This low magnification
view of the ileum shows, from left to right: (1) The
mucosa, which is covered by a simple columnar epithelium supported
by a lamina propria of loose connective tissue. Finger-like extensions of
the mucosa penetrate the lumen of the intestine as villi,
which consist of a fattened cylinder of lamina propria covered by epithelium.
At the bases of the villi are tubular intestinal glands, called the crypts
of Lieberkuhn, which are lined by simple columnar epithelium and surrounded
by lamina propria. The muscularis mucosae of smooth muscle, which is very
thin and even obliterated in places by lymphoid tissue, forms the inner boundary
of the mucosa. (2) The submucosa, a thin layer
of loose connective tissue often containing, as shown here, Peyer's
patches, which are aggregations of lymphoid cells located to one side
of the ileum (usually the side of the ileum opposite to the attachment to
the mesentery). The patches are often so large they span the width of the
submucosa and mucosa and distort the crypts and villi, as seen here. The lighter-staining
areas in the center of some of the Peyer's patches are germinal centers.
Also present in the submucosa are ganglia, called Meissner's
plexus. (3) The muscularis externa, composed of an inner circular
and outer longitudinal layer of smooth muscle. (4) The
serosa, a thin layer of loose connective tissue covered
by a mesothelium.
