Characterization of maternally derived neural fold (NF) in the early YS. (A) At 8 dpc (0-8 S), a population of CD45⁺GFP^– cells expressed Mac-1 and the terminally differentiated Mφ marker F4/80. In embryos resulting from a CD45.1 × CD45.2 cross, only the maternal allele was detected. These Mφ are thus maternally derived. (B-M) LHF embryo (B-G) and 0S embryo (H-M), stained with F4/80, CD45, and Bodipy-ceramide (C, E-G; I, K-M) or hybridized with a βH1 embryonic globin riboprobe (D, J). The red spots in the DIC picture (B, H) correspond to the confocal software–generated projection of F4/80⁺CD45⁺cells, as seen in the merged figure (C, I). YS-F4/80⁺CD45⁺cells are mainly located close to the NF and below the YS-BI compared with its position in LHF embryo hybridized with βH1 riboprobe (D, J). White arrowheads in J indicate the YS-BI level anteriorly because its caudal aspect is also visible. F4/80⁺CD45⁺cells are located within the YS mesoderm (E-G, higher magnification of the square in panel C; K-M, higher magnification of the square in panel I). NF indicates neural fold; YSC, yolk-sac cavity.