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Anus & perianal area

General

Anatomy & histology

Authors: Charles Ladoulis, M.D., Nat Pernick, M.D.

Last author update: 1 May 2012

Last staff update: 13 October 2021

Copyright: 2002-2024, PathologyOutlines.com, Inc.

PubMed Search: Anus perianal area anatomy

Table of Contents

Anatomy | Histology | Diagrams / tables | Microscopic (histologic) images

Cite this page: Ladoulis C., Pernick N. Anatomy & histology. PathologyOutlines.com website. https://www.pathologyoutlines.com/topic/anusanatomy.html. Accessed December 30th, 2024.

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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