[Histogenesis of the omental milk spot of the mouse (author's transl)].

The mouse omental milk spots can be classified into two types, type I and II. The type I spots are seen in the margin of the fatty tissue along the omental vessels, and the type II spots are scattered on the omental peritoneum. In this study, histogenesis of the two types of milk spots was examined with the light and electron microscope. 1. Milk spots are not yet observed at birth, but macrophages are already intermingled with mesenchymal cells along the omental vessels and within the omental peritoneum. 2. Fat cells first appear along the omental vessels at 1 to 2 days of age and they then form fatty tissue at 3 to 5 days. Mononuclear cells such as lymphocytes, lymphoid cells and macrophages are scattered within the fatty tissue. At 6 to 10 days capillary network is developed along the margin of the fatty tissue, and lymphocytes, lymphoid cells and macrophages are accumulated along capillaries. Thus type I milk spots are formed. Lymphocytes and lymphoid cells are frequently seen also within the capillary lumens. They are sometimes seen crossing the capillary walls. Macrophages in the deeper zone of milk spots are generally small, but macrophages in the superficial zone are large and well differentiated in appearance. 3. The proportions of constituent cells in type I milk spots were examined at various ages. In spots in early life, macrophages, lymphoid cells and lymphocytes are relatively few, occupying 7 to 8% in proportion, respectively. Then lymphocytes increase in number with age, and they are more than 50% after 1 month of age. Lymphoid cells remain almost unchanged in proportion, being 7 to 11%. Macrophages constitute 2 to 3% after 1 month. 4. Type II milk spots appear later than type I spots, and they are first seen at 12 days of age, especially on the dorsal layer of the omentum. They then increase in number with age. 5. Functional significance of the milk spots was discussed in relation to their histogenesis.