Laminar organization of the early developing anterior hypothalamus.

The bHLH-PAS transcription factor SIM1 is required for the development of neurons of the anterior hypothalamus (AH). In order to dissect this developmental program, we compared gene expression in the AH of E12.5 Sim1(+/+) and Sim1(-/-) littermates using an oligonucleotide-based microarray. Our analysis identified 48 genes that were downregulated and 8 genes that were upregulated. We examined the expression pattern of 10 of the identified genes--Cart, Cbln1, Alcam, Unc-13c, Rgs4, Lnx4, Irx3, Sax1, Ldb2 and Neurod6--by in situ hybridization in E12.5 embryos. All of these genes are expressed in domains that are contained within that of Sim1 and their expression is changed in Sim1(-/-) embryos as predicted by the microarray analysis. Classical dating studies have established that the hypothalamus follows an "outside-in" pattern of neurogenesis, with neurons of the lateral hypothalamus being born before the medial ones. Analysis of the genes identified in this microarray study showed that the developing AH is characterized by different layers of gene expression that most likely correspond to distinct waves of neurogenesis. In addition, our analysis suggests that Sim1 function is required for the production or the survival of postmitotic neurons as well as for correct positioning of AH neurons.

[Light and electron microscopic studies of the greater epithelial ridge and its relationship to the developing tectorial membrane in the cochlear duct of the guinea pig (author's transl)]. / Licht- und elektronenmikroskopische Untersuchungen des grossen Epithelwulstes und seiner Beziehungen zu der sich entwickelnden Membrana tectoria im Ductus cochlearis von Meerschweinchenfeten.

In early stages of fetal development (36th day, 3rd turn) the thickening of the epithelium at the basal side of the cochlear duct forms two ridges. Later in fetal development the laterally situated lesser epithelial ridge forms the major part of the organ of Corti, whereas the medially situated greater epithelial ridge contributes only a small part to this organ. The medial part of the greater ridge consists of the columnar inner supporting cells, which bear a border of closely packed microvilli at their upper surface. Up to the time of the opening of the internal spiral sulcus in the 48th day of fetal development, there is a close spacial relationship between microvilli and filaments of the tectorial membrane. We conclude that the inner supporting cells contribute to the formation of the tectorial membrane. However, thus far we cannot entirely exclude a different possibility, that the inner supporting cells absorb material of the tectorial membrane. During the opening of the sulcus spiralis internus the inner supporting cells become considerably smaller, some of them undergo complete destruction by cytolysis, with pyknosis and karyorrhexis.