Stage 1

Developmental timeline and staging designations for Smed embryogenesis. Timeline: days (d) post egg capsule deposition at 20˚C. Gray bars indicate time windows for RNA-Sequencing samples. Double headed arrows: time windows for Stages (S) S1-S8.



▻ Overview

▻ Stage Description

▻ Images






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Stage Description


Zygote(s) and yolk cells are packaged into egg capsules laid by the parent. The single cell stage can be visualized in histological cross-sections: spherical zygotes, ~ 25-30 µm in diameter, are surrounded by a halo of yolk cells reminiscent of the corona radiata. During early cleavage divisions blastomeres cycle asynchronously, without retaining direct cell-cell contacts, within a yolk syncytium. Embryos are readily apparent in histologically stained sections but may be difficult to locate in live, unstained specimens.


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Figure 1 - figure supplement 1: Histological cross-sections of Stage 1 embryos. A-D: Four independent examples of the single cell stage (S1), either metaphase II arrested oocytes or zygotes, in paraffin embedded, hematoxylin and eosin stained sectioned egg capsules fixed at 1-2 days post egg capsule deposition (20˚C). The entire capsule is shown in D. Black arrowheads: metaphase II arrested oocytes or zygotes, surrounded by a corona of yolk cells. Scale bars: 25 µm (A-C); 100 µm (D).



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