Since the discovery that epidermal growth factor (EGF) can accelerate opening of the eyelids, the EGF receptor (EGF-R) has been extensively studied and is now considered to be a prototype tyrosine kinase receptor. Binding of EGF or of transforming growth factor-alpha (TGF-alpha) or other related factors activates the receptor and induces cell proliferation and differentiation. Although it is not found on haematopoietic cells, the EGF-R is widely expressed in mammals and has been implicated in various stages of embryonic development. Here we investigate the developmental and physiological roles of this receptor and its ligands by inactivating the gene encoding EGF-R. We find that EGF-R-/- mice survive for up to 8 days after birth and suffer from impaired epithelial development in several organs, including skin, lung and gastrointestinal tract.

The striatum has a central role in many neurobiological processes, yet little is known about the molecular control of its development. Inroads to this subject have been made, due to the discovery of transcription factors, such as the Dlx genes, whose expression patterns suggest that they have a role in striatal development. We report that mice lacking both Dlx-1 and Dlx-2 have a time-dependent block in striatal differentiation. In these mutants, early born neurons migrate into a striatum-like region, which is enriched for markers of the striosome (patch) compartment. However, later born neurons accumulate within the proliferative zone. Several lines of evidence suggest that mutations in Dlx-1 and Dlx-2 produce abnormalities in the development of the striatal subventricular zone and in the differentiation of striatal matrix neurons.

The Dlx homeobox gene family is expressed in a complex pattern within the embryonic craniofacial ectoderm and ectomesenchyme. A previous study established that Dlx-2 is essential for development of proximal regions of the murine first and second branchial arches. Here we describe the craniofacial phenotype of mice with mutations in Dlx-1 and Dlx-1 and -2. The skeletal and soft tissue analyses of mice with Dlx-1 and Dlx-1 and -2 mutations provide additional evidence that the Dlx genes regulate proximodistal patterning of the branchial arches. This analysis also elucidates distinct and overlapping roles for Dlx-1 and Dlx-2 in craniofacial development. Furthermore, mice lacking both Dlx-1 and -2 have unique abnormalities, including the absence of maxillary molars. Dlx-1 and -2 are expressed in the proximal and distal first and second arches, yet only the proximal regions are abnormal. The nested expression patterns of Dlx-1, -2, -3, -5, and -6 provide evidence for a model that predicts the region-specific requirements for each gene. Finally, the Dlx-2 and Dlx-1 and -2 mutants have ectopic skull components that resemble bones and cartilages found in phylogenetically more primitive vertebrates.

Integrin receptors for extracellular matrix receptors are important effectors of cell adhesion, differentiation, and migration in cultured cells and are believed to be critical effectors of these processes during development. To determine when 131 integrins become critical during embryonic development, we generated mutant mice with a targeted disruption of the [31 integrin subunit gene. Heterozygous mutant mice were normal. Homozygous loss of ~1 integrin expression was lethal during early postimplantation development. Homozygous embryos lacking B1 integrins formed normal-looking blastocysts and initiated implantation at E4.5. However, the E4.5 Ill-null embryos in situ had collapsed blastocoeles, and whereas the trophoblast penetrated the uterine epithelium, extensive invasion of the decidua was not observed. Laminin-positive endoderm cells were detected in the inner cell mass area, but endoderm morphogenesis and migration were defective. By E5.5 ~l-null embryos had degenerated extensively. In vitro analysis showed that trophoblast function in Ill-null peri-implantation embryos was largely normal, including expression of tissue-specific markers, and outgrowth on fibronectin-and vitronectin-coated, although not on laminin-coated substrates. In contrast, the inner cell mass region of 131-null blastocyst outgrowths, and inner cell masses isolated from [31-null blastocysts, showed highly retarded growth and defective extraembryonic endoderm morphogenesis and migration. These data suggest that 131 integrins are required for normal morphogenesis of the inner cell mass and are essential mediators of growth and survival of cells of the inner cell mass. Failure of continued trophoblast development in 131-null embryos after inner cell mass failure could be attributable to either an intrinsic requirement for 131 integrins for later stages of trophoblast development, or to the lack of trophic signals from the 131-null inner cell mass.[Key Words: 131 integrins; e~V-integrins; trophoblast; extraembryonic endoderm; survival; migration] Received April 4, 1995; revised version accepted June 15, 1995.Cell--extracellular matrix (ECM) interactions play critical roles in morphogenesis and in the regulation of gene expression (Damsky and Werb 1992;Hynes 1992Hynes , 1994Adams and Watt 1993;Ashkenas et al. 1994;Cross et al. 1994). The integrin family of heterodimeric transmembrane glycoproteins constitutes the major class of receptors mediating cell-ECM interactions. These receptors link the ECM to the internal cytoskeleton and to intracellular signaling pathways. In this role, they mediate cell adhesion and migration, and transduce mechanical and informational signals from the complex extracellular environment, thereby influencing both cytoarchitecture and gene expression. Integrin heterodimeric receptors for ECM can be classified into two major families: {lJ those containing the 131 subunit, and, {2} those containing the oLV subunit. There is extensive apparent redundancy in the ligandbinding preferences of these integrins. For exampl...

We present genetic evidence that integrins regulate epithelial-mesenchymal interactions during organogenesis. Mice with a mutation in the alpha8 gene do not express the integrin alpha8 beta1 and exhibit profound deficits in kidney morphogenesis. In wild-type animals, inductive interactions between the ureteric epithelium and metanephric mesenchyme are essential for kidney morphogenesis. In alpha8 mutant homozygotes, growth and branching of the ureteric bud and recruitment of mesenchymal cells into epithelial structures are defective. Consistent with these phenotypes, alpha8 expression is induced in mesenchymal cells upon contact with the ureter. Since none of its previously identified ligands appears likely to mediate the essential functions of alpha8 beta1 in kidney morphogenesis, we have used an alpha8 beta1-alkaline phosphatase chimera to localize novel ligand(s) in the growing ureter. The distribution of these ligand(s) makes them strong candidates for regulators of kidney morphogenesis.