The earliest invasion of macrophages into the developing brain and eye of the tree shrew Tupaia belangeri

The earliest occurrence of macrophages was investigated in the brain and op tic anlagen of the tree shrew Tupaia belangeri. Nineteen serially sectioned embryos, belonging to five phases of programmed neuroepithelial cell death previously found during optic cup formation, were used. Macrophages were i dentified by structural criteria and by labelling with the lectin Griffonia simplicifolia I-B-4. Macrophages, most probably derived from the yolk sac, are present in the perineural vessels of the phase 1 embryo (V-shaped opti c evagination). Within this compartment, their number increases up to phase 4 (advanced invagination) and drops during phase 5. This first wave of mac rophages is followed by a second one occurring within the perineural mesenc hyme and within the neuroepithelium of the brain and eyes from phase 3 onwa rds. In the phase 4 embryos, a considerable rise in the number of intravent ricular macrophages is noted. During phase 5 (far advanced invagination), m arked vascularization of the brain starts, and a peak of macrophages is not ed in the neuroepithelium and in the ventricular lumen of the brain. This s patiotemporal pattern suggests that, in Tupaia, the earliest macrophages ar e simultaneously shifted from perineural vessels into the neuroepithelial w alls of the developing brain and, at earlier stages than previously describ ed in other vertebrate species, of the eye anlagen.