Fig. 6. Microscopic and histological assessment of extra- and intraembryonic vascular formation in MLC2a-null mutants and wild-type littermates. (A-I) Whole-mount assessment of wild-type (+/+) and MLC2a-null (–/–) yolk sacs at ED8.5 (A,F), ED9 (B,G), ED9.5 (C,H) and ED10 (D,I). (E,J) Paraffin wax-embedded sections from wild-type (E) and MLC2a-null (J) yolk sacs at ED10 were stained for nuclei and cytoplasm with Hematoxylin and Eosin, respectively. Separation of mesodermal and endodermal layers was observed in the MLC2a-null yolk sac (J). (K-P) Whole-mount immunostaining of wild-type (+/+) and MLC2a-null (–/–) yolk sacs for Flk1 at ED8.5 (K,N), and PECAM at ED9 (L,O) and ED9.5 (M,P). (Q-T) Whole-mount immunostaining of wild-type (Q,R) and MLC2a-null (S,T) embryos for PECAM at somite pair 24. White arrows point out the lack of organization of cranial and intersegmental vessels in MLC2a-null embryos, relative to controls.