B2. Conception, implantation

Fertilisation (conception) is the process during which a sperm cell fuses with the oocyte. This occurs within 1 day of ovulation, usually in the ampullary region of the fallopian tube. Before sperm cells can fuse with the oocyte, they must undergo two processes, capacitation and the acrosome reaction. After these reactions, the sperm cell has fused with the oocyte, forming a zygote, a diploid cell with 46 chromosomes. At this point, the oocyte completes meiosis II.

The zygote then splits into two cells, each called a blastomere. The zygote is now at the 2-cell stage, and it will continue cleavage to continue to multiply the number of cells. When the zygote is at the 16-cell stage, it is known as a morula.

As all this happens, the zygote is travelling from the ampullary region of the fallopian tube to the uterine cavity. Approximately during the fourth day the zygote will enter the uterine cavity, and at this time the zona pellucida disappears. When it reaches the uterine cavity, fluid from the uterine cavity enters between the cells and form a single large cavity in the zygote, the blastocoele. At this point the zygote is known as a blastocyst.

Now that the zona pellucida is gone the blastocyst can begin implantation. During the sixth day after fertilization the finger-like projections of trophoblast cells starts to penetrate between the epithelial cells of the uterine mucosa, the endometrium. Implantation occurs most commonly into the anterior or posterior wall of the uterus.

Fertilisation/Conception

Fertilisation, also called conception, is the process by which a sperm cell fuses with the oocyte. This process most commonly occurs in the ampullary region of the fallopian tube. It occurs when insemination occurs during the 2 days preceding or on the day of ovulation.

The journey of a sperm cell

Spermatozoa may remain viable in the female reproductive tract for several days, but the oocyte is viable only for 12 – 24 hours, and so fertilization itself most commonly occurs within 1 day of ovulation. Movement of sperm from the cervix to the uterine tube is mostly due to muscular contractions of the uterus and uterine tube, not due to the spermatozoa’s own propulsion.

After reaching the isthmus of the uterine tube the sperm stop and wait. At ovulation the sperm become motile again, causing them to swim towards the ampulla, where they will meet the ovulated oocyte.

Capacitation and the acrosome reaction

Before sperm cells can fertilize the oocyte, they must undergo two processes, capacitation and the acrosome reaction.

During capacitation the sperm loses a glycoprotein coat and seminal plasma proteins on their surface. Capacitation allows sperm to pass through the corona radiata of the oocyte.

The acrosome reaction occurs when the sperm has passed through the corona radiata and bound to the zona pellucida. The acrosome is a small structure on the head of the spermatozoon which contains enzymes. During the acrosome reaction these enzymes are released from the acrosome, causing the zona pellucida at that point to be degraded. This allows the spermatozoon’s cell membrane to fuse with the oocyte’s cell membrane, forming a zygote. The zygote is diploid with 46 chromosomes.

The zona reaction

As soon as one spermatozoon has reached the oocyte’s cell membrane, the zona reaction occurs. This hardens the zona pellucida, making sure that only one sperm can fertilize the egg.

After fusion between the oocyte’s and the spermatozoon’s plasma membrane

As soon as the spermatozoon has entered the oocyte the oocyte finishes meiosis II, forming the mature ovum. The genetic material of the mature ovum forms the female pronucleus. The head of the spermatozoon forms the male pronucleus. DNA is then replicated in both pronuclei. After this replication, the pronuclei fuse, making them 2n4c.

The sister chromatids are then split longitudinally at the centromere and moved to opposite sides of the cell. The cell is then divided in two at the middle, yielding two genetically identical 2n2c cells, still enclosed within the zona pellucida. The zygote is now in the two-cell stage, which is reached approximately 30 hours after fertilization. Each of these cells are known as blastomeres.

Cleavage

Cleavage is the process where the zygote rapidly divides by mitosis. Because the zygote is still confined by the zona pellucida its volume and size remain unchanged as the cells multiply.

The two-cell stage is reached approximately 30 hours after fertilization, and 10 hours later the zygote is in the four-cell stage.

When the 16-cell stage is reached, the zygote is now a morula. This occurs 3 days after fertilization. At this stage not all cells are in contact with the zona pellucida, separating the blastomeres into the inner and outer cell mass. The inner cell mass will give rise to the embryoblast and later the embryo itself, while the outer cell mass will form the trophoblast and later the placenta.

Blastocyst formation

Approximately during the fourth day the zygote will enter the uterine cavity, and at this time the zona pellucida disappears. Fluid from the uterine cavity enters between the cells and form a single large cavity in the zygote, the blastocoele. At this point the zygote is known as a blastocyst. The trophoblast lines the outside of the blastocyst.

Implantation

The uterus is now in the secretory phase. Now that the zona pellucida is gone the blastocyst can begin implantation. During the sixth day after fertilization the finger-like projections of trophoblast cells starts to penetrate between the epithelial cells of the uterine mucosa, the endometrium. Implantation occurs most commonly into the anterior or posterior wall of the uterus.

The endometrium swells, increasing the glycogen stores. The trophoblast starts to secrete enzymes to dissolve away the wall of the endometrium. Paracrine hormones are also secreted, which increase the number of capillaries, allowing more oxygen and nutrients into the area.