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The Anal Canal Epi- Thelium at the Pectinate Line

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The Anal Canal Epi- Thelium at the Pectinate Line Okajimas Folia Anat., Jpn., 64 (4): 203-222, October, 1987 Scanning Electron Microscopic Observations on the Epithelium Lining the Mouse Anorectal Region By Takashi NAKANO and Hiroshi MUTO Department of Anatomy, Aichi Medical University, Nagakute, Aichi 480-11, Japan -Received for Publication, June 26, 1987- Key words: Epithelium, Pectinate line, Rectum, Anal canal, Mouse Summary: In most cases, the rectal epithelium was abruptly changed into the anal canal epi- thelium at the pectinate line. In some cases, however, just proximal to the pectinate line there existed the transitional zone between the rectal epithelium and the anal canal one The epi- thelium occupying the transitional zone consisted of cuboidal cells with bulging surfaces or flattened cells. It is considered that the epithelium represents various stages which the rectal columnar epithelium transforms into the stratified squamous one. The epithelium lining the mouse anal canal showed an irregular paving-stone-like appearance, which was a typical surface appearance of stratified squamous epithelium, throughout from the anal margin to the pecti- nate line. The width of the microridges in the distal part of the anal canal was about doubled compared to that in the proximal part. It is suggested that the epithelium in the distal part is keratinized, while that in the proximal part is nonkeratinized. The junction between the rectal epithe- importance described above, there is still lium and the anal canal one, denominating some uncertainty regarding the epithelium the anorectal junction, represents the comb- involving the anorectal junction and the anal like "pectinate line". The pectinate line is canal. This paper reports some of the fin- the former site of the anal membrane which dings of the epithelium lining the anorectal embryonically divided the ectodermal pro- region, and is expected to contribute to- ctodeum from the entodermal hindgut. It wards further understanding of histology has been pointed out that some types of of this region. carcinoma involving the anorectal junction show the wide variation in histopatholo- Materials and Methods gical pattern, which can not readily be explained on the basis of origin either from The mucous membrane in the anorectal the columnar epithelium lining the rectum region was removed from 30 ddN mice (12 or from the stratified squamous one covering male and 18 female) under a dissecting the anal canal (Grinvalsky and Helwig, 1956; microscope. Scanning electron microscopic Takano, 1978). Furthermore, the epithelium specimens were fixed with glutaraldehyde lining the human anal canal is closely related in phosphate buffer, postfixed with 2% to the diseases and congenital abnormalities osmium tetroxide, immersed in 2% tannic in the anorectal region. acid, and stained with 2% osmium tetroxide. In spite of its embryological and clinical They were dehydrated in graded ethanol. 203 204 T. Nakano and H. Muto After replacement with isoamyl acetate junction between the rectal epithelium and and drying at critical point with liquid the anal canal one (the anorectal junction) CO2 , they were coated with gold and can be defined, because the rectal epithelium observed under a Jeol-U3 scanning electron with a dull appearance is abruptly changed microscope. into the anal canal epithelium which shows a shinky appearance when viewed in oblique light. Results Examination in the SEM reveals that the bases of the anal columns form the anal The rectal epithelium near the anorectal papillae of various size. The anal papillae junction. give a somewhat serrated or comblike The surface of the rectal epithelium is appearance at the anorectal junction, being divided by deep furrows to form the anal the pectinate line (Fig. 6). The anal valves columns (Fig. 1). At higher magnifications, are absent. In most cases, at the pectinate the surface of the rectal epithelium presents line the rectal epithelium is abruptly a remarkably geometrical pattern comprising changed into the anal canal epithelium (Fig. numerous polygonal units, each with a 6). Occasionally, just proximal to the central crypt surrounded by epithelial pectinate line there exists the transitional cells and goblet cells. The goblet cells are zone between the rectal epithelium and the arranged radiately about the central crypt anal canal one. In some areas of the tran- (Fig. 2). The crypt is approximately 5-15 sitional zone, the superficial cells are cu- gm in diameter, and is surrounded by boidal, of various size and dissected by epithelial cells which appear bent down into depressions between the adjacent cells. the crypt, like a funnel. Many goblet cells They bulge out from the surface to a varying are interspersed among the epithelial cells degree, presenting a cobblestone appearance on the crypt wall (Fig. 3). (Figs. 7 and 8). In other areas, the superficial The junction of the adjacent epithelial cells appear to be relatively flat (Fig. 