Introduction

Key Events of Human Development during the first week (week 1) following fertilization or clinical gestational age GA week 3, based upon the last menstrual period.

The first week of human development begins with fertilization of the egg by sperm forming the first cell, the zygote. Cell division leads to a ball of cells, the morula. Further cell division and the formation of a cavity in the ball of cells forms 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 (spermatozoa and oocyte) by the process called gametogenesis. Chromosomal content is then 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, morula then the blastula is moving along the uterine horn into the uterus for implantation in the uterine wall.

Implantation also begins in this first week, but will be covered in Week 2 notes, as the implantation process is completed by the end of the second week.

Human blastocyst week 1 movies, 3 above movies together in single table.

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).

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[2], where the male pronucleus initially contains methylcytosine (5mC, red) oxidises to form hydroxymethylcytosine (5hmC, green).

5mC - 5-methylcytosine (red). 5hmC - 5-hydroxymethylcytosine (green) formed by enzymatic oxidation of 5mC.

Mouse zygote mitosis[2]

Cleavage of the zygote forms 2 blastomeres and is cleavage with no cytoplasm synthesis.

Cell division within these cells is initially synchronous (at the same time), then becomes asynchronously (at different times).

Carnegie stage 2

Carnegie stage 3

Two forms of cellular junctions Figure 21-69. The blastula

Blastocyst Hatching - zona pellucida lost, ZP has sperm entry site, and entire ZP broken down by uterine secretions and possibly blastula secretions. Uterine Glands - secretions required for blastocyst motility and nutrition

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.

The following figure is from a recent study[4] using video and genetic analysis of in vitro human development during week 1 following fertilization.

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.[5] 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.

See Week 1 - Abnormalities

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

Monozygotic 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.

Table based upon: Twinning. Hall JG. [6]

Embryo Week: Week 1 | Week 2 | Week 3 | Week 4 | Week 5 | Week 6 | Week 7 | Week 8 | Week 9

Cite this page: Hill, M.A. (2019, October 20) Embryology Week 1. Retrieved from https://embryology.med.unsw.edu.au/embryology/index.php/Week_1

Link:
Week 1 - Embryology