Adult intestinal epithelium consists of a sheet of single-cell thickness which is morphologically highly organized into tubular invaginations (crypts) and finger-like projections (villi). Proliferation of the cells is confined to the base of the crypts, from which cells migrate to the villi, where they are shed. The villi are formed during embryogenesis from a multilayered epithelium. In mice, crypts develop at about the time of birth from the epithelium between the villi, which by this stage is no longer multilayered. So far it has remained unknown how many progenitor cells contribute to each crypt, and whether they develop by the proliferation of already committed progenitors, or as a result of local inductive tissue interactions. Here, we have used mouse aggregation chimaeras as an experimental system to demonstrate immunohistochemically that the epithelium of individual crypts in small and large intestine of adult mice is always composed of cells of a single parental type. We have confirmed that this result is not an artefact of the chimaeric system by examining female mice that are mosaic for the X-linked alleles Pgk-1a and Pgk-1b. We conclude that the epithelium of each adult crypt is derived from a single progenitor cell.

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In mammals there are two forms of alkaline phosphatase, one of which is widely distributed in a variety of tissues, and one of which is confined to intestine. Levamisole (1tetramisole) inhibits the nonintestinal form of the enzyme, but is without effect on the intestinal form. We have exploited this difference by using conjugates made with calf intestinal alkaline phosphatase for immunohistochemical demonstration of H2 antigens in frozen sections of mouse

The immunohistological demonstration of H2 antigens in cryostat sections of a wide variety of mouse tissues is reported. The purpose in developing the method was to use H2 antigens as cellular markers in studies of mouse chimeras. Monoclonal anti-H2 antibodies were used, either with a hapten-sandwich technique using biotin or arsanilate, or as direct enzyme conjugates. The direct antibodyenzyme conjugates were simpler to use, provided an intensity of specific staining which was comparable to that obtained with the hapten-sandwich systems, and gave fewer problems of background when simultaneous double stain

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