TRACHEA - Slide 98 ("cracked" background = methacrylate embedding medium).
Diagnostic in the identification of the upper respiratory tree is the presence of hyaline cartilage in the wall. The C-shaped cartilage rings of the trachea are thickest anteriorly and taper off posteriorly, with interlacing bands of smooth muscle (called the trachealis muscle) bridging the gap between the two ends. Another diagnostic feature of trachea is the pseudostratified ciliated columnar epithelium (or respiratory epithelium) lining the lumen.Slide 98 (monkey) shows a complete (or nearly completed) transverse cut through a cartilage ring; if you see separate pieces of cartilage, the section has been sufficiently tangentially cut to catch pieces of neighboring rings. Determine the antero-posterior axis of the tracheal lumen. The smooth muscle in the posterior wall may overlap the ends of the cartilage ring because of contraction at the time of fixation. In the mucosa look for the typical pseudostratified epithelium, underlaid by a loose connective tissue lamina propria. Note the epithelial goblet cells, the ciliated surface, the unusually thick basement membrane, and the high population of eosinophils in the lamina propria.
The lamina propria merges with the somewhat denser connective tissue submucosa, which contains mucous or serous glands and many blood vessels. Although there are great numbers of elastic fibers in the submucosa, they are not differentially visible in these H&E preparations. The cartilage is surrounded by a dense, collagenous perichondrium. The outermost layer of the tracheal wall is the connective tissue adventitia.