Abstract
The new concept of first level and second level lymphatic organs, and the fact that all first level lymphatic organs defined so far are lymphoepithelial, justifies a new close look at all types of lymphoepithelial relationships. In this investigation the life span of lymphocytes within different epithelia of mice have been studied with the aid of continuous labelling with H3-thymidine and subsequent autoradiography. The majority of the lymphocytes within the jejunal epithelium seems to be short-lived. About half of the lymphocytes within the epithelium covering Peyer’s patches and in epidermis also seem to be short-lived, while the other half is long-lived. Most of the liver lymphocytes seem to be long-lived, as are the blood lymphocytes. The results are discussed in the light of other recent findings about lymphocytes within epithelia, which we have called theliolymphocytes.

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