Three distinctive steps in Peyer's patch formation of murine embryo

Investigation of the process of Peyer's patch (PP) formation has been hampered by difficulties in identifying its initial step in the embryo. In this study, we overcame this problem by means of whole-mount immunohistochemistry using mAb against the molecules which were expressed in the cells accumulated at the site of PP development. This method was sensitive enough to distinguish a minute cell cluster in the developing gastrointestinal tract, We analyzed the time course of the expression of various surface markers and found that PP formation proceeds through three successive steps. The first is the appearance of a VCAM-1(+) cell cluster at 15.5 days postcoitus (d.p.c.). Histological examination of the VCAM-1(+) clusters suggested that VCAM-1(+) cells represent a stromal component. The second step is characterized by the accumulation of round cells expressing Ia, IL-7R or CD4 at 17.5 d.p.c. Lymphocytes expressing CD3 or B220 were detected only in the final step which started at 18.5 d.p.c. Using any of these markers, the aggregation was initially detected on the upper jejunum and it extended to the colon as the number of clusters increased. At the neonatal stage, the number reached up to eight or nine, irrespective of the antibodies used for the detection. In the aly/aly mutant mouse, where no lymph nodes or PP are found in the adult, none of these three steps was detected. On the other hand, in the SCID mouse that is defective in the formation of mature lymphocytes, the first and second step proceeded, whereas the third step was undetectable. These findings suggest that the progression of each step is indeed regulated by different mechanisms.