ANAT2341 Lab 2 - Week 1

From Embryology
ANAT2341 Lab 2: Introduction | Fertilization | Week 1 | Week 2 | Online Assessment | Group Project

Week 1


Week 1 and 2 development

Key Events of Human Development during the first week (week 1) following fertilization or Clinical week 3 (LMP).

The first week of human development begins with fertilization of the egg by sperm forming the zygote, followed by early cell division forming the blastocyst. These notes also cover events before fertilization formation of both the egg and sperm, gametogenesis.

Initially, there is a halving of chromosomal content in the gametes, which is restored by fertilization, allowing genetic recombination to occur. This is then followed by a series of cell divisions without cytoplasmic growth. During this first week the egg, then zygote, then the blastula is moving along the uterine horn into the uterus for implantation in the uterine wall.

Overview of development of the 1-chambered conceptus (blastocyst)

Week1 summary.jpg

Ovulation is the initial release of oocyte.

Follicular fluid and fimbriae together aid oocyte movement into infundibulum then the ampulla of the uterine tube.

Sperm deposited in the vagina then enter the uterus, mature (capacitation), then actively migrate along the uterine tube.

Fertilization generally occurs in the ampulla region of the tube.

Following fertilization, repeated rounds of cell division occur without growth forming initially a solid ball of cells (morula), which cavitates to form the 1-chambered conceptus (blastocyst). Liberation of the blastocyst from the zona pellucida the allows attachment (adplantation) to the uterine wall.

Following ovulation the empty follicle within the ovary now forms the corpus luteum.

Note - the day timings shown above for the first week are approximate and may vary by several days for events following fertilization.

Week 1 - Formation of Zygote

  • male and female pronuclei, 2 nuclei approach each other and nuclear membranes break down
  • DNA replicates, first mitotic division
  • sperm contributes centriole which organizes mitotic spindle
Human zygote two pronuclei 22.jpg Mouse zygote pronuclei 01.jpg
Human zygote pronuclei Mouse zygote pronuclei[1]

Movie - Pronuclear Fusion | Movie - Parental Genomes

Conceptus - term refers to all material derived from this fertilized zygote and includes both the embryo and the non-embryonic tissues (placenta, fetal membranes).

Blastocyst (right) hatching from zona pellucida (left)

CSt3.jpg Human carnegie stage 3 label.jpg

Week 1 Movies

Follicle 001 icon.jpg
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Spermatozoa animation icon.jpg
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Spermatozoa motility icon 01.jpg
 ‎‎Spermatozoa Motility
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Fertilization 002 icon.jpg
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Fertilization 001 icon.jpg
 ‎‎Mouse Fertilisation
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Pronuclear fusion 001 icon.jpg
 ‎‎Pronuclear Fusion
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Week1 001 icon.jpg
 ‎‎Week 1
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Human Blastocyst (day 3 to 6)
 ‎‎Day 3 to 6
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Human blastocyst day 5-6.jpg
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Human blastocyst hatching movie icon.jpg
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Secretory Phase Stage 1 Early zygote.jpg Fertilization, Zygote, Secretory Phase
Stage 2 Stage2.jpg Week1 001 icon.jpg Morula, Blastula
  Week1 001 icon.jpg
Human embryo day 5.jpg
Stage 3 CSt3.jpg

Blastocyst Hatching (zona pellucida lost) Blastocyst (free floating)

