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
Cite this page: Ladoulis C., Pernick N. Anatomy & histology. PathologyOutlines.com website. https://www.pathologyoutlines.com/topic/anusanatomy.html. Accessed November 28th, 2024.
Anatomy
Anal canal:
Classic anatomic definition of anal canal:
Clinical AJCC definition of anal canal:
Histologic definition of anal canal:
Anal valves:
Anal cushions:
Anal verge:
Musculature of anal canal
Internal anal sphincter:
Intersphincteric longitudinal muscle:
External anal sphincter:
Regional lymph node drainage:
- 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
Images hosted on other servers:
Coronal section of rectum and anal canal
Columns of Morgagni and the anal valves
Inner wall of the lower end of the rectum and anus
Median sagittal section of male pelvis
Median sagittal section of female pelvis
Microscopic (histologic) images
Images hosted on other servers:
Anal / rectal junction and sphincter
