The laboratory shrew, Suncus murinus, which lacks such gut associated lymph organs as the appendix and Peyer's plates, was recently demonstrated (KUBO and ISOMURA, 1996) to possess a pair of anal tonsils at the end of its rectum. The present paper deals with the development of this lymphoid organ as observed by light and electron microscopy. The anal tonsil was characterized by the initial postnatal development. On neonatal Day 1, a pair of epithelial crypts formed at the dorsal boundary between the anus and the ostium urogenitoanale. On Day 2 after birth, lymphocytes began to accumulate in the subepithelial mesenchymal tissue under the crypt. From Day 3 on, the lymphocytes increased to form a lymph nodule, from which, on Day 5, some lymphocytes began to penetrate into the crypt epithelium. The crypt and the nodule were fused together between Days 6 and S. A germinal center-like structure was observed on Day 20 after birth. Around Day 40, the invading cells comprised cellular units consisting of large and small lymphocytes and plasma cells. High endothelial venules were observed in the parafollicular area at this time. These findings indicate that the anal tonsil originates from an accumulation of lymphocytes in the mesenchymal tissue close to a particular epithelium of the crypt, presumably in response to antigens in foods; the tonsilar structure is then gradually completed by fusion of the lymphoid and epithelial elements. This paper further reports on an electron microscope finding on Day 8 where the anal tonsillar crypt epithelium was seen to contain some basal-granulated cells of the open type.
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