Lymphocyte subpopulations in the liver, spleen, intestines, and mesenteric nodes: an immunohistochemical study using human fetuses at 15-16 weeks.

The roles of the liver and intestines in lymphocyte differentiation in human fetuses were assessed by immunohistochemical analysis of the thymus, bone marrow, liver, spleen, intestines, and lymph nodes of 15-16 week human fetuses using primary antibodies against IgM, CD3, CD7, CD8, CD10, CD20, CD45RO, HLA-DR, and CD68. The density of immunoreactive lymphocytes was high in the thymus and lymph nodes, but much lower in the bones, liver, spleen, and intestines. The medulla of the thymus contained IgM-positive mature B lymphocytes as well as CD20-positve B lymphocytes. In contrast, CD10-positive immature B lymphocytes were restricted in the cortex. There were no site-dependent differences among axillary, mediastinal, mesenteric, and pelvic lymph nodes. CD68-positive cells were observed at all sites examined. Many HLA-DR-positive round cells were present in the thymus, with fewer in the liver and spleen. The absolute number of lymphocytes was estimated to be ≥10-fold higher in lymph nodes than in liver. Although limited by analysis of only one fetal stage, these findings suggest that mesenteric nodes are likely to be more important than the liver, spleen, and intestines for lymphocyte proliferation and differentiation in human mid-term fetuses.