Purpose-The generation of non-viable homozygous null mouse embryos from heterozygote null/+ breedings can be highly resource consuming, with only 25% of the embryos in the litter being null mutants. We hypothesized that 1) we could double the number of homozygous null mouse embryos in a litter without reducing litter size using Hypoxanthine-guanine phosphoribosyltransferase-Cre (Hprt-Cre) (which is active in the female germ line at the time of fertilization) and 2) these homozygous null mutants would be identical to mutants generated through traditional null/+ breedings.Methods-To test this hypothesis we used a conditional allele Fgfr2IIIb flox . This allele when recombined is identical to the Fgfr2IIIb null allele. An F1 generation of Fgfr2IIIb rec/+ ; Hprt Cre/+ females was created by mating Fgfr2IIIb +/+ ; Hprt cre′/cre females to a Fgfr2IIIb flox/flox male. The F1 females were then mated to a Fgfr2IIIb flox/flox male. F2 embryos were genotyped and the morphology and histology of the lungs, intestine, limbs and brain was analyzed.Results-The Hprt-Cre mating strategy results in 51% of pups being genotypic homozygous null embryos (85/166) versus 23% for the standard null/+ approach (38/167). These embryos did not express the Fgfr2IIIb transcript and were phenotypically identical to null embryos generated through standard null/+ breedings. Conclusions-TheHprt-Cre mating strategy increases the number of homozygous mutant embryos in a litter without decreasing litter size. Embryos generated through this approach are phenotypically identical to those from standard heterozygous breedings. We recommend this Publisher's Disclaimer: This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. , and cleft palate [9,10] have been described. Generation of homozygous null embryos through null/+ (n/+) heterozygous breedings can be highly resource consuming. When employing the n/+ breedings for one gene, a single homozygous null (n/n) embryo is generated for every 4 embryos. Thus, a litter of 10 will yield 2 or 3 homozygous null mutant embryos. Generating compound homozygous null mutants for 2 or 3 genes with this process becomes even less efficient as the frequency of homozygous null mutants decreases to 1 in 16 embryos and 1 in 64 embryos, respectively. Researchers also face increasing pressure to minimize the number of animals used while maintaining, or even increasing, the quality of basic science research. This pressure is driven in part by both the animal rights movement [11,12] To test the efficiency of the Hprt-Cre strategy, we determined the number of phenotypically null embryos in a litter as compared ...

Purpose Duodenal atresia in humans has been hypothesized to arise from a failure of the duodenal lumen to recanalize after formation of an endodermal plug. Recently, mutations in the Fibroblast Growth Factor Receptor 2 gene have been shown to cause atretic defects of the duodenum in mice (Fgfr2IIIb). However, work in rats suggests that murine species do not form an endodermal plug during normal duodenal development. These lines of data led us to hypothesize that mice are able to form a duodenal atresia in the absence of an endodermal plug. To test this hypothesis we examined duodenal development in wild-type and Fgfr2IIIb-/- embryos. Methods Paraffin sections were generated for either hematoxylin and eosin, E-cadherin or terminal deoxynucleotidyl transferase mediated X-dUTP nick end labeling (TUNEL) staining from Fgfr2IIIb-/- and wild-type embryos between Embryonic Days (E) 10.5 and E14.5. Sections were photographed and reconstructed into 3-dimensional display using Adobe Photoshop and Amira Visage software. Results Normal mouse duodenum does not form an endodermal plug, although a plug does form in the pyloric region of the stomach at E14.5. Fgfr2IIIb-/- embryos experience significant apoptosis in the duodenal region at E10.5, followed by the disappearance of the endoderm in the atretic precursor by E11.5. Thereafter, the mesoderm of the atretic precursor involutes over the next 2 days in the absence of further apoptosis. Interestingly, an endodermal plug was not observed at any point during the formation of a duodenal atresia. Conclusions These results suggest that duodenal atresia in the Fgfr2IIIb-/- model does not arise from persistence of an epithelial plug. Rather it appears to result from the loss of the endoderm due to apoptosis very early in development.

Introduction Intestinal atresias have long been hypothesized to result from either failure of recanalization of the intestinal lumen or in utero vascular accidents. Recent work in animal models is now calling for a reassessment of these widely held paradigms. Purpose In this review, we will examine the data that led to the original hypotheses and then evaluate more recent work challenging these hypotheses. Furthermore, we will discuss how defining the mechanism of atresia formation in animal models may provide insight into early intestinal development and the mechanism of lengthwise intestinal growth. Conclusion Such insight will be critical in developing regenerative therapies for patients with intestinal failure.

