The development of retinal projections to the dorsal lateral geniculate nucleus (dLGN) and superior colliculus (SC) has been studied in fetal and neonatal mice of the pigmented C57BL/6 strain, using the anterograde transport of tritiated proline and horseradish peroxidase (HRP). Retinal efferents are present contralaterally just beyond the chiasm at E14. By E16 they have grown into both dLGN and SC. Ipsilateral fibers are limited to the proximal optic tract at E16; their growth into dLGN and SC is delayed until E18-birth. During the first 2 postnatal days, an early population of ipsilateral fibers invades the dLGN. Most of these fibers grow in or around the medio-dorsal sector of the dLGN, i.e., the future binocular segment. Fibers are also present, but at lower densities, in the ventral half of the nucleus and thereafter become dispersed or are lost, without at any stage becoming dense. Some denser labeling is also present ipsilaterally in the outer rim of dLGN, just below the optic tract, and later disappears. On the third postnatal day, the ipsilateral fibers establish a deep and denser projection along the medial and dorsal borders of dLGN; this projection overlaps part of the crossed projection, which at this age extends to the whole nucleus. The segregation of each projection starts on the fourth postnatal day, when crossed fibers begin to disappear from the small region of uncrossed projection. This process goes on for another 4 days. During this period, the ipsilateral fibers withdraw from the deepest layer of dLGN, and their terminal density increases gradually; by the eighth postnatal day, both projections are already well separated. Dense crossed projections first appear near the surface of the SC at birth. Prior to this, retinal fibers course throughout neurons of the collicular plate and underneath the pia. The uncrossed fibers invade the SC between birth and P3. They are located preferentially in the anterior and medial aspect of the SC. Subsequently, there occurs a diminution in the laminar and tangential extent of these projections, simultaneously with an intensification of the ipsilateral input to several small, longitudinally oriented clusters located deep to the crossed projections.

is rigorously restricted to the sites where Placenta and yolk sac from 8-to 17-day-old (E8-E17) mouse embryos/fetuses were investigated for the presence of in vitro clonogenic progenitors. At E8-E9, the embryonic body from the umbilicus caudalwards was also analysed. Fetal liver was analysed beginning on E10. At E8, between five and nine somite pairs (sp), placenta, yolk sac and embryonic body yielded no progenitors. The first progenitors appeared at E8.5 at the stage of 15 sp in the yolk sac, 18 sp in the embryonic body, 20 sp in the placenta and only at E12 in the fetal liver (absent at E10, at E11 not determined). Progenitors with a high proliferation potential that could be replated for two months, as well as the whole range of myeloid progenitors, were found at all stages in all organs. However, the earliest of these progenitors (these yielding large, multilineage colonies) were 2-4 times more frequent in the placenta than in the yolk sac or fetal liver. In the fetal liver, late progenitors were more frequent and the cellularity increased steeply with developmental age. Thus, the fetal liver, which is a recognized site for amplification and commitment, has a very different hematopoietic developmental profile from placenta or yolk sac. Placentas were obtained from GFP transgenic embryos in which only the embryonic contribution expressed the transgene. 80% of the colonies derived from these placental cells were GFP + , and so originated from the fetal component of the placenta. These data point to the placenta as a major hematopoietic organ that is active during most of pregnancy.

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