Anus & perianal area

General

Anatomy & histology



Topic Completed: 1 May 2012

Minor changes: 13 October 2021

Copyright: 2002-2021, PathologyOutlines.com, Inc.

PubMed Search: Anus perianal area anatomy

Nat Pernick, M.D.
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Cite this page: Ladoulis C., Pernick N. Anatomy & histology. PathologyOutlines.com website. https://www.pathologyoutlines.com/topic/anusanatomy.html. Accessed December 6th, 2021.
Anatomy
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 rectums 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 Hiltons 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
  • 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
Histology
  • Lacks a peritoneal covering
  • Three histologic types: glandular (proximal), transitional (also called intermediate, cloacogenic) and keratinized or nonkeratinized squamous (distal)
  • Anal glands and transitional zone epithelium are CK7+ / CK20-, different from colorectal carcinoma (CK7- / CK20+, Arch Pathol Lab Med 2001;125:1074)
  • Notes: ganglion cells are normally absent 1 - 2 cm above dentate line (important for Hirschsprung's disease biopsies); multinucleated stromal cells are common (may be fibroblasts)

Proximal colorectal zone:
  • Top of puborectalis to dentate line
  • Glandular and transitional mucosa
  • 1 - 2 cm long
  • Similar to rectal mucosa but with shorter more irregular crypts, more smooth muscle fibers in lamina propria

Anal transitional zone (ATZ):
  • 0.3 cm to 1.1 cm
  • Zone between uninterrupted columnar mucosa above and uninterrupted squamous epithelium below
  • Wrinkled, glistening appearance
  • Transitional epithelium resembles urothelium (small basal cells with nuclei perpendicular to basement membrane, columnar, cuboidal, polygonal or flat) with 4 - 9 cell layers, minimal mucin production
  • Not highly specialized and may incorporate features of both urothelium and squamous epithelium (Hum Pathol 1978;9:579)
  • Contains anal glands in submucosa, also endocrine cells, rare melanocytes
  • Expresses CK7 and CK19 but not CK20

Lower distal zone:
  • Dentate line to squamous mucocutaneous junction: nonkeratinizing squamous epithelium without skin appendages, without glands
  • Contains melanocytes
  • Anal papillae contain squamous mucosa that joins rectal mucosa
  • Squamous mucosa merges with perianal skin (with keratin, hair follicles and apocrine glands) at anal verge / anal margin
Diagrams / tables

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Coronal section of rectum and anal canal

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Columns of Morgagni and the anal valves

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Inner wall of the lower end of the rectum and anus

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Median sagittal section of male pelvis

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Median sagittal section of female pelvis

Microscopic (histologic) images

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Anal / rectal junction and sphincter

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