Figure 4.

Normal expression of extraembryonic markers in huntingtin deficient embryos. Whole mount in situ hybridization analysis at E7.5 of markers of the extraembryonic tissues reveals grossly normal expression in the absence of huntingtin. Hnf4, expressed in the visceral endoderm at the junction of embryonic-ectoderm junction (A), is normal in mutant embryos, although the signal is slightly higher (B). Similarly, the expression of Pem transcripts is maintained in mutant embryos (D) similar to normal embryos (C), although Pem is expressed in the abnormal lopsided overhang of visceral endoderm over the anterior of the mutant embryos. Expression of extraembryonic signaling molecules is unaffected by the loss of huntingtin, as evidenced by the expression of Bmp4 (E,F) in the extraembryonic ectoderm, and Lefty1 and Dkk1 (I-L) in the AVE in mutant embryos. Bmp4 is not localized, however, to a ring of extraembryonic ectoderm in mutant embryos (F) as in normal embryos (E). Primitive germ cells (PCGs) are induced normally in both wild-type (G) and mutant embryos (H), suggesting the Bmp4 signaling from the extraembryonic ectoderm to the epiblast is normal. Lefty1 expression appears disorganized in mutant embryos (I) compared to wild-type embryos (J). In contrast, the anterior expression of Dkk1 in the AVE in mutant embryos (L) matches the wild-type expression pattern (K). Despite normal AVE formation, head folds fail to form in mutant embryos, even when cultured in nutrient rich media for 24 hours. Wild-type E7.5 embryos, when cultured in 75% rat serum, develop somites (M), heart (white arrow, N) and head folds (blue arrow head, N) in culture. In contrast, huntingtin deficient embryos continue to live in culture but do not form headfolds, heart or somites (O). Embryos are shown in a lateral view (A-F, I-J) with anterior oriented to the left. Embryos in (G,H,K,L) are shown in an anterior view with proximal oriented up.