Table 1.

Summary of different techniques used to distinguish microglia from BMDC in the CNS

Method	Description	Advantages	Disadvantages
IR bone marrow transplantation 44, 48	Whole body irradiation followed by labelled bone marrow rescue	High levels of chimerism, rapid chimerism	Lethal dose radiation. Fulminant systemic inflammatory response, BBB damage
Head‐shielded bone marrow transplantation 16, 17, 47	As above with lead shielding of cranium.	Similar to above with protection from BBB damage	Higher doses of irradiation required. Significant systemic inflammatory response
Parabiosis 54	Surgical anastomosis of labelled donor and recipient blood circulations	Minimal systemic inflammatory response. No perturbation of BBB	Lower levels of chimerism (~50%). Technically challenging
NMT	Single injection of busulfan followed by bone marrow transplantation	Technically simple. High levels of chimerism, minimal systemic inflammatory response and perturbation of BBB	Longer time to chimerism (12 weeks)
Genetic lineage tracing studies 23, 59	Transgenic mice expressing labelled cell populations or genetically engineered conditional labelling of specific myeloid lineages	Highly accurate lineage dependent labelling of different cell types and myeloid subpopulations	Costly and technically challenging. Nonconditionally labelled single gene marker systems (e.g. CX3CR1 labelled transgenic mice) are less reliable at distinguishing microglia from macrophages in inflammatory environments