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

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.


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 totipotency 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 totipotency of these populations.

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


It is the repeated mitotic divisions of the zygote, resulting into a rapid increase in the number of cells that are called blastomeres.

Site: The uterine tube medial to the ampula.

Human development before implantatioon

DIVISION OF THE ZYGOTE into 2 daughter cells, the blastomeres, takes place by 30 hours. Further divisions follow rapidly upon one another, forming progressively smaller and smaller blastomeres: 4 are seen in 40-50 hours, 8 by 60 hours, and 12-16 by day 3 or 4

The 12-16 blastomere stage, arrived at by cleavage of the fertilized ovum, is a solid ball resembling a mulberry and is called a morula (morula stage). As it forms, the morula enters the uterine cavity from the tube.

Stages of cleavage

a- Morula: It has 16 cells(blastomeres). It has mulberry appearance. It is surrounded by Zona pellucida. It enters the uterus nearly 3 days after fertilization.

b- ABOUT DAY 4, fluid enters the morula from the uterine cavity and occupies the intercellular spaces. The fluid-filled spaces fuse to form a single, large cavity, the blastocele, and the morula is now called a blastocyst (blastocyst stage)

Week 1 abnormalities

Dizygotic Twinning

Monoygotic Twinning

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