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The milky spots (MS), originally described by Ranvier as taches laiteuses, are found on the greater omentum but also in other peritoneal regions, as well as on the pleura and pericardium. They represent aggregations of mesenchymal tissue surrounding blood vessels. These small whitish regions are covered by mesothelium, and within the mesothelial layer are scattered macrophage-like cells. The blood supply of MS is provided by arterioles that give rise to capillary network formed by fenestrated or continuous endothelial cells. Most MS possess also lymphatic vessels, with extremely thin endothelial cells. The most frequent cells in MS are the macrophages, followed by lymphocytes and mast cells. Typically, the macrophages are located in the periphery, while the lymphocytes-in the center of MS. Additional structural elements are plasmocytes, adipocytes, fibroblasts, rounded fibroblast-like cells (undifferentiated mesenchymal cells), as well as collagen, reticular and elastic fibers. The nerve fibers innervating MS are located under the mesothelium and among the free cells. Despite their small size, the MS are a significant organ, functioning at both normal and pathological conditions. Under inflammatory conditions (peritonitis), MS act as the first line of defense, and dramatically change their number, size and structure. MS are also involved in extramedullary hemopoiesis. They are the first target of intraperitoneal (intrapleural) metastases, and appear an important target in the development of immunotherapeutic strategies against malignant diseases.

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