Ontogeny of milky spots in the fetal lamb omentum.

Milky spots in the fetal lamb omentum were observed by light and electron microscopy under normal conditions and after intraperitoneal carbon injection in utero. Rudimentary milky spots first appeared as small aggregations of cells along capillaries of the omental branch of the right ruminal artery at 72 days of gestation. At 116 days of gestation, macrophages were detected immunohistochemically in the milky spots. At 125 days of gestation, T cells were detected in the milky spots, but B cells were absent. Under the conditions induced by intraperitoneal injection of carbon suspension, openings were observed between the omental mesothelial cells, and macrophage aggregations appeared on the surface of the omentum through the openings. At 148 days of gestation (newborn), the milky spots were noted as black spots because of aggregation of carbon laden macrophages. The present study demonstrates that milky spots are present at fixed sites in the fetal lamb omentum by the middle of the term, and that at birth the macrophages on the spots already possess phagocytotic ability. The fetus develops the ability to protect the peritoneal cavity by producing peritoneal macrophages from the milky spots in the greater omentum even in utero.