Page 611 - Atlas of Histology with Functional Correlations

Figure 15.16 | Anorectal Junction (Longitudinal

Section)

The portion of the anal canal above the anorectal junction (7) represents the
lowermost part of the rectum. The part of the anal canal below the anorectal

junction (7) shows the transition from the simple columnar epithelium (1) to
the stratified squamous epithelium (8) of the skin. The change from the rectal
mucosa to the anal mucosa occurs at the anorectal junction (7).

The rectal mucosa is similar to colon. The intestinal glands (3) are shorter
and spaced farther apart with the lamina propria (2) more prominent, diffuse

lymphatic tissue more abundant, and lymphatic nodules (11) more numerous.

The muscularis mucosae (4) and the intestinal glands (3) terminate at the
anorectal junction (7). The lamina propria (2) of the rectum is replaced by the
dense irregular connective tissue of the anal canal (9). The submucosa (5)

merges with the connective tissue of the anal canal, a highly vascular region. The
internal hemorrhoidal plexus (10) of veins lies in the mucosa of the anal canal.
Blood vessels from this region continue into the submucosa (5) of the rectum.

The circular smooth muscle layer of the muscularis externa (6) increases in

thickness in the upper region of the anal canal and forms the internal anal
sphincter (6). Lower in the anal canal, the internal anal sphincter (6) is replaced
by skeletal muscles of the external anal sphincter (12). External to the external
anal sphincter (12) is the skeletal levator ani muscle (13).

FIGURE 15.16 ■ Anorectal junction (longitudinal section). Stain: hematoxylin
and eosin. Low magnification.

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