Week One of Development

Fertilization

Fertilization occurs when female and male gametes meet and form a zygote. In order for fertilization to occur, the following processes need to take place:

  1. Capacitation

    • This is the process of maturation of sperm cells which occurs when they are deposited into the vagina. The male sperms become hyperactive and motile and are able to make their way towards the female egg.

  2. Ovulation

    • Ovulation is the process by which a female egg is released from the ovaries. The egg passes through the uterine tubes and has approximately 12-24 hours to encounter a male sperm such that it can be fertilized.

  3. Acrosomal reaction

    • This occurs when the male sperm encounters the female egg, the acrosomal reaction occurs as the sperm attempts to breakdown outer layer of the egg in order to penetrate it.

  4. Cortical reaction

    • This process allows for the modification of the outer layer of the female egg in order to prevent penetration by other male sperm.

  5. Zygote formation

    • A fertilize female egg (by a male sperm) is also known as a zygote.

Fertilization typically occurs in the ampulla region of the uterine tube.

Cleavage

Cleavage occurs approximately 24 hours after fertilization. Here, the while maintaining its size, the cells in the zygote begin doubling and replicating.

The zygote goes from the 2 cell stage, to the 4 cell stage, to the 8 cell stage, to the 16 cell stage and so forth.

As the size of the cell remains unchanged, the cells within the zygote become smaller.

Morula

By the 16 cell stage of cleaving, the ball of cells now looks significantly different from when it was a zygote, at this stage, this packed ball of cells is known as a morula. Meanwhile, the cells within the morula are now called blastomeres.

The cells within the morula being organizing themselves as they prepare for the next stage of development.

Blastulation

Blastulation occurs around 4 days after fertilization.

At this stage of development, the morula has made its way to the uterus and the cells within it start to arrange themselves into an outer cell mass (trophoblast) that surrounds the periphery and an inner cell mass (embryoblast) in the middle.

The cells of the outer cell mass (trophoblast) pull in luminal fluid into the morula from the uterine cavity. The fluid takes up space in the morula and this space is known as the blastocoel. This causes the cells of the inner cell mass to be pushed to one corner of the cavity and a blastocyst forms.

The cells of the outer cell mass (trophoblast) will give rise to the structures necessary for the support and growth of the embryo. Through further development, the trophoblast will differentiate into the cytotrophoblast and the syncytiotrophoblast.

The cells of the inner cell mass (embryoblast) will give rise to the embryo itself. Through further development, the embryoblast will differentiae into the epiblast and hypoblast (or the bilaminar disc).

Implantation

Approximately 5 days after fertilization, the blastocyst hatches and frees itself from the zona pellucida it was confined in, this allows it to grow in size and interact with the walls of the uterus.

The blastocyst attaches itself and begins implanting into the uterus.

Clinical Notes:

  1. Ectopic pregnancies - these occur when implantation occurs outside the uterus, such as in the uterine tubes, or the recto-uterine pouch (pouch of Douglas), or even in the intestines.