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The ileal Peyer's patch (IPP) of young sheep is a site of diversification of the primary antibody repertoire and where involution takes place at young age. Tissue samples from the ileum were collected in 134 animals aged from 1 month to 6 years, and IPP follicle phenotypes were characterised. We describe a new type of ileal lymphoid follicles that became relatively more frequent during involution, and had numerous intrafollicular T-cells and BAQ44A+ B-cells and large interfollicular T-cell areas. As opposed to classical IPP follicles in which the BAQ44A+ cells were confined to the narrow follicle-neck region, the novel atypical ileal lymphoid follicle had these cells distributed throughout the follicle. The relative distribution of cell types in the typical IPP follicle remained fairly constant during involution. Many animals older than 9 months (64/92) still had had typical IPP follicles and even sheep 4 years and older (5/9) had IPP-type follicles.

Aggregates of lymphocytes were demonstrated from 70 days gestation (term 150 days in sheep) in the proximal colon and rectum. Immunoperoxidase staining for 5'-bromo-2'-deoxyuridine incorporation and IgM, indicated that the lymphocyte population of lymphoid follicles in fetal sheep colon was actively dividing and surface IgM positive. Enzyme histochemistry for 5'-nucleotidase showed that the lymphocytes developed in a meshwork of positive reticular cells, suggestive of developing follicles. Follicle aggregates were distributed in a characteristic pattern in lambs, with major accumulations in the ascending colon and in the rectum. In adult sheep a partial atrophy of follicle aggregates was observed. The microscopic structure of large intestinal aggregates showed similarities to the jejunal Peyer's patch (PP), with broad follicles containing a prominent corona and wide interfollicular areas in older lambs. The apparent deep penetration of crypts into the lymphoid tissue proper, which is a frequently reported phenomenon for colon follicles, was dependent on the contractile state of the mucosa, as judged from its absence in specimens where the intestinal wall had been stretched before fixation.