Stage 4 Adplantation
Stage 5 Stage5 bf11b.jpg Week2 001 icon.jpg Implantation


  • adplantation - Initial adhesion of blastocyst (released from zona pellucida) to uterine wall. Adplantation is followed by implantation.
  • ampulla - longest segment (approximately 2/3 of overall length) of uterine tube (oviduct or Fallopian tube). Medial segment forming the remainder of the tube is called the isthmus.
  • antrum- (Latin, a cave), cavity; a nearly-closed cavity or bulge. In the ovary this refers to the follicular fluid-filled space within the follicle.
  • blastocyst - the developmental stage following morula, as this stage matures, the zona pellucia is lost allowing the coceptus to adplant and then implant into the uterine wall.
  • capacitation the process by which sperm become capable of fertilizing an egg, requires membrane changes, removal of surface glycoproteins and increased motility.
  • cavitates - to form a space within a solid object.
  • coel - a term meaning “cavity” (blastocoel, embryonic coelom, extra-embryonic coelom).
  • conceptus the product of conception, that is all the structures derived from the zygote. This includes not only the embryo, but also the placental and membrane components formed from the conceptus.
  • fertilization The penetration of the egg by the sperm and the resulting combining of genetic material that develops into an embryo. The union of two haploid gametes to form a diploid cell or zygote.
  • fimbriae (Latin, fimbria = a fringe) finger-like projections at the ovarian end of uterine tube. At ovulation they sit over the ovary to aid egg movement into the uterine tube.
  • follicular fluid - the fluid found in the antrum of an antral follicle (secondary follicle). Secreted by cells in the wall of the follicle, this fluid is released along with the oocyte at ovulation.
  • infundibulum - funnel-shaped initial segment of uterine tube (oviduct or Fallopian tube) opening into peritoneal cavity and connected to the ampulla. The peritoneal opening sitting over the ovary.
  • morula (Latin, morula = mulberry) early stage in development (week 1) where the cells have divided to produce a solid mass of cells (12-15 cells) with a "mulberry" appearance. This stage is followed by formation of a cavity in the mass (blastocyst stage).
  • oocyte - (egg or ovum) female germ cell.
  • ovulation- release of the oocyte from the mature follicle.
  • triploidy - in humans, three sets of 23 chromosomes instead of 2 (diploid) combine to form the embryo. This occurs mainly by fertilization of a single egg by two sperm and less frequently by a diploid egg or sperm. Most human triploids abort spontaneously, with very rare survival to term.
  • tetraploidy - in humans, four sets of 23 chromosomes instead of 2 (diploid) due to a failure of the first mitotic division after fertilization, these fertilization events do not development.
  • uterine tube - (also called oviduct or Fallopian tube) the laterally paired tubes that connect the ovary to the uterus. Is the site for oocyte fertilization and initial development of the conceptus.
  • uterine wall - the site of normal blastocyst implantation.
  • zona pellucida- glycoprotein shell that surrounds the oocyte through to blastula stage of development
  • uterus- site of embryo implantation and development. Uterine wall has 3 layers; endometrium, myometrium, and perimetrium.
  • zona pellucida- extracellular layer lying directly around the oocyte underneath follicular cells. Consists of glcosaminoglycans and glycoproteins (ZP1, ZP2, ZP3).

ANAT2341 Lab 2: Introduction | Fertilization | Week 1 | Week 2 | Online Assessment | Group Project

Additional Information


Zygote pronuclei stages 01.jpg

Within the early zygote, at the 2 pronuclei stage, the male pronucleus is "reprogrammed" by the demethylation of the paternal genome. Image sequence shows the mouse zygote at pronuclear stages[1], where the male pronucleus initially contains methylcytosine (5mC, red) oxidises to form hydroxymethylcytosine (5hmC, green).

  • PN1/PN2 (Early)
  • PN3 (Mid)
  • PN4/PN5 (Late)

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[2] 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.[3] 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[3] Early human telomere length[3]


  1. 1.0 1.1 <pubmed>21321204</pubmed>| PMC2132672 | PNAS
  2. <pubmed>20890283</pubmed>| Nat Biotechnol.
  3. 3.0 3.1 3.2 <pubmed>20573647</pubmed>| PMC2930518 | Mol Hum Reprod.

ANAT2341 Lab 2: Introduction | Fertilization | Week 1 | Week 2 | Online Assessment | Group Project

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Cite this page: Hill, M.A. (2024, April 16) Embryology ANAT2341 Lab 2 - Week 1. Retrieved from

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