Faculty Bibliography

Characterizing embryonic circulatory physiology requires accurate cardiac output and flow data. Despite recent applications of high-frequency ultrasound Doppler to the study of embryonic circulation, current Doppler analysis of volumetric flow is relatively crude. To improve Doppler derivation of volumetric flow, we sought a preliminary model of the spatial velocity profile in the mouse embryonic dorsal aorta using ultrasound biomicroscopy (UBM)-Doppler data. Embryonic hematocrit is 0.05-0.10 so rheologic properties must be insignificant. Low Reynolds numbers (<500) and Womersley parameters (<0.76) suggest laminar flow. UBM demonstrated a circular dorsal aortic cross section with no significant tapering. Low Dean numbers (<100) suggest the presence of minimal skewing of the spatial velocity profile. The inlet length allows for fully developed flow. There is no apparent aortic wall pulsatility. Extrapolation of prior studies to these vessel diameters (300-350 microm) and flow velocities (~50-200 mm/s) suggests parabolic spatial velocity profiles. Therefore, mouse embryonic dorsal aortic blood flow may correspond to Poiseuille flow in a straight rigid tube with parabolic spatial velocity profiles. As a first approximation, these results are an important step toward precise in utero ultrasound characterization of blood flow within the developing mammalian circulation

Huntingtin is an essential protein that with mutant polyglutamine tracts initiates dominant striatal neurodegeneration in Huntington's disease (HD). To assess the consequences of mutant protein when huntingtin is limiting, we have studied three lines of compound heterozygous mice in which both copies of the HD gene homolog (Hdh) were altered, resulting in greatly reduced levels of huntingtin with a normal human polyglutamine length (Q20) and/or an expanded disease-associated segment (Q111): Hdh(neoQ20)/Hdh(neoQ20), Hdh(neoQ20)/Hdh(null) and Hdh(neoQ20)/Hdh(neoQ111). All surviving mice in each of the three lines were small from birth, and had variable movement abnormalities. Magnetic resonance micro-imaging and histological evaluation showed enlarged ventricles in approximately 50% of the Hdh(neoQ20)/Hdh(neoQ111) and Hdh(neoQ20)/Hdh(null) mice, revealing a developmental defect that does not worsen with age. Only Hdh(neoQ20)/Hdh(neoQ111) mice exhibited a rapidly progressive movement disorder that, in the absence of striatal pathology, begins with hind-limb clasping during tail suspension and tail stiffness during walking by 3-4 months of age, and then progresses to paralysis of the limbs and tail, hypokinesis and premature death, usually by 12 months of age. Thus, dramatically reduced huntingtin levels fail to support normal development in mice, resulting in reduced body size, movement abnormalities and a variable increase in ventricle volume. On this sensitized background, mutant huntingtin causes a rapid neurological disease, distinct from the HD-pathogenic process. These results raise the possibility that therapeutic elimination of huntingtin in HD patients could lead to unintended neurological, as well as developmental side-effects

Recent studies suggest that neurons born in the developing basal forebrain migrate long distances perpendicularly to radial glia and that many of these cells reach the developing neocortex. This form of tangential migration, however, has not been demonstrated in vivo, and the sites of origin, pathways of migration and final destinations of these neurons in the postnatal brain are not fully understood. Using ultrasound-guided transplantation in utero, we have mapped the migratory pathways and fates of cells born in the lateral and medial ganglionic eminences (LGE and MGE) in 13.5-day-old mouse embryos. We demonstrate that LGE and MGE cells migrate along different routes to populate distinct regions in the developing brain. We show that LGE cells migrate ventrally and anteriorly, and give rise to the projecting medium spiny neurons in the striatum, nucleus accumbens and olfactory tubercle, and to granule and periglomerular cells in the olfactory bulb. By contrast, we show that the MGE is a major source of neurons migrating dorsally and invading the developing neocortex. MGE cells migrate into the neocortex via the neocortical subventricular zone and differentiate into the transient subpial granule neurons in the marginal zone and into a stable population of GABA-, parvalbumin- or somatostatin-expressing interneurons throughout the cortical plate

The contribution of early cell lineage to regional fate in the mammalian forebrain remains poorly understood. Previous lineage-tracing studies using retroviral methods were only begun at mid-neurogenesis and have suffered from region-specific retroviral silencing. We have been able to study cell lineage in the telencephalon from the onset of neurogenesis by using ultrasound backscatter microscopy to label the forebrain neuroepithelium and a modified retroviral lineage library to overcome regional silencing. Our studies suggest that by embryonic day 9.5, forebrain clones are primarily restricted to territories within anatomically demarcated regional boundaries, such as the cortex, striatum and hypothalamus. In addition, we observed a subset of clones that appeared to be composed entirely of glia. These observations suggest that both regional and cell-type restrictions exist within progenitor populations before the first forebrain cells become postmitotic