E12 injection-survival to E13
Figure 6 shows a sagittal-slice of an E13 embryo 24 hrs after its mother was injected with 3H‑thymidine on E12. The slice is filled with reduced silver grains distributed evenly throughout—that is “noise” and it is due to a positive chemographic reaction between the photographic emulsion and methacrylate. We realized later that interaction occurred after we had collected many methacrylate-embedded embryos; unfortunately, we had to discarded most of these specimens, but we did keep a few E12 to E14 embryos because paraffin embedding does not have good preservation of early embryonic tissue. The labeled cells appear darker because they have more radioactivity. The effect of the noise is to mask the signal labeling. However, we can still learn something from analyzing the labeling patterns in this slice.
24 hr after an E12 injection virtually every cell is labeled both inside and outside the NEP. That is because (1) these cells have divided at least once, and diluted the label that was absorbed on E12, and (2) the lack of unlabeled cells indicates virtually no neurogenesis occurs before E12. Note that the heaviest labeled cells are outside the NEP 24 hours after an E12 injection, especially outside the basal ganglionic NEPS. These are postmitotic neurons, generated on E12, that will settle in the magnocellular basal telencephalic nuclei. The oldest neurons in the telencephalon are in these nuclei.