The appendix (vermiform appendix) is typically described as a wormor fingerlike structure [L. vermis, worm; forma, form]. It arises from the cecum (the first segment of the large intestine; the others, in order, are the ascending, transverse, and descending colon; the sigmoid colon; the rectum; and the anal canal) and forms a blind-ending tube ranging from 2.5 cm to as much as 13 cm in length (average length of ~8 cm). Because it is a blind-ended pouch, intestinal contents may be trapped or sequestered in the appendix, often leading to inflammation and in-fection. In infants and children it is both relatively and absolutely longer than in adults and contains numerous lymphatic nodules, suggesting that it has an immunologic role. Recent evidence indicates that it (and the cecum and terminal ileum) may be the “bursa equivalent” in mam-mals, i.e., the portion of the immature immune system in which potential B lymphocytes achieve immunocompetence (equivalent to the bursa of Fabricius in birds). The wall of the appendix is much like that of the small intestine, having a complete longitudinal layer of muscularis externa, but it lacks both plicae circulares and villi. Thus, the mucosa is similar to that of the colon, having simple glands. Even this resemblance is often obliterated, however, by the large number and size of the lymphatic nodules that usually fuse and extend into the submucosa. In later life, the amount of lymphatic tissue in the appendix regresses, and there is a consequent reduction in size. In many adults, the normal structure is lost, and the appendage is filled with fibrous scar tissue.
Appendix, human, H&E ×25. mucosa (Muc), submucosa (Subm), muscularis externa (ME ), and serosa (S ) are identified.
Cross section of an appendix from a preadolescent, showing the various structures composing its wall. The lumen (L),
Appendix, human, H&E ×80; inset ×200.
This micrograph is a higher magnification of the boxed area in figure above. It reveals the straight tubular glands (Gl ) that extend to the muscularis mucosae. Below is the submucosa (Subm) in which the lymphatic nodules (LN ) and considerable diffuse lymphatic tissue are present. Note the distinct germinal centers (GC ) of the lymph nodules and the cap region (Cap) that faces the lumen. The more superficial part of the submucosa blends and merges with the mucosal lamina propria because of the numerous lymphocytes in these two sites. The deeper part of the submucosa is relatively de void of lymphocyte infiltration and contains the large blood vessels (BV ) and nerves. The muscularis externa (ME ) is composed of a relatively thick circular layer and a much thinner outer longitudinal layer. The serosa (S ) is only partially included in this micrograph.
The inset is a higher magnification of the rectangular area in lower figure. Note that the epithelium of the glands in the appendix is similar to that of the large intestine. Most of the epithelial cells contain mucin; hence, the light appearance of the apical cytoplasm. The lamina propria, as noted, is heavily infiltrated with lymphocytes, and the muscularis mucosae at the base of the glands is difficult to recognize (arrows).
KEY BV, blood vessel Cap, cap of lymphatic nodule GC, germinal center Cl, gland L, lumen LN, lymphatic nodule ME, muscularis externa Muc, mucosa S, serosa Subm, submucosa arrows, muscularis mucosae at base of glands