Fig. 1. Liver is specified in Hex^-/- embryos but fails to grow because of a cell proliferation defect. (A) RT-PCR analysis of RNA from ventral and dorsal foregut domains of 10-somite stage (S) embryos (lanes 3-8). Albumin and transthyretin were expressed in the hepatic primordium of Hex^-/- embryos at 10S (n=8, two representative samples depicted). L, embryonic liver mRNA; no RT, not reverse transcribed RNA; v, ventral; d, dorsal. (B) In situ hybridization for Albumin and -fetoprotein on isolated ventral endoderm from 2-6S embryos cultured for 48 hours with cardiac mesoderm. Beating cardiac domains are outlined in red and sometimes lie above or below endoderm domains; all purple staining is positive, with a subset denoted by arrows. (C-E) Embryos were exposed in vivo to BrdU for 2 hours, harvested, fixed, sectioned and stained for BrdU incorporation (purple nuclei). Blue boxes denote lateral gut endoderm; red boxes denote hepatic endoderm. (F,G) Quantitation of BrdU-positive endoderm cells relative to total cells in the hepatic (red bars) and lateral (blue bars) endodermal domains. A total of 24 sections from wild-type (n=3) and Hex^-/-(n=3) embryos were evaluated; P value was determined by the homoscedastic one tailed t-test. A lower proliferation rate was detected in the hepatic bud of Hex^-/- embryos. (H,I) TUNEL assay in the hepatic bud of 18S embryos. The regions with a white dotted outline correspond to the blue and red boxed regions in panels D,E. No evidence for enhanced apoptosis was detected in Hex^-/- embryos; note the positive staining in the neural tube and amnion in both embryos.