Fig. 2. Nephrogenesis arrests and cells in the peripheral zone die in the absence of FGF8. Marker analysis in (A,B,Q-T) cryosections or (C-P,U,V) vibratome sections of kidneys at the stages indicated. (A-L,U,V) In situ hybridization for the genes indicated. The Fgf8 (FL) probe we used contained the full-length coding sequence, and therefore detected Fgf8 RNA produced by the Fgf8^null allele. (M-P) Immunofluorescence assays for PAX2 (green) to identify the developing nephrons and collecting ducts, co-stained with LysoTracker (LysoT, red) to identify regions containing dying cells. (Q-T) Immunofluorescence assays for PAX2 (green) and Calbindin (CB, blue), which identifies collecting ducts, and for TUNEL staining (red), which detects dying cells. Arrowheads point to nascent nephrons, which are present in normal kidneys and also in Fgf8-MM-KO kidneys at E13.5 (A,B) and E14.5 (Q,R), but not at E16.5 (S,T). Note that the nascent nephrons in Fgf8-MM-KO kidneys (B,R) have formed an epithelial structure surrounding a lumen, i.e. they have reached the renal vesicle stage.