The relationship between the histological localization of alkaline phosphatase activity and appearance of lymphocytes in lymphocytic tissue of the embryonic and neonatal rabbit

Abstract

Abstract Various lymphocytic organs (thymus, spleen, appendix and lymph node) were removed from embryonic and neonatal rabbits between the eighteenth day of gestation and the tenth postnatal day. These tissues were studied morphologically and histochemically using a variety of procedures in order to determine the manner of lymphocyte formation in relation to the pattern of histochemical differentiation in these lymphocytic tissues. Of particular interest was the consistent development of alkaline phosphatase activity in non‐thymic lymphocytic organs one to two days prior to the appearance of lymphocytes in these tissues. The regional development of alkaline phosphatase activity in these non‐thymic lymphocytic organs corresponded with the sites of appearance of the first lymphocytes in these organs and with the regions of subsequent lymphocytic proliferation during the developmental stages of the spleen, lymph nodes and appendix of the rabbit. Specifically, alkaline phosphatase activity localized in the mesenchymal cells surrounding the developing arteries of the spleen, in the developing cortical region of the lymph nodes and in the lamina propria of the domed elevations of the appendix. In contrast, neither the thymus nor the surrounding connective tissue of the thymus exhibited demonstrable alkaline phosphatase activity during the early development of the thymus into a lymphocytic organ. Significantly, the first cells of the lymphocytic series to develop in the spleen, lymph nodes and appendix in the embryonic rabbit were of small or medium size in contrast with the thymus where lymphoblasts were the first recognizable cells of the lymphocytic series. These observations suggest that alkaline phosphatase is intimately associated with the development of lymphocytes in potential non‐thymic lymphocytic organs. The mechanisms of alkaline phosphatase action, although not clear, may act to induce the in situ transformation of lymphocytes, provide a suitable environment for lymphopoiesis or serve as a chemotactic agent, attracting blood‐borne lymphocytes to these regions.