2010 BGD Practical 3 - Early Cell Division

From Embryology

Practical 3: Oogenesis and Ovulation | Gametogenesis | Fertilization | Early Cell Division | Week 1 | Implantation | Week 2 | Extraembryonic Spaces | Gastrulation | Notochord | Week 3 | Quiz



Following fertilization, the zygote undergoes a series of mitotic cell divisions during the first week of development leading to the formation of a hollow ball of cells, the 1-chambered conceptus or blastocyst (defined by the presence of this cavity the blastocoel).

Practical Audio

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BGD Cycle A 2010 Audio - Dr Mark Hill Monday 12th May 2010 12-2pm G2G4.

Note - this is a live unedited recording from the practical and may contain errors in either descriptions or content.

listen Part 5 | download (1.2 Mb MP3 9:26)

From Oocyte to Blastocyst


First cell divisions of the zygote forming initially 2 blastomeres and continuing to divide to form the morula.

This early mitosis is a unique embryonic cell cycle (M, S, M phases) compared to adult (M, G1, S, G2, M phase). With virtually no G1 or G2 phases this results in a reduction in cytoplasmic volume of each daughter cell with each cell division. See also Human oocyte to blastocyst

Facts: Week 1, 2-3 days, size 0.1-0.2 mm Features: zona pellucida, blastomeres

Stage 2 Day 2 Stage 2 Day 3 Human_embryo_day_5.jpg
Human Embryo (day 2) Human Embryo (day 3) Human Embryo (day 5)

Embryo mitosis icon.jpg

Blastocyst Hatching

By the end of the first week the blastocyst now consists of a ball of cells containing a large hollow fluid-filled (blastoceol) space.

Trophoblast Layer - Continued expansion of the blastoceol and cell division has led to the single layer of cells located at the periphery now being pressed against the inflexible zona pellucida wall and becoming flattened (squamous). This peripheral layer of cells is now called the trophoblast layer. Later in development this later will be involved in implantation and form part of the placenta.

Inner Cell Mass - On one wall of the blastoceol there is present a second layer of non-flattened cells. These inner cells are called the inner cell mass. Later in development from this layer the embryo will be formed.

Hatching - a combination of lysins (from the blastocyst or the uterus) and physical expansion, reduces the thickness and weakens the zona pelludica wall in preparation for hatching. Typically the blastocyst will hatch through a small opening (potentially at the site of fertilization) in the zona pellucida. The blastocyst is now ready to begin implantation.

Assisted Hatching - in vitro fertilization techniques use zona thinning, zona drilling, zona slitting or laser-assisted hatching to help blastocyst hatching and increase the probability of implantation occurring.

CSt3.jpg Zona pellucida (left) with Blastocyst (right) hatching through a small opening in the wall. (More? Carnegie stage 3)

Links: MBoC - Mouse Blastocyst Development


Practical 3: Oogenesis and Ovulation | Gametogenesis | Fertilization | Early Cell Division | Week 1 | Implantation | Week 2 | Extraembryonic Spaces | Gastrulation | Notochord | Week 3 | Quiz

Additional Information

Dizygotic Twinning

Dizygotic twins (fraternal, non-identical) arise from separate fertilization events involving two separate oocyte (egg, ova) and spermatozoa (sperm).

Monoygotic Twinning

Monoygotic twins (identical) produced from a single fertilization event (one fertilised egg and a single spermatazoa, form a single zygote), these twins therefore share the same genetic makeup. Occurs in approximately 3-5 per 1000 pregnancies, more commonly with aged mothers. The later the twinning event, the less common are initially separate placental membranes and finally resulting in conjoined twins.

Week Week 1 Week 2
Day 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14
Cell Number 1 1 2 16 32 128 bilaminar
Event Ovulation fertilization First cell division Morula Early blastocyst Late blastocyst


Implantation starts X inactivation
Follicle 001 icon.jpg Early zygote.jpg Human embryo day 2.jpg Human embryo day 3.jpg Human embryo day 5.jpg CSt3.jpg Week2 001 icon.jpg

Twin Type








Table based upon: Twinning. Hall JG. Lancet. 2003 Aug 30;362(9385):735-43. Review. PMID: 12957099


  • bilaminar- having 2 layers
  • blastocyst- the developmental stage following morula, as this stage matures, the zona pellucia is lost allowing the conceptus to adplant and then implant into the uterine wall.
  • blastomeres-the cells resulting from the initial rounds of mitotic division of the zygote. These cells become smaller (in cytoplasmic volume) with each division.
  • corona radiata- Layer of follicle cells of cumulus oophorus remaining attached to zona pellucida of oocyte after ovulation.
  • inner cell mass- the clump of cells found inside the blastocyst. These cells will go in to form the embryo, these are the "stem cells" (we here about in the media) that are totipotential, they can form any tissue in the embryo. Mature oocyte-the female germ cell released at ovulation from the ovary.
  • morula - (L. morus = mulberry) early stage of development (12-15 cells) Followed by formation of a cavity in the mass (blastocyst stage). (More? Week 1 Notes)
  • parental genomes- the male (sperm) and female (oocyte) DNA which contributes to the embryo's cells.
  • polar bodies- 3 exclusion bodies which contain the DNA not used by the embryo. Contributed to initially by the meiotic division of the oocyte.
  • pronuclei- the male (sperm) and female (oocyte) nuclei within the fertilized oocyte, prior to their combination to form the new embryo's nuclei.
  • trilaminar embryonic disc- the 3 layered embryo stage.
  • Trophoblasts- (Gr. trophe = nutrition) outer layer of cells on blastocyst that will generate the embryonic part of the placenta.
  • uterine wall - the site of normal blastocyst implantation.
  • zona pellucida- glycoprotein shell that surrounds the oocyte through to blastula stage of development.
  • Zygote- The first cell stage following fertilization of the oocyte by the sperm. This is the first cell of the conceptus which will divide into blastomeres.

Online Resources

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2010 BGD: Lecture 1 | Lecture 2 | Practical 3 | Practical 6 | Practical 12

Cite this page: Hill, M.A. (2024, April 24) Embryology 2010 BGD Practical 3 - Early Cell Division. Retrieved from https://embryology.med.unsw.edu.au/embryology/index.php/2010_BGD_Practical_3_-_Early_Cell_Division

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