1234 Int J Colorectal Dis (2010) 25:1231–1242 Fig. 2 a–m Series of sagittal sections through the anorectum of a 9- to 10-week-old human fetus. a–e RZ, f–I ARZ and ATZ, j–m DL, SZ and PS positive for CK 18 (Fig. 2k) and strongly positive for CK 7 (Fig. 2l). Whereas negative reaction was found for CDX2 (not shown), CK 14 affinity was first seen and restricted to basal layers (Fig. 2m) in the future non-keratinizing zone. Twelve- to fifteen-week-old fetuses Rectal zone In 12-week-old fetuses, the rectal surface was covered by transitional villi, lined by a one- to two-layered columnar epithelium with goblet cells (Fig. 3a). The epithelium showed positive reaction for CK 18 (Fig. 3b) and negative reaction for CK 7 (Fig. 3c) as well as for CK14 (not shown). The labelling for CDX2 was positive, especially in the epithelium covering the bottoms between the villi (Fig. 3d). Labelling for CK 20 (Fig. 3e) was particularly positive in the surface epithelium and was spread in the same area as CDX2. Anorectal zone This area was characterized by vanishing villi (Fig. 3a, f). CK 18 (Fig. 3b, g) and CK 7 (Fig. 3c, h) labelling was positive; some scattered CDX2-positive cells (Fig. 3d, i) and CK 20, respectively, (Fig. 3e, j) were found. Anal transitional zone Whereas the epithelia and their immunoreactivity did not change (Fig. 3f–j) in comparison to the younger stage, the differentiation of the columnar region into anal columns and sinuses was evident.