Background Homozygous null mutation of the fibroblast growth factor receptor 2IIIb (Fgfr2IIIb) gene in mice results in 42% of embryos developing duodenal atresias. Retinaldehyde dehydrogenase 2 (Raldh2, a gene critical for the generation of retinoic acid) is expressed in the mouse duodenum during the temporal window when duodenal atresias form. Raldh2 is critical for the normal development of the pancreatoduodenal region; therefore, we were interested in the effect of a Raldh2 mutation on duodenal atresia formation. To test this, we rendered Fgfr2IIIb−/− embryos haploinsufficient for the Raldh2 and examined these embryos for the incidence and severity of duodenal atresia. Methods Control embryos, Fgfr2IIIb−/− mutants, and Fgfr2IIIb−/−; Raldh2+/− mutants were harvested at embryonic day 18.5, genotyped, and fixed overnight. Intestinal tracts were isolated. The type and severity of duodenal atresia was documented. Results A total of 97 Fgfr2IIIb−/− embryos were studied; 44 had duodenal atresias, and 41 of these presented as type III. In the 70 Fgfr2IIIb−/−; Raldh2+/− embryos studied, a lesser incidence of duodenal atresia was seen (15 of 70; P = .0017; Fisher exact test). Atresia severity was also decreased; there were 12 embryos with type I atresias, 3 with type II atresias, and 0 with type III atresias (P < 2.81E–013; Fisher exact test). Conclusion Haploinsufficiency of Raldh2 decreases the incidence and severity of duodenal atresia in the Fgfr2IIIb−/− model. The ability to alter defect severity through manipulation of a single gene in a specific genetic background has potentially important implications for understanding the mechanisms by which intestinal atresias arise.

Purpose The etiology of intestinal atresia remains elusive but has been ascribed to a number of possible events including in utero vascular accidents, failure of recanalization of the intestinal lumen and mechanical compression. Another such event that has been postulated to be a cause in atresia formation is disruption in notochord development. This hypothesis arose from clinical observations of notochord abnormalities in patients with intestinal atresias as well as abnormal notochord development observed in a pharmacological animal model of intestinal atresia. Atresias in this model result from in utero exposure to Adriamycin, wherein notochord defects were noted in up to 80% of embryos that manifested intestinal atresias. Embryos with notochord abnormalities were observed to have ectopic expression of Sonic Hedgehog (Shh) which in turn was postulated to be causative in atresia formation. We were interested in determining whether disruptions in notochord development or Shh expression occurred in an established genetic model of intestinal atresia and utilized the Fibroblast Growth Factor Receptor 2IIIb homozygous mutant (Fgfr2IIIb−/−) mouse model. These embryos develop colonic atresias (100% penetrance) and duodenal atresias (42% penetrance). Methods Wild-type and Fgfr2IIIb−/− mouse embryos were harvested at E10.5, E11.5, E12.5 and E13.5. Whole mount in situ hybridization was performed on E10.5 embryos for Shh. Embryos at each time point were harvested and sectioned for H&E staining. Sections were photographed specifically for the notochord and resulting images reconstructed in 3-D using Amira software. Colons were isolated from wild-type and Fgfr2IIIb−/− embryos at E10.5, then cultured for 48 hours in matrigel with FGF10 in the presence or absence of exogenous SHH protein. Explants were harvested, fixed in formalin and photographed. Results Fgfr2IIIb−/− mouse embryos exhibit no disruptions in Shh expression at E10.5, when the first events in atresia formation are known to occur. Three-dimensional reconstructions failed to demonstrate any anatomical disruptions in the notochord by discontinuity or excessive branching. Culture of wild-type intestines in the presence of Shh failed to induce atresia formation in either the duodenum or colon. Cultured Fgfr2IIIb−/− intestines developed atresias of the colon in either the presence, or absence, of Shh protein. Conclusions Although disruptions in notochord development can be associated with intestinal atresia formation, in the Fgfr2IIIb−/− genetic animal model neither disruptions in notochord development nor the presence of exogenous Shh protein are causative in the formation of these defects.