Talk:2010 BGD Practical 3 - Early Cell Division

From Embryology

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Monoygotic Twinning

Week 1
Week 2
Day 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14
Cell Number 1 1
Event Ovulation fertilization First cell division Morula Early blastocyst Late blastocyst


Implantation starts X inactivation
Monoygotic Twin Type Diamniotic







(a,b,e,f- human) (c- mouse) (d- worm)

(a) Mature oocyte with zona pellucida and surrounded by follicular cells (corona radiata). These cells provide oocyte protection, are the first barrier to sperm, and following are shed fertilization.

(b) Zygote showing 2 polar bodies and male and female pronuclei prior to their fusion. Note that corona radiata has been lost,but that zona pellucia is still present.

(c) Mixing of 2 parental genomes contained within each pronuclei during initial divisions of the zygote. Oocyte genome labelled blue, sperm genome labelled green.

(d) Early cell division (4-cell stage) of zygote, continued divison to 16 cell stage forms the morula stage (solid ball of cells).

(e) Movie showing cell division from 2 blastomeres (showing mitotic spindle) to late morula stage. Note the cell division occurs within the zona pellucida with a sequential reduction in cytoplasmic volume. Nuclei are shown by coloured ring overlays.

(f) Blastocyst stage with flattened outer shell of trophoblasts and inner cell mass (visible as a dark spot). Inner cell mass will give rise to the bilaminar then the trilaminar embryonic disc.