Fig. 3. Phosphorylated serine 1 on histone H4/H2A. (A-L) Light and fluorescent micrographs of mouse immature oocytes, fertilized eggs and early embryonic stages stained for the Ph(Ser1)H4/H2A modification (E-H, red) and nucleic acid (I-L, green). Chromatin staining is present at all stages of pre-attachment development. (B,F,J) In fertilized eggs, both the metaphase II plate (F, asterisk) and decondensing sperm nuclei (F, arrow) stain positive for this modification. (D,H,L) In the blastocyst note that levels of this modification in inner cell mass blastomeres (identified by arrow in D) appears to be lower than those seen in blastomeres of the trophectoderm.