Histogenesis of the mouse colonic mucosa with special reference to obliteration and re-opening of the colonic lumen

Abstract

Temporary obliteration of the colonic lumen has been known prior to the formation of primary villi since an early light microscope (LM) study. We examined this process with a transmission electron microscope (TEM) using the descending colon of ICR mouse fetuses on days 13-18 of gestation (E13-E18, vaginal plug=E0). Pieces of the descending colon were doubly fixed in 2.5% glutaraldehyde and 1% OsO4 and embedded in an epoxy resin. Semi-thin sections stained with toluidine blue were examined by LM. Ultrathin sections doubly stained with uranyl acetate and lead citrate were examined by TEM.On day E13,the colonic mucosa is lined with a single layer of undifferentiated epithelial cells (FIG. 1a), which were mutually joined by the primary junctional complex around the colonic lumen. Mitotic cells face the lumen and are connected with neighboring cells by the junctional complex. The epithelium looks stratified and the colonic lumen is extremely narrowed on days E15-E16 so that it cannot be identified by LM examination (FIG. 1b). A secondary junctional complex consisting of macula occludens with attaching microfilaments and desmosomes is formed between two epithelial cells apart from the colonic lumen. A cavity with microvilli is formed intracellularly and it subsequently opens with in the macula occludens of the secondary junctional complex to form the intra-epithelial cavity (FIG. 2, 3). A mitotic cell faces the intra-epithelial cavity and is joined with neighboring cells by these condary junctional complex (FIG. 4).