muscle fibres (arrowheads). In three dimensions these knobs are parts of networks as illustrated in Figure 10.14.

These alveolar ducts open into groups of interconnected alveoli (Al).

A large vessel (V) next to the bronchiole is also labelled.

Coloration: Hématoxyline ferrique-É
Grossissement: X60