The Facts About Zygotes

Fertility and conception have a whole language of their own. Words like gamete, zygote and embryo may become part of your everyday conversation if you seek fertility treatment or learn about conception. So let's start at the very beginning.

When a sperm meets an egg (also known as an ovum) and fertilizes it, the egg becomes a zygote. In short, a zygote is a fertilized egg.

The zygote begins as a single cell that contains all 46 chromosomes to create a human being (23 from the sperm and 23 from the egg). The zygote is a brief phase lasting only four days before it moves on to the next stage and becomes a blastocyst.

Around five to six days after fertilization, the zygote divides through a process known as mitosis. It then becomes a cluster of cells known as a blastocyst. The blastocyst is a rapidly dividing ball of cells, eventually becoming the structures that protect and support the developing fetus.

The blastocyst travels down the fallopian tube for several days, before it implants in the wall of the uterus. Once implanted, the blastocyst divides again, and half of the cells become your baby and half the placenta. Around 10 to 12 days after fertilization, the blastocyst develops into an embryo.

The zygote is not an embryo. It becomes an embryo around two weeks after fertilization. Some of the cells in the blastocyst develop into an inner layer of membranes, also known as the amnion, forming the amniotic sac.

Once the amniotic sac is formed (around day 10 or 12), the blastocyst is considered an embryo.

The embryonic stage lasts until eight weeks after fertilization—about 10 weeks of pregnancy—when the embryo becomes known as a fetus.

Now that we've established what a zygote, blastocyst and embryo are, where does a gamete fit in?

A gamete is what is often referred to as "sex cells." It is essentially the reproductive cells of an organism. Female gametes are eggs (or ovum) and male gametes are sperm. It's these two cells that come together to make a zygote.

In humans, gametes are also haploid cells. This means they carry a single set of unpaired chromosomes that carry genetic information. Both sperm and egg cells carry 23 chromosomes.

When the sperm and egg cells come together and fertilization happens, male and female gametes fuse, producing a diploid cell. The two sets of 23 chromosomes in the gamete haploid cells pair up to form the 46 required to make a human organism.

Let's talk about twins for a moment. Where do they fit into this picture and how do they come about?

Identical twins (monozygotic twins) form when a single fertilized egg (a zygote) splits and develops two embryos with exactly the same genetic information.

Fraternal twins (dizygotic twins) form when two eggs are fertilized by two sperm. The two embryos formed are genetically unique.

Factors that influence the likelihood of having twins include:

• Age. Hormonal changes that happen nearer to menopause can stimulate your ovaries to release more than one egg at a time.
• Assisted reproductive technology (ART). Certain fertility medications and procedures can increase your chance of having twins.
• Genetics. You're more likely to have fraternal twins if you are a fraternal twin or have siblings who are fraternal twins.
• Previous pregnancies. The more pregnancies you have had, the higher the chances are that you'll conceive twins.
• Race. Black women have the highest incidence of twins. Asian women have the lowest.

Fertility and conception require a healthy sperm to find a healthy egg and for fertilization to happen. There are many factors that influence egg and sperm quality, including:

• Age
• Alcohol/drug use
• Certain medications
• Certain medical conditions
• Diet
• Environmental factors
• Genetics
• Hormone problems
• Lifestyle
• Obesity
• Smoking
• Stress

 To give yourself the best possible chance of having healthy eggs or sperm:

• Stop drinking alcohol or cut down
• Eat a healthy, balanced diet
• Exercise regularly
• Maintain a healthy weight
• Reduce stress and get a good night's sleep
• Stop smoking
• Stop using any illegal drugs

Always see a doctor if you have any concerns about your fertility so you can strategize some lifestyle changes that can benefit your chances of conception.

The zygote contains all the genetic information (46 chromosomes) required to form either a baby boy or a baby girl. Half the chromosomes come from the mother's egg and half from the father's sperm. The genetic information passed on from the egg and sperm cells determines the sex.

All egg cells have an X chromosome (female diploid cells are XX). Sperm cells can carry either an X or Y chromosome (male diploid cells are XY.)

It's actually the sperm that determines the sex of the baby. One X chromosome always comes from the egg and the second—which can be X or Y—comes from the sperm. If the sperm that fertilizes an egg has an X chromosome, the baby will be a girl (XX genotype). If it has a Y chromosome, it will be a boy (XY genotype).

It may seem that after ejaculation into the vagina, the sperm aimlessly swim through your cervix, uterus and fallopian tubes to try to find an egg. But the process is much more sophisticated than that.

Eggs actually release an attractant chemical—a transport calcium protein—to lure the sperm to find them. The tails of the sperm have receptors that can trace the calcium back to the egg, and these receptors also help the sperm tails