Fig. 4.1
Histological zones of the anal canal
Histological Zones of the Anus
The anal canal is divided into three separate histological zones according to the type of lining mucosa: the colorectal zone, the anal transitional zone and the squamous zone [9 11]. These zones can be roughly mapped onto the anatomical landmarks of the anus, but there is some variation both between and within individuals [21].
The Colorectal Zone
The anal colorectal zone is lined by large intestinal-type mucosa and blends imperceptively with the lower rectal mucosa. The mucosa of the colorectal zone typically shows more crypt architectural distortion and shorter crypts than more proximal large intestinal mucosa with frequent branched/bifid crypts, and there are often mild mucosal prolapse-type features in this area (these include mild angulation of crypt outlines and mild fibromuscularisation of the lamina propria with vertically orientated bands of smooth muscle extending up between crypts). There are also changes in the type of mucin with goblet cells of the anorectum predominantly producing sialomucins rather than the sulphomucins found in the colorectum [6]. The colorectal zone starts at around the level of puborectalis and is typically 1–2 cm in length. Its mucosa merges with the mucosa of the anal transitional zone above, but close to, the dentate line.
The Transitional Zone
The anal transitional zone is lined by transitional-type mucosa and merges with the squamous zone below. There is an abrupt change from transitional- to colonic-type mucosa at the junction with the colorectal zone above. In some areas the transitional zone may be absent with large intestinal-type mucosa of the colorectal zone directly abutting squamous mucosa of the squamous zone. Other names for the transitional zone include the “intermediate zone” and the “cloacogenic zone”. The anal transitional zone typically starts just above the dentate line, approximately 10 mm above the lower border of the internal sphincter, and is on average 5 mm in length, ranging between 3 and 11 mm [21]. The anal transition zone contains anal ducts which connect with the anal glands (see The Anal Blood Supply).
The anal transitional epithelium is normally 4–9 cell layers thick. The basal cells are small and perpendicularly arranged to the basement membrane. Cells at the surface of the anal transitional mucosa can be columnar, cuboidal, flattened or polygonal. The columnar cells produce small volumes of mucin. Foci of squamous differentiation and occasional colonic-type glands may also be present. Endocrine cells are scattered in the transitional epithelium, and occasional melanocytes may be present [11] but Langerhan’s cells are absent [12]. Similar to transitional epithelium in other regions, such as the bladder, anal transitional epithelium stains positively with CK 19 [24]. The transitional zone merges with the anal squamous zone below typically just above the level of the dentate line.
The Squamous Zone
The anal squamous zone (sometimes referred to as the “smooth” zone) anatomically corresponds to the pecten of the anal canal [10]. It is lined by non-keratinising squamous epithelium which merges at its lower border with perianal skin. The squamous zone lacks well-formed dermal papillae and skin appendages/adnexal structures. The squamous zone also lacks mucin-producing cells but melanocytes are present. As the epithelium approaches and merges with perianal skin, there is a gradual change in its cellular constituency with an increase in the number of melanocytes [4] and Langerhan’s cells [12].
Mucosa-Associated Lymphoid Tissue of the Anus
There is mucosa-associated lymphoid tissue distributed throughout the large intestine but the amount varies markedly depending on the location. Lymphoid follicles are seen in the mucosa of the anal canal from the dentate line upwards [15].
In the anorectal region, the mucosa-associated lymphoid tissue can form a polypoidal structure which is known as a rectal tonsil [8]. This is more commonly seen in young children and adolescents. The aetiology of this is uncertain in most cases but the literature suggests that some of these cases may be linked to anorectal infections such as chlamydia [5]. Knowledge of this “normal” structure is important to avoid its misdiagnosis as lymphoproliferative disease.
The Anal Glands and Ducts
The anal glands drain via ducts which open into the anal transitional zone. Approximately 80 % of anal glands lie in the submucosa; however, the remainder of the anal glands are situated more deeply in the anal wall within the internal sphincter, the intersphincteric space and even the external sphincter [19]. Infection within anal glands is considered to be the main aetiological factor in the development of perianal fistulae [18].
The epithelial lining of anal glands is very similar to that of the anal transitional zone but a characteristic feature of the epithelium is the presence of intra-epithelial microcysts [11].
The Anal Musculature
The anal muscular wall is composed of the internal and external anal sphincters. The internal anal sphincter is a continuation of the muscularis propria of the rectum to which it has a similar structure although it is considerably thicker. The external anal sphincter is formed of striated muscle, is said to be composed of superficial and deep components and surrounds the internal sphincter.
The Anal Blood Supply
The anus has a rich arterial blood supply which is predominantly derived from the superior rectal artery (a direct continuation of the inferior mesenteric artery) but also receives contributions from the middle and inferior rectal arteries (branches of the internal iliac/pudendal arteries). In the upper anal canal, above the dentate line, the anal submucosa contains a rich vascular plexus known as the anorectal vascular plexus which includes numerous arteriovenous connections. This is not evenly distributed throughout the upper anal canal but concentrated to form three anal cushions (left lateral, right anterior and right posterior) which have erectile tissue-type properties and are thought to be involved in the maintenance of continence. The anal cushions are also thought to be important in the pathogenesis of haemorrhoids [1, 22].
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