2010 BGD Practical 3 - Early Cell Division

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


Introduction

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).

From Oocyte to Blastocyst

(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.

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.

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Zona pellucida (left) with Blastocyst (right) hatching through a small opening in the wall.

(More? Carnegie Stage 3)

Online Textbooks

MBoC - Mouse Blastocyst Development

Glossary

Terms

  • 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

Glossary Links

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

Cite this page: Hill, M.A. (2021, October 22) 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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© Dr Mark Hill 2021, UNSW Embryology ISBN: 978 0 7334 2609 4 - UNSW CRICOS Provider Code No. 00098G