7). cells is easily identified by distinct ridges. The surfaces of individual cells in the tran- Such ridges revealed epithelial cells to be sitional zone have microvilli and micro- polygonal in surface outlines with irregular ridges, of about 0.1-0.15 jim in width. pentagonal and hexagonal shapes predo- Various transitional forms between the minating. The surfaces of the epithelial microvilli and the microridges are observed; cells are covered with distinct microvilli, e.g., several microvilli lined up closely in of about 0.1-0.15 /IM in diameter (Figs. 4 a row to form a rosary-like disposition and 5). Hemispherical projections on the (Fig. 9). surface may represent the goblet cells bulging with mucus (Fig. 4). The goblet cells, which have presumably discharged The anal canal epithelium their contents, appear as holes 5 pm in At low magnification, the anal canal diameter (Figs. 2, 4 and 5). Occasionally, epithelium shows a typical surface appea- septal walls which divide the mucus in the rance of stratified squamous epithelium; filled goblet cells into various sections are i.e. the superficial cells appear to be re- seen in the holes (Figs. 4 and 5). latively flat and are arranged in an irregular paving-stone-like appearance with some cells partially overlying those beneath (Fig. 10). The anorectal junction Furthermore, superficial cells indicating Under the dissecting microscope, the Epithelium in the Anorectal Region 205 various stages of desquamating process are 1971; Atsumi, 1978; Ito, 1983) or that of observed. the proximal end of the internal anal sphin- Higher magnifications reveal that the cter muscle (Fenger, 1979; Fenger and surface of the anal canal epithelium is some- Knoth, 1981). On the other hand, many what different according to the region of clinicians regarded the pectinate (dentate) location. In the proximal part of the anal line as the proximal border of the anal canal, the surfaces of individual superficial canal, since the types of carcinoma derived cells have microridges of about 0.1-0.2 pm from the bowel proximal to and distal to width. In some cells, the microridges with the line differ (Hollinshead, 1962). Further- constricted portions are found giving an more, Onitsuka (1970) and Takano (1978) impression of a fusion of microvilli standing defined the proximal end of the "surgical close together (Fig. 11). In other cells, the anal canal" at the level of the anorectal microridges consist of fine linear ledge. ring, while that of the "anatomical anal Although the microridges are occasionally canal" at the pectinate line. The pectinate arranged parallel to the surface boundaries line represents the former site of the anal in the periphery of the cell, there is no membrane, which embryonically divided the specific pattern of arrangement. A few entodermal hindgut from the ectodermal branchings and anastomoses exist (Fig. proctodeum. Furthermore, the two parts 12). In the distal part of the anal canal, of the bowel proximal to and distal to the however, the microridges are approximately pectinate line have a different sensory 0.3-0.4 Am in width, being about twice the innervation, blood supply and lymphatic size of those in the proximal part (Figs. 13 drainage. and 14). Branchings and anastomoses are In this study, the term "anal canal" was more frequent. defined as the duct extending from the anal As the surface is traced distally, the margin to the pectinate line, considering microridges begin to show a tendency to not only the anatomical but the embryo- form adhesion and, therefore, the outlines logical findings. are not so- distinct. Near the anal margin, the surface shows relatively smooth appea- rance (Fig. 15). However, no typical honey- Anorectal junction As discussed above, in this study the term comb appearance of interconnections of anal canal is used for the duct extending microridges surrounding depressions are from the anal margin to the pectinate line; observed. i.e. the pectinate line corresponds to the At the anal margin, the anal canal epithe- lium is changed to the perianal skin with junction between the rectum and the anal numerous hairs (Fig. 16). canal. In the human fetus and newborn infant, at the pectinate line there is a sharp transition between the simple columnar Discussion epithelium lining the rectum and the strati- fied squamous one covering the anal canal Term "anal canal" (Hollinshead, 1962). However, in the adult For the purpose of this study, the limit the issue is more confused. Stonesifer et of the anal canal is of special importance. al. (1960) and Henrich (1980) reported that Some investigators defined that the human the simple columnar epithelium was directly anal canal extended as far as the level of the changed into the stratified squamous one at anorectal ring (Johnson, 1914; Gotoh et al., or near the pectinate line. On the other 206 T. Nakano and H. Muto hand, Goligher et al. (1955), Walls (1958) transitional zone consisted of cuboidal and Onitsuka (1970) reported that the cells with bulging surface or flattened stratified squamous epithelium could extend cells.
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