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Appearance of monocytes and macrophages in the embryonic thymus of the mouse: light and electron microscopic studies.

Development of the embryonic thymus was studied by light and electron microscopy, particularly with reference to the appearance of macrophages. The thymus anlage appeared at 12 days of gestation. Then, at 14 days, the anlage was invaginated by mesenchymal septa carrying small blood vessels. In the embryonic thymus, macrophages and monocytes were present as early as 15 days of gestation. Monocytes as well as macrophages laden with phagocytic inclusions first appeared in the perivascular space or very close to blood vessels at the cortico-medullary junction. The perivascular space was continuous with the mesenchyme surrounding the thymus. From 15 to 17 days, monocytes could often be seen in the perivascular space. The origin of thymus macrophages was discussed in relation to a route for movement of macrophage precursors into the embryonic thymus.