Study on the Blood Cells and Hemocytopoietic Tissues of the Hagfish, Eptatretus Burgeri. 3. Lymphoid Cells Within the Epithelial Layers

In the previous papers, lymphoid cells resembling mammalian lymphocytes are found in the peripheral blood and the promitive spleen of the hagfish (Tomonaga 1973). It is now a very important problem in comparative immunology whether true lymphocytes exist in the hagfish, which is the thymusless promitive vertebrate. As a step in the solution of it the distribution of lymphoid cells in the epithelial layers of the hagfish were examined in this study using light and electron microscopes, for it is well known that lymphocytes enter into the epithelial layers as one of the characteristics of them. Animals used in this study were Japanese hagfish, Eptatretus burgeri, which were the same as used in the previous study. To examine the occurrence of lymphoid cells in the epithlial layers, serial sections of the whole body of a small hagfish and sections of various organs from many hagfish varing in body length were made. As a result, numerous lymphroid cells were found within the epithelial layer of the intestine. Predominant distribution of the lymphoid cells were a basal region below the level of the epithelial cell nuclei. Electron microscopic figures of these lymphoid cells (Fig. 3) are characterized by showing pale cytoplasm compared with that of thecirculatory blood. This finding coincide with the observation of Andrew (1965) in mammals. Lymphoid cells are also seen at the level of the epithelial cell nuclei and in the apical region of the epithelial layer in a relatively low frequency. In the basal region mitotic figures of lymphoid cells are observed (Fig. 1). On the contrary, lymphoid cells in the apical region show degenerating figures (Fig.2). Degenerated lymphoid cells are also observed in the lumen of the intestine (Fig.2). These findings suggest that some of lymphoid cells within the intestinal epithelial layer of the hagfish proliferate in the basal region and migrate to the apical region, where many of them degenerate, and further to the lumen of the intestine. It is not clear that some lymphoid cells within the epithelial layer reenter the lamina propria as Meader and Landers (1967) and Fichtelius (1969) claimed. Lymphoid cells in the epithelial layer in the hagfish were also observed in other organs than the intestine, especially in the gill (Fig. 4). It should be noticed that numerous neutrophil granulosytes are also seen within the epithelial layers of the hagfish. The function of lymphoid cells within the epithelial layers may play important roles in the defense mechanisms of the body. Insummary, it was found that hagfish, thymusless promitive vertebrate, have lymphoid cells resemnling mammalian lymphosytes not only in their structure but also in their character. These findings may also provide a high possibility of the existence of true lymphocytes in the hagfish, though further research is necessary.