[Morphological studies of the omental milk spots in the mouse: light and electron microscopy (author's transl)]

In adult dd-mice of both sexes ranging in age from 2 to 8 months, the omental milk spots were studied by light and electron microscopy. 1. Omental milk spots could morphologically be classified into two types, namely, type I and type II. 2. Type I spots were seen on the margin of the fatty tissue along the omental artery and vein. This type of spots were supplied with capillary vessels. 3. Type II were scattered on the omental peritoneum, particularly in abundance on the dorsal layer of the omentum. The spots of this type generally had no vascular supply. Type II milk spots on the dorsal layer were significantly more numerous in females than in males. 4. The two types of milk spots were composed mainly of macrophages, lymphoid cells and lymphocytes, although the constituents were different in proportion between the two types. The type I spots had abundant macrophages especially in the superficial zone. Lymphoid cells and lymphocytes were relatively abundant in the deeper zones. The type II were also abundant in macrophages which were scattered diffusely in the spots. 5. Macrophages had slightly PAS positive cytoplasm which contained PAS positive granules and vacuoles. Macrophages in the superficial zone of the spot were large with abundant lysosomes and vacuoles. Lymphoid cells were more or less similar in appearance to lymphocytes, but their cytoplasm was more abundant than that of lymphocytes, and had lysosomes and bundles of microfilaments. In addition, large macrophages were also distributed diffusely within the fatty tissue along the omental vessels and between the opposed peritoneium of the omentum. They had abundant light cytoplasm with numerous projections on the surface. The organelles were generally well developed. The cytoplasm had abundant lysosomes, coated vesicles and small tubules. Lymphocytes were generally small lymphocytes. 6. The endothelial cells of capillaries in type I spots were fenestrated. 7. The functional significance of the two types of spots was discussed on the basis of the structure.