All orientations of respiratory bronchioles were included; however, the generation of each airway level was clearly identified based on position from the terminal bronchiole and/or first-generation respiratory bronchiole.
To increase the power of this analysis, only cases with three or more complete sets of respiratory bronchioles consisting of contiguous first-, second-, and third-generation bronchioles were used.
Parenchymal tissues arising from the apico-posterior and apico-anterior regions were examined for membranous and respiratory bronchioles.
These particles were particularly abundant and heavy in the adventitial wall of membranous bronchioles (Figure 4) and within the walls of first-generation respiratory bronchioles.
The distribution of particles and histologic features of the membranous and respiratory bronchioles were further analyzed in respiratory bronchioles, in longitudinal profile, which could be identified as either first, second, or third generation (Figure 5).
The results of the histologic and particle analysis of respiratory bronchioles by order of generation are given in Figure 6.
As a result of this sampling procedure, the importance of terminal and respiratory bronchioles as sites for particle retention and the association of particle retention with subtle but quantifiable changes in tissue remodeling have been clearly established.
Predominant pathological effects were also confined to the epithelial and interstitial tissue compartments of the respiratory bronchioles, forming the transitional zone between the conducting airways and gas-exchange regions of the lungs.
Analytical electron microscopic studies by Churg and colleagues (23) have revealed that the concentration of particles in respiratory bronchioles can be 25-100 times greater than the concentration of particles in the mainstem bronchus (23).
This region of the lung is anatomically distinct, as the respiratory bronchioles have some of the same properties of conducting airways as well as serving a gas-exchange function (25).
Furthermore, we show that the principal sites of deposition of ambient particles and associated tissue remodeling are the terminal bronchioles and first-generation respiratory bronchioles.