We have used the chicken retina as a model for investigating cell cycle regulation and cell fate commitment during central nervous system development. This thesis focuses on the characterization of and commitment to the horizontal cell fate in the retina. Horizontal cells are interneurons that provide intraretinal signal processing prior to information relay to the brain. We have identified molecular markers that selectively distinguish the three subtypes of horizontal cells, previously described in the chicken retina based on morphology. Subtype specific birth-dating revealed that horizontal cell subtypes are generated consecutively by biased progenitors that are sensitive to the inhibitory effects of follistatin. Follistatin stimulates proliferation in progenitors by repressing the differentiation signal of activin. Initially, injection of follistatin led to a decrease in committed horizontal cells but as the inhibitory effect dissipated it resulted in an increased number of horizontal cells. During development committed horizontal cell progenitors migrate to the vitreal side of the retina where they become arrested in G2-phase for approximately two days. When the arrest is overcome the horizontal cell progenitors undergo ectopic mitosis followed by migration to their designated layer. The G2-phase arrest is not triggered or maintained by any of the classic G2-arrest pathways such as DNA damage or stress. Nevertheless, we show that the cyclin B1-Cdk1 complex has a central role in maintaining this G2-phase arrest. Two transcription factors, FoxN4 and Ptf1a, are required for the generation of horizontal cells. We show that these factors are also sufficient to promote horizontal cell fate. Overexpression of FoxN4 and Ptf1a resulted in an overproduction of horizontal- and amacrine cells at the expense of ganglion- and photoreceptor cells. We identified Atoh7, a transcription factor required for the generation of ganglion cells, as a Ptf1a transcriptional target for downregulation. Our data support a common horizontal/amacrine lineage separated from the ganglion/photoreceptor lineage by the action of Ptf1a. In conclusion, these data describe several novel characteristics of horizontal cells enhancing our understanding of neural development and cell fate commitment.

GaP ist ein Halbleiter mit einer großen Bandlückenenergie und infolgedessen transparent im größten Teil der sichtbaren Wellenlängen. GaP hat außerdem die kleinste Gitterfehlanpasung zu Si (weniger als 0.4%). Das macht GaP ein interessantes Material für monolithische Integration zu III–V Lichtsender auf Si. Diese Arbeit ist eine Untersuchung über die strukturellen und optische Eigenschaften von (Al, Ga) (As, P) Heterostrukturen auf GaP (001) -Substrat aufgewachsen. Die Einflüsse des PH3 Fluss und Wachstumstemperatur untersucht auf dem Kristallqualität und Oberflächenqualität von AlGaP/GaP Heterostructure. Experimentelle Ergebnisse deuten darauf hin, dass eine Wachstumstemperatur von 490 oC und ein geknackter (engl. cracked) PH3 Fluss von 2.7 sccm zur besten AlGaP Qualität und gleichzeitig zur guten GaP Qualität führen. Um die ineffiziente Lichtemission von GaP zu überwinden wurde GaAs in der GaP-Matrix gewachsen. Die Entstehung der Quantenpunkte wurde durch die 3.7% Gitterfehlanpassung zwischen GaAs und GaP für GaAs Nenndicke über 1,2 ML. Die optischen Messungen zeigen zwei Peaks im Bereich von 1,7 bis 2,1 eV und die Lumineszenz auf Raumtemperatur für 2,7 und 3,6 ML-Proben. Die hohe Energieemission wird der indirekten Rekombination in den dünnen Quantentröge oder kleine gespannte Quantenpunkte zurückzuführen, Während die niedrige Energie Emission ist aufgrund der direkten Elektron-Loch- Rekombination in der entspannten Quantenpunkte. Die Wirkung von Al wird untersucht auf die energetische Bandausrichtung und auf die elektronische Struktur der (Al,Ga)As Quantenstrukturen. Die optische Spektren zeigten einen blaue Verschiebung (engl. blue shift) mit wachsendem Al-Inhalt und die höchste missionsenergie für die (Al,Ga)As/GaP- Heterostruktur war 2.17 eV die zum indirekten Typ-II-Rekombination zusammenhängt. / Transparency of GaP due to the large indirect bandgap energy and its small lattice mismatch with Si make GaP an interesting candidate for optoelectronic devices in visible wavelength. This thesis is an investigation on the structural and optical characteristics of (Al,Ga)(As,P) heterostructures grown on GaP (001) substrates. The influences of the PH3 flux and growth temperature are studied on the crystal and surface quality of AlGaP/GaP heterostructure. The results indicate the narrow growth window of PH3 = 2.7 sccm and growth temperature = 490oC as the optimized conditions. To overcome the inefficient light emission of indirect GaP, direct bandgap GaAs was grown as the quantum structures in the GaP matrix. The QD formation is driven by the 3.7% lattice mismatch between GaAs and GaP for GaAs nominal thickness above 1.2 ML. The optical measurements show two peaks in the range of 1.7 to 2.1 eV and the luminescence up to room temperature for 2.7 and 3.6 ML samples. The high energy emission is attributed to indirect carrier recombination in the thin quantum wells or small strained quantum dots, whereas the low energy red emission is due to the direct electron-hole recombination in the relaxed quantum dots. The influence of the Al content on the band alignment and electronic structure of (Al,Ga)As quantum structures is studied. The optical spectra illustrate the blueshift of the radiative emission with increasing the Al content and the highest emission energy of 2.17 eV is observed for the (Al,Ga)As/GaP system that is related to the indirect type-II radiative recombination.