The cellular interactions that control the differentiation of dorsal cell types from neural progenitors have been examined in neural plate explants. Certain genes that are expressed in the dorsal neural tube are initially expressed uniformly within the neural plate and appear to achieve their dorsal restriction through a Sonic hedgehog (SHH)-mediated repressive signal from the notochord. The acquisition of definitive dorsal cell fates, however, requires a contact-dependent signal from the epidermal ectoderm. BMP4 and BMP7 are expressed in the epidermal ectoderm, and both proteins mimic its inductive activity. BMP4 and a related gene, DSL1, are subsequently expressed by cells in the dorsal neural tube. The differentiation of dorsal cell types, therefore, appears to be initiated at the neural plate stage and to involve the opponent activities of a BMP-mediated dorsalizing signal from the epidermal ectoderm and a SHH-mediated ventralizing signal from the notochord.

Distinct neuronal cell types are generated at characteristic times and positions in the dorsal horn of the spinal cord. We provide evidence that the identity and pattern of generation of dorsal neurons depend initially on BMP-mediated signals that derive from the epidermal ectoderm and induce dorsal midline cells of the roof plate. Roof plate cells provide a secondary source of TGFbeta-related signals that are required for the generation of distinct classes of dorsal interneurons. These inductive interactions involve both qualitative and quantitative differences in signaling by TGFbeta-related factors and temporal changes in the response of neural progenitor cells.

Recently, a pituitary-specific enhancer was identified within the 5' flanking region of The glycoprotein hormones are a family of heterodimeric proteins which consist of a common a subunit noncovalently associated with a hormone-specific 13 subunit (37). The glycoprotein hormones include the pituitary hormones, luteinizing hormone, follicle-stimulating hormone, and thyroid-stimulating hormone. In addition, some species also synthesize a chorionic gonadotropin within the placenta. Within a species, the glycoprotein hormones all share a common a subunit, while the unique ,B subunits specify the biological activity of the heterodimer. In the pituitary gland, luteinizing hormone and follicle-stimulating hormone are synthesized within cells which are designated gonadotropes, while thyroid-stimulating hormone is synthesized in thyrotropes. Thus, the glycoprotein hormone a-subunit gene is expressed within two different cell types in the pituitary and in some species within the placenta. The mechanisms mediating the tissue-specific expression of the a-subunit gene have been the focus of a large number of studies. While a number of DNA elements which are important for at-subunit expression in the placenta have been identified (3,4,11,12,25,28,34,46), much less is known concerning requirements for expression in the pituitary. It has been demonstrated that reporter genes containing various * Corresponding author. Mailing address: