1236 lium at the bottom of the sinuses. Weak positive reaction for CK 18 was restricted to the cranial part of this zone (Fig. 4j), whereas CK 7-labelled cells appeared throughout the whole transitional zone (Fig. 4k). Positive staining for CK 14 was found in the basal cell layers of the squamous epithelium (Fig. 4l), especially at the bottom of the sinuses. CK 20 and CDX2 reactions were negative (data not shown). Squamous zone The epithelial border between the transitional zone and anal stratified squamous epithelium stood out clearly and marked the DL (Fig. 4l). The anal squamous epithelium labelled weakly or negatively for CK 18 (Fig. 4m) and CK 7 (not shown) and positively in its basal layers for CK 14 (Fig. 4n) which were also CK 14 positive at the bottom of the anal sinuses. Perianal skin The border between non-keratinizing (SZ) and keratinizing epithelium (PS) became evident (Fig. 4l) due to first signs of keratinization in the 16th week. Labelling for CK 18 (Fig. 4m) and CK 7 were mostly negative, but CK 14 labelled positive in the basal layers of all stratified squamous epithelia (Fig. 4n). Twenty-five-week to thirty-three-week-old fetuses Rectal zone The epithelium of the regular crypts (Fig. 5a) labelled positive for CK 18 (Fig. 5b). CDX2 expression was observed at the bottom of the crypts and partially in the walls (Fig. 5c). Strong labelling for CK 20 was evident within the walls and at the surface, but not at the bottom of the crypts (Fig. 5d). Anorectal zone The labelling within the epithelium of the irregular crypts (Fig. 5e) was similar to that of the RZ concerning CK 18 (Fig. 5f), CDX2 (Fig. 5g) and CK 20 (Fig. 5h), but in contrast remarkably positive for CK 7 (not shown). Anal transitional zone Regular columns and sinuses were seen (Fig. 5i). Intensive CK 7-positive staining was detected (Fig. 5j). Squamous zone and perianal skin The basal cell layers of the non-keratinizing and the keratinizing epithelia (Fig. 5k) were positive for CK 14 (Fig. 5l). Int J Colorectal Dis (2010) 25:1231–1242 Fig. 4 a–k Series of sagittal sections through the anorectum of a b 16-week-old human fetus (a–h) and a 20-week-old fetus (i–n). a–d RZ, e–h ARZ and ATZ, i–k ATZ and DL, l–n SZ and PS Postnatal stages/adult No structural differences in the anorectal epithelia (Fig. 6a, e, i, k) were found. Cytokeratin 18, 20 and 7 (Fig. 6b, d, f, h, j) as well as CDX2 (Fig. 6c, g) expressions showed no differences compared to the late fetal stages. Panepithelial CK 14 expression was found in the SZ and PS (Fig. 6l). Ultralow rectal tumours Seventeen tissue probes from patients with clinical-staged low rectal adenocarcinoma were investigated (Fig. 7a). The tumour tissue was strongly positive for CK 18 (Fig. 7b), negative for CK 14 (not shown) and in most cases (15) negative for CK 7 (Fig. 7c). The two CK 7-positive samples showed sphincter infiltration. All rectal adenocarcinomas displayed positive labelling for CDX2 (Fig. 7d). Anal tumours In correspondence with our findings in healthy adult anal epithelia, the examined anal squamous cell carcinomas (Fig. 7e) of the four investigated probes were negative for CK 18 (Fig. 7f) and CK 7 (Fig. 7g) but remarkably positive for CK 14 (Fig. 7h). They all were negative for CDX2 (not shown). Discussion The present study was undertaken to examine the develop- ment and differentiation of the anorectal epithelia and to compare the results with the histomorphological character- istics of anorectal tumours. We report that: a. Due to the cytokeratin profile and to the expression of CDX2, the different zones of the anorectum can be recognized early in human prenatal life; b. Epithelial differentiation runs in a craniocaudal direction; c. The ARZ is an area of epithelial changes; and d. We explain why CK7, CK20 and CDX2 can simulta- neously be expressed in rectal adenocarcinomas. Following the detailed investigation of Johnson [5], our study is the first to contribute to the histogenesis of the anorectal epithelia. Moreover, we compare the situation in human prenatal stages with adult specimens as well as with ultralow rectal and anal carcinoma. In the last decade, some morphological studies concerning the early development of the human anorectum were published that—in connection