Enhanced engraftment of transplanted microglia through host microglia depletion in the embryonic brain.

Transplantation of microglia into the embryonic brain could provide a powerful approach to trace microglial fate and test their functional contribution during development. However, embryonic microglial transplantation remains technically challenging because only a limited number of donor cells can be obtained and transplantation efficiency is low. Here, we investigated whether transient depletion of endogenous microglia could improve the engraftment of transplanted microglia in the embryonic mouse brain. Pregnant mice were fed a diet containing a colony-stimulating factor 1 receptor inhibitor (PLX5622) in advance to reduce resident microglia in the embryos. CD11b-positive cells isolated from embryonic day (E) 12.5 CX3CR1-GFP donor brains were transplanted into the lateral ventricles of E12.5 embryos, and brains were analyzed at E14.5. In embryos fed a normal diet, most transplanted cells remained in the ventricular space, whereas PLX5622-treated embryos showed a marked increase in GFP-positive cells within the brain parenchyma, suggesting improved engraftment. Furthermore, early microglial depletion caused enlargement of cavities in the cortico-striato-amygdalar boundary (CSA), a developmental boundary region in the ganglionic eminence. Microglial transplantation significantly reduced the size of these cavities. These results suggest that transient depletion of host microglia enhances embryonic microglial transplantation and that transplanted microglia can partially restore structural abnormalities caused by microglial loss.