LUNG - Slide 99 and 100

Check both slides under low power to see whether they contain sections of some of the larger intrapulmonary segments of the respiratory tree.  Look also at the relative degree of inflation of the alveoli.  The lung has a very open, lacy appearance that is easy to identify when fully inflated, but you want to be able to recognize it in any condition.  The presence of bronchioles, etc., is diagnostic.  When you find likely areas with low magnification, check each one with high power for details of walls and cell contents.  Although your sections are not stained specifically for it , you should realize that there are a great many elastic fibers through  the respiratory walls.

In identifying different segments of the respiratory tree, look for the following features, using oil for cellular details in the alveoli:

1.            intrapulmonary bronchus - has pieces of hyaline cartilage, interlacing bundles of smooth muscle, submucosal glands (probably) and a definite connective tissue adventitia.  In successively smaller bronchi the mucosal epithelium changes from pseudostratified ciliated columnar type, with goblet cells.  Cartilage plates are smaller and more scattered.

2.            bronchiole - has no cartilage, a fair amount of smooth muscle, and hardly any surrounding connective tissue.  The epithelium gradually becomes lower and loses both goblet cells and cilia.  A  terminal bronchiole is simply the last one before the respiratory bronchioles begin.

3.            respiratory bronchiole - has a low simple cuboidal epithelium and a thin coat of interlaced smooth muscle and elastic fibers.  More important, it has thin outpocketings in its wall: the first respiratory alveoli, where gaseous exchange can take place.  Try to find (with lower power) some area where the respiratory tree is cut in such a fashion as to show continuity from a respiratory bronchiole to alveolar ducts, alveolar sacs, and alveoli.  Often this will be toward the outer edge of your section.

4.            alveoli - have a simple squamous epithelial lining and only the smallest amount of reticular and elastic connective tissue sandwiched between the epithelial surfaces of adjacent alveoli.  These fibers support numerous capillaries which make a vascular meshwork between alveoli.  Where the tissue is favorable, look for engorged capillaries.  The nuclei you see in the alveolar walls belong variously to endothelial cells, simple squamous lining cells, septal cells, and alveolar macrophages (the "dust" cells).  (See terminology below.)

Slide 99 (rabbit) is particular good for alveolar capillaries with well preserved red blood cells showing their typical biconcave shape.  There are also many eosinophils inside and outside the capillaries.  (There has been some extravasation of  RBC's during handling of tissue also.)  Note the presence of quite large macrophages, which lie free in the alveolar spaces and can be clearly seen.