Fig. 2. Morphological analyses of Ext2 mutant embryos. In all sections,
embryos are viewed laterally and the anterior side of the embryo is towards
the left. (A) At E6.5, wild-type embryos have initiated the formation of a
primitive streak and show an elongated egg cylinder. (B)
Ext2–/– embryos have underdeveloped
extra-embryonic structures and do not elongate. There are no signs of
primitive streak formation. (C,D) Whole-mount embryos collected at E7.5. (C)
Some Ext2–/– mutants show little or no
elongation compared with wild-type littermates, but show a normal
ectoplacental cone. (D) Other Ext2–/– mutants
are only slightly smaller in overall size than wild-type embryos. (E-H)
Hematoxylin and Eosin staining of longitudinal sections of wild-type (E) and
Ext2–/– (F-H) embryos. (E) At E7.5, a
wild-type embryo has well-defined embryonic and extra-embryonic structures. A
layer of mesodermal cells is visible between ectoderm and endoderm. (F)
Ext2–/– mutant (class 1) with a two-layered,
rounded egg cylinder and no visible signs of extra-embryonic ectoderm,
endoderm or mesoderm. (G) Ext2–/– mutant
(class 2) with a two-layered egg cylinder that is about half the size of the
wild-type embryo with significantly underdeveloped extra-embryonic structures.
(H) A third group of Ext2–/– mutants (class 3)
shows further elongation of a two-layered egg cylinder. Arrowhead in H
indicates the formation of a headfold. (I-L) TUNEL assay examining cell death
in wild-type E7.5 embryos (I) and class 1 (J), class 2 (K) and class 3 (L)
E7.5 Ext2–/– mutants. Little or no cell death
is observed in wild type (I) or class 2
Ext2–/– mutants (K). The extensive cell death
in class 1 mutants shows this embryo is no longer viable (J). Cell death is
visible in the headfold of class 3 mutants (L). cho, chorion; ecp,
ectoplacental cone; eec, embryonic ectoderm; een, embryonic endoderm; ems,
embryonic mesoderm; pen, parietal endoderm (attached to inner aspect of
Reichert's membrane); ps, primitive streak; xec, extra-embryonic ectoderm;
xen, extra-embryonic endoderm; xms, extra-embryonic mesoderm. Scale bars: 50
µm in A,B; 100 µm in E-H; 140 µm in I-L.