This image of the duodenum
seen in cross section is formed from a composite of several micrographs. The duodenum,
the first and shortest portion of the small intestine, exhibits the four-layered
structural organization as the other regions of the intestine. The innermost layer,
which faces the lumen (Lu), is the mucosa
(Mu). Finger-like extensions of the mucosa, called villi
(Vi), extend into the lumen and increase the surface area
for absorption. The bases of the villi are surrounded by the openings of intestinal
glands, also called crypts
of Lieberkuhn (Cr). These branched tubular glands extend
all the way to the muscularis
mucosae (MM), the thin layer of smooth
muscle that marks the boundary between the mucosa and the underlying submucosa
(Su). The submucosa of the duodenum differs from the rest
of the small intestine in that it contains large numbers of tubuloalveolar glands,
called Brunner's glands (BG). Isolated aggregations
of lymphoid
cells (Ly) may be present, such as the one seen on the left
side of this image. The lymphoid cells extend from the submucosa into the mucosa,
breaching the muscularis mucosae. The
outer wall of the duodenum is formed by the muscularis
externa (ME), which consists of two layers of smooth muscle
- an inner circumferential layer and an outer longitudinal layer. The outermost
layer is the serosa
(Se),
which consists of a thin layer of loose connective tissue covered by a simple
squamous epithelium of mesothelial cells.
