Int J Colorectal Dis (2010) 25:1231–1242 1235 Fig. 3 a–o Series of sagittal sections through the anorectum of a 12-week-old human fetus. a–e RZ, f–j ARZ and ATZ, k–o DL, SZ and PS Squamous zone and perianal skin The squamous epithelium (Fig. 3k) of the anal region showed only weak reactivity for CK 18 (Fig. 3l), superficial cells were still positive for CK 7 (Fig. 3m), and basal cell layers were strongly positive for CK 14 in the anal squamous epithelia (Fig. 3n) and in the sinuses. No labelling was observed for CDX2 (not shown) and for CK 20 (Fig. 3o). In the SZ skin appendages were first seen about the 14th week. Sixteen- to twenty-four-week-old fetuses Rectal zone The transitional villi were vanishing and regular crypts appeared. The overall single-layered columnar epithelium contained numerous goblet cells (Fig. 4a) and showed positive and consistent labelling for CK 18 (Fig. 4b), whereas positive reaction for CDX2 (Fig. 4c) seemed to be more concentrated at the bottom of the crypts. CK 20-positive reaction was found within the wall of the crypts (Fig. 4d). Anorectal zone As in the earlier stages, the ARZ was characterized by an irregular surface with columnar epithelium and some interspersed goblet cells (Fig. 4e), positive labelling for CK 18 (Fig. 4f), CK 7 (Fig. 4g), CK 20 (Fig. 4h), but only few CDX2 positive cells (not shown). Anal transitional zone The major feature of the transitional zone consisted of a layered cuboidal epithelium (Fig. 4i) and stratified squamous epithe-