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Anus and perianal area

General

Anatomy


Reviewers: Charles Ladoulis, M.D. (see Reviewers page)
Revised: 13 April 2013, last major update May 2012
Copyright: (c) 2002-2012, PathologyOutlines.com, Inc.

General
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Anal canal:
● Tubular structure 3-4 cm long
● Derived from cloaca (distal hindgut) and arises at level of prostatic apex, is directed downward and backward, and ends at anus
● Boundaries are proximal and distal margins of internal sphincter muscle, and includes part of rectum
● Embryologically divided by urogenital septum (cloacal membrane) into anterior GU and posterior GI compartments, and separated from perianal ectoderm by anal membrane, which ruptures at week 7 of gestation

Classic anatomic definition of anal canal:
● Between proximal and distal margins of internal sphincter muscle, which includes part of rectum

Clinical AJCC definition of anal canal:
● Begins at puborectalis sling at apex of anal sphincter complex (palpable as anorectal ring, but difficult for pathologists to identify)
● Ends at squamous mucocutaneous junction with perianal skin; includes 1-2 cm of rectal-type glandular mucosa and possibly transitional mucosa at dentate line

Histologic definition of anal canal:
● Anal transitional zone and squamous epithelium down to the perianal skin; cannot be identified by clinicians
● Note: “columns”, “valves” and “sinuses” below are macroscopic landmarks, which may not correspond precisely to microscopic structures

Anal columns of Morgagni: longitudinal folds just distal to dentate line, analogous to lower rectum’s rectal columns of Morgagni; less pronounced in adults
Anal papillae: raised toothlike projections on anal columns; extend proximally into rectum
Anal sinuses of Morgagni: depressions between anal columns
Anal crypts of Morgagni: minute pockets with anal valves as boundary; site of discharge of anal glands

Anal valves:
● Also called semilunar valves or transverse plicae
● Connect distal ends of anal columns
● Identifiable in children, often obscured in adults

Anal cushions:
● Normal structures of anal canal that contribute to anal closure by close apposition to each other
● Contain blood vessels, connective tissue, smooth muscle; vessels contain abundant smooth muscle
● Resemble erectile tissue due to numerous arteriovenous communications

Anal verge:
● Also called Hilton’s line or anal margin;
● Junction between anal canal and anal skin
● Mucosa contains cutaneous adnexae

Corpus cavernosum recti: network formed by peculiar vessels with a complex convoluted appearance
Dentate (pectinate) line: midpoint of anal canal, formed by anal valves; circumferential musculature of canal


Musculature of anal canal

General
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Muscularis mucosa: continues from rectum through upper anal transitional zone
     ● Presence of muscle fibers in lamina propria indicates mucosal prolapse syndrome
Musculus submucosae ani: fibers from intersphincteric longitudinal muscle which pass through internal sphincter and from the internal sphincter itself; form a network around the vascular plexus

Internal anal sphincter:
● Continuation of circular muscle of rectum, but thicker (5-8 mm); ends 5-19 mm below dentate line

Intersphincteric longitudinal muscle:
● Between internal and external sphincters
● Contains fibers from longitudinal muscle layer of rectum and levator ani muscles
● Distally breaks up into septa that diverge fan-wise through subcutaneous layer of external sphincter and ends in corium, which forms characteristic corrugation of perianal skin

External anal sphincter:
● Consists of superficial, subcutaneous and deep parts; provides voluntary control of defecation

Regional lymph node drainage:
Above dentate line - anorectal, perirectal, paravertebral nodes
Below dentate line - superficial inguinal nodes

Arterial supply: superior, middle and inferior rectal arteries
Venous supply: superior rectal vein

Diagrams
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Diagrams

End of Anus and perianal area > General > Anatomy


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