2011 Lab 2 - Week 1

From Embryology
2011 Lab 2: Introduction | Week 1 | Week 2 | Week 3 | Online Assessment | Group Project

Molecular Changes

There are several important changes that occur in this new diploid cell beginning the very first mitotic cell divisions and expressing a new genome.

The oocyte arrested in meiosis is initially quiescent in terms of gene expression, and many other animal models of development have shown maternal RNAs and proteins to be important for early functions.

The new zygote gene expression is about cycles of mitosis and maintaining the toptipotency of the stem cell offspring cells.

The morula gene expression supports the formation of two populations of cells the trophoblast (trophectoderm) and embryoblast (inner cell mass), each having different roles in development, while maintaining the toptipotency of these populations.

Current research is now also pointing to non-genetic mechanisms or epigenetics as an additional mechanism in play in these processes.

Links: Cell Division - Mitosis | Molecular Development - Epigenetics

Genome Expression

The following figure is from a recent study[1] using video and genetic analysis of in vitro human development during week 1 following fertilization.

Model human blastocyst development.jpg

  • EGA - embryonic genome activation
  • ESSP - embryonic stage–specific pattern, four unique embryonic stage–specific patterns (1-4)
Links: Figure with legend | Blastocyst Quicktime Movie | Blastocyst Flash Movie

Telomere Length

A recent paper has measured telomere length in human oocyte (GV, germinal vesicle), morula and blastocyst and found changes in this length in preimplantation embryos.[2] Telomeres are the regions found at the ends of each chromosome and involved in cellular ageing and the capacity for division. The regions consist of repeated sequences protecting the ends of chromosomes and harbour DNA repair proteins. In the absence of the enzyme telomerase, these regions shorten during each cell division and becoming critically short, cell senescence occurs.

Early human telomeres.jpg Early human telomere length.jpg
Early human telomeres[2] Early human telomere length[2]

Sex Determination

--Mark Hill 13:21, 9 May 2011 (EST) These concepts will be covered later in detail with BGDB - Sexual Differentiation.

Mammalian sex determination is regulated by chromosomes.

  • Females have two X chromosomes. (XX)
  • Males have a single X and a small Y. (XY)
  • The X and Y chromosome are morphologically and functionally different from each other.
  • Evolutionary studies have shown that the Y was once the homologous pair for X.
  • It is only in the last 5 years that we have some idea about how these two types of chromosomes may be regulated and genes of importance located upon them.

X chromosome

Human idiogram-chromosome X.jpg

In females - the main scientific problem was understanding gene dosage, only one copy of X chromosome is needed to be genetically active the other copy is inactivated (More? X Inactivation.

About the X Chromosome

  • 155 million base pairs
  • In contrast to the Y chromosome, the X chromosome contains about 5% of the haploid genome and encodes house-keeping and specialized functions.
  • Genes such as Wnt-4 and DAX-1 necessary for initiation of female pathway ovary development
  • An early discovery (1961) was that in order to have correct levels of X chromosome gene/protein expression (gene dosage), females must "inactivate" a single copy of the X chromosome in each and every cell. The initiator of the X inactivation process was discovered (1991) to be regulated by a region on the inactivating X chromosome encoding an X inactive specific transcript (XIST), that acts as RNA and does not encode a protein.
  • The genetic content of the X chromosome has been strongly conserved between species because these genes have become adapted to working as a single dose - Ohno's law
  • X inactivation occurs randomly throughout the embryo, generating a mosaic of maternal and paternally derived X chromosome activity in all tissues and organs. This can be seen in the fur colour of tortoiseshell cats.

Y chromosome

Human idiogram-chromosome Y.jpg

In males - the main scientific problem was understanding what on the Y chromosome determined "maleness", and how this is done.

About the Y Chromosome

  • 59 million base pairs, hypervariable in length, mostly non-functional repeats
  • Current known protein-coding genes = 48 including SRY
  • SRY encodes a 204 amino acid protein that is a member of the HMG (High mobility group) box class of DNA-binding proteins. Transcription factors bind to specific sites of DNA and regulates the transcription (expression) of other genes.


  1. <pubmed>20890283</pubmed>| Nat Biotechnol.
  2. 2.0 2.1 2.2 <pubmed>20573647</pubmed>| PMC2930518 | Mol Hum Reprod.

Glossary Links

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Cite this page: Hill, M.A. (2024, April 22) Embryology 2011 Lab 2 - Week 1. Retrieved from https://embryology.med.unsw.edu.au/embryology/index.php/2011_Lab_2_-_Week_1

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© Dr Mark Hill 2024, UNSW Embryology ISBN: 978 0 7334 2609 4 - UNSW CRICOS Provider Code No. 00098G