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Fig. 8. Early progeny of the N teloblast. (A) Lineage diagram showing divisions of
the N teloblast. After its birth (stage 6a, 
22 hours AZD), the N
teloblast divides every 90 minutes, generating a column of progeny that mostly
follow the ns and nf (segmental founder cell) fates in exact alternation. But
the fourth cell (n') contributes exclusively to the provisional
epithelium, and the fates of cells born prior to n' were not known with
certainty. (B) Pseudocolored 3D reconstruction of an nGFP-labeled n bandlet,
48 hours after the N teloblast was injected. The clone of the anteriormost
cell contains four nuclei of equal size (yellow), unlike either nf or ns, so
this is designated n°. The three cells posterior to this have each divided
once, in manner characteristic of the nf (green) or ns (red) clones. The
nucleus in the sixth position (broken outline) has entered mitosis before the
primary blast cell just ahead of it, also indicative of the nf (green) and ns
(red) fates. By this time, the n' micromere clone contains six cells
(blue), superficial to the bandlet. (C) Equivalent view of an older embryo, in
which the N teloblast was re-injected with RDA after the birth of n° (all
nuclei are shown as yellow). By cl.ag. 96 hours, the n° clone comprises
70-80 nuclei, distinguished by their smaller size and less intense
fluorescence. Nuclei of the RDA labeled nf and ns blast cells are larger,
brighter and surrounded by RDA fluorescence (red). The n° clone is now
flanked by cells derived from the anteriormost nf clone (arrows). Fissures
(arrowheads) have formed between the posterior edges of the nf clones and the
anterior edges of the adjacent ns clones. (D) Ventral view of the
subesophageal ganglion and presegmental tissue of an embryo (146 hours
AZD) in which the left N teloblast was injected with RDA shortly before the
birth of its first ns blast cell, i.e. after it had already produced cell
n° and the first nf blast cell. The embryo was counterstained with Hoechst
33258 (green). Broken outlines indicate the edges of the four neuromeres
(R1-R4) in the subesophageal ganglion. Neuromere R1 contains the same
complement of labeled neurons as neuromeres R2-R4, even though its anterior
edge is unlabeled. (E) Pseudocolored 3D representation of the RDA-labeled
cells in an embryo equivalent to that shown in D. (F-H) Pseudocolored images
(processed as in C) of an embryo in which the N teloblast was injected with
RDA after the birth of n° (i.e. one cell cycle earlier than in D and E).
The N teloblast had also been injected with nGFP mRNA prior to the birth of
n°. As a result, the n° clone is labeled with nGFP only, and the first
RDA-labeled clone is descended from the first nf cell. (F) Ventral view,
comparable with E, showing only the RDA labeled cells; broken outline
surrounds the first nf clone. (G) The same image as in F, with the addition of
the n°-derived nuclei (yellow). (H) Side view (ventral to left) of the
image shown in G. Scale bar: 10 µm.
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