Sometimes, in a cross section through the intestine, a plica displays a clear cross-sectional profile such as that shown here. Note, again, that the submucosa (SM ) constitutes the core of the plica. Although many of the villi in this figure present profiles (V ) that would be expected if the villus were a fingerlike projection, others clearly do not. In particular, one villus (marked with three asterisks) shows the broad profile of a longitudinally sectioned leaf like villus. If this same villus were cut at a right angle to the plane shown here, it would appear as a finger like villus.
separated by numerous lymphocytes close to the intestinal glands (Gl ). Clearly, the lymphocytes of the nodule are on both sides of the muscularis mucosae and, thus, within both the mucosa and the submucosa.
In places, the lymph nodule is covered by the intestinal epithelium. Whereas the nature of the epithelium cannot be appreciated fully in the light microscope, electron micrographs (both scanning and transmission) have shown that among the epithelial cells are special cells, designated M cells, that sample the intestinal content (for antigen) and present this antigen to the lymphocytes in the epithelial layer.
KEY Gl, intestinal glands LN, lymphatic nodules ME, muscularis externa MM, muscularis mucosae MM??, presumptive location of muscularis mucosae SM, submucosa V, villi asterisks, leaf like villus
PLATE 61 • ILEUM MMMMMM??MM??GIGIMEMESMSMLNLNV LN SM ME V MEMEME SMSMSM V GI MM?? MM GIGIGI ** ** **
The principal functions of the colon are reabsorption of electrolytes and water and elimination of undigested food and other waste. The mu-cosa has a smooth surface; neither plicae circulares nor villi are present. Numerous simple glands (crypts of Lieberkn) extend through the full thickness of the mucosa. The glands, as well as the surface, are lined with a simple columnar epithelium that contains goblet cells, absorp-tive cells, and enteroendocrine cells but does not normally contain Paneth cells. Here, too, stem cells are restricted to the bottoms of the glands (crypts), and the normal zone of replication extends about one third of the height of the crypt.
Colon, monkey, H&E ×30.
A cross section through the large intestine is shown at low magnification. It shows the four layers that make up the wall of the colon: the mucosa (Muc), the submucosa (SubM ), the muscularis externa (ME ), and the serosa (S ). Although these layers are the same as those in the small intestine, several differences should be noted. The large intestine has no villi, nor does it have plicae circulares. On the other hand, the muscularis externa is arranged in a 622 distinctive manner, and this is evident in the photomicrograph. The longi-
Mucosa, colon, monkey, H&E ×140.
The mucosa, shown at higher magnification, contains straight, unbranched, tubular glands (crypts of Lieberkn) that extend to the muscularis mucosae (MM). The arrows identify the openings of some of the glands at the intestinal surface. Gener-
Lamina propria, colon, monkey, H&E ×525.
This figure reveals the muscularis mucosae (MM) and the cells in the lamina propria (LP), many of which can be recognized as lymphocytes and plasma cells. The smooth muscle cells of the muscularis mucosae are arranged in two layers. Note that the smooth muscle cells marked by the arrowheads
Intestinal glands, colon, monkey, H&E ×525.