New neurons are continually recruited throughout adulthood in certain regions of the adult mammalian brain. How these cells mature and integrate into preexisting functional circuits remains unknown. Here we describe the physiological properties of newborn olfactory bulb interneurons at five different stages of their maturation in adult mice. Patch-clamp recordings were obtained from tangentially and radially migrating young neurons and from neurons in three subsequent maturation stages. Tangentially migrating neurons expressed extrasynaptic GABAA receptors and then AMPA receptors, before NMDA receptors appeared in radially migrating neurons. Spontaneous synaptic activity emerged soon after migration was complete, and spiking activity was the last characteristic to be acquired. This delayed excitability is unique to cells born in the adult and may protect circuits from uncontrolled neurotransmitter release and neural network disruption. Our results show that newly born cells recruited into the olfactory bulb become neurons, and a unique sequence of events leads to their functional integration

Both in vivo and in vitro, the addition of soluble ephrin-B1 results in a loss of the metameric migratory pattern and a disorganization of neural crest cell movement. These results demonstrate that Eph-family receptor tyrosine kinases and their transmembrane ligands are involved in interactions between neural crest and sclerotomal cells, mediating an inhibitory activity necessary to constrain neural precursors to specific territories in the developing nervous system.

Stimulation of B lymphocytes with a combination of lipopolysaccharide (LPS) and interleukin‐4 (IL‐4) induces germline transcription of and subsequent switching to the epsilon heavy chain constant region (C epsilon) gene. Mature germline C epsilon transcripts contain a non‐coding exon (I epsilon exon) spliced to the C epsilon exons. To distinguish between the potential roles of germline transcription and those of germline transcripts in regulating the class switch process, we replaced the LPS‐ and IL‐4‐inducible I epsilon promoter and exon in ES cells with an LPS‐inducible E mu enhancer/VH promoter expression cassette. Wildtype, heterozygous or homozygous mutant ES cells were injected into RAG‐2 deficient blastocysts to generate somatic chimeras in which all B cells derived from ES cells. In contrast to normal B cells, heterozygous and homozygous mutant B cells had substantial transcription through the epsilon switch recombination region (S epsilon) following treatment with LPS alone and, under these conditions, both underwent low level switching (10‐ to 100‐fold less than wildtype cells stimulated with LPS + IL‐4) to IgE production. Heterozygous mutant cells underwent switching to IgE at essentially wildtype levels when stimulated with LPS and IL‐4. However, homozygous mutant cells still showed extremely low levels of switching to IgE upon LPS and IL‐4 stimulation. Analyses of hybridomas from heterozygous mutants indicated that the mutation is cis‐acting and normal switching to other isotypes indicated that it is specific for IgE. Thus transcription per se generates low levels of class switch recombination in the absence of I region sequences. However, we demonstrate for the first time that, for optimal efficiency, the process requires the presence of the intact I region and/or I region promoter in cis, implicating factors beyond transcription through the S region in the regulation of class switching.

Embryonic axis elongation is a complex multi-tissue morphogenetic process responsible for the formation of the posterior part of the amniote body. How movements and growth are coordinated between the different posterior tissues (e.g. neural tube, axial and paraxial mesoderm, lateral plate, ectoderm, endoderm) to drive axis morphogenesis remain largely unknown. Here, we use quail embryos to quantify cell behavior and tissue movements during elongation. We quantify the tissue-specific contribution to axis elongation using 3D volumetric techniques, then quantify tissue-specific parameters such as cell density and proliferation. To study cell behavior at a multi-tissue scale, we used high-resolution 4D imaging of transgenic quail embryos expressing fluorescent proteins. We developed specific tracking and image analysis techniques to analyze cell motion and compute tissue deformations in 4D. This analysis reveals extensive sliding between tissues during axis extension. Further quantification of tissue tectonics showed patterns of rotations, contractions and expansions, which are consistent with the multi-tissue behavior observed previously. Our approach defines a quantitative and multi-scale method to analyze the coordination between tissue behaviors during early vertebrate embryo morphogenetic events.