The ontogenetic development of macrophage subpopulations and Ia-positive non-lymphoid cells in gut-associated lymphoid tissue of the rat.

The ontogenetic development of macrophage subpopulations and Ia-positive non-lymphoid cells was studied in gut-associated tissue in fetal and neonatal Wistar rats. A two-step immunoperoxidase method was carried out on cryostat sections, a panel of monoclonal antibodies being applied and aimed specifically at rat macrophages (ED1, ED2 and ED3) and at Ia antigen (Ox4). The first ED1-positive macrophages appeared in the liver on Day 15 (gestational age), and they did not express Ia. In developing mesenteric lymph nodes and in the gut wall, macrophages were found for the first time on Day 17 and 18, respectively, of gestation. These early macrophages were also ED1-positive. Until birth, ED2 recognized few cells in the gut-associated tissue; on the day of birth this subpopulation showed a sudden and considerable increase. The distribution pattern of ED3-positive macrophages appeared to be the same in fetal and in adult rats; it was confined to lymph nodes and Peyer's patches. Ia-positive non-lymphoid cells appeared in the abdomen early in ontogeny. The first Ia-positive cells displayed dendritic features and were found on Day 15 of fetal life in the mesenchymal tissue between intestinal loops. A few days later, many Ia-positive cells with a dendritic appearance were demonstrable in the gut wall and in developing mesenteric lymph nodes. Their number increased rapidly during the following days. Based on these results, the existence of two differentiation lines for dendritic cells and classical macrophages is discussed. The function of early Ia-positive cells in the abdomen is suggested as not being an antigen-presenting one.