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Fig. 1 | BMC Developmental Biology

Fig. 1

From: The asymmetric cell division machinery in the spiral-cleaving egg and embryo of the marine annelid Platynereis dumerilii

Fig. 1

Polarization of the P. dumerilii Zygote upon Fertilization. a Mature Oocytes of P. dumerilii: Peripheral lipid droplets (yellow) surround the central oocyte nucleus (light blue) exhibiting a slight concentration within the equatorial plane. b Ooplasmic segregation triggered by fertilization. Cortical lipid droplets and yolk granules stream towards the vegetal pole while clear cytoplasm, including the female pronucleus, is segregated towards the animal pole establishing an A/V polarity axis. b‛ Completion of the first and second meiotic cell division generates two polar bodies defining the animal pole. Male (red) and female (light blue) pronuclei fuse forming the nucleus of the zygote. b Asymmetry of the first mitotic spindle: The smaller aster of one spindle pole maintains its proximity to an animal cortical domain foreshadowing the first asymmetric mitotic cell division. c Timeline of early events that polarize the zygote upon fertilization in P. dumerilii. Mass exocytosis of cortical granules generating a surrounding jelly coat occupies the first 40 min post fertilization (mpf). Ooplasmic segregation leads to a redistribution of cytoplasm along the A/V axis, completion of meiotic division at the animal pole with the formation of the first and second polar body at ~60 and ~80 mpf, and subsequent fusion of pronuclei. Asymmetries in spindle pole sizes and unipolar spindle attachment to the animal cortex forecast the first asymmetric cell division parallel to the A/V axis generating a smaller AB cell and a larger CD cell (see Fig. 2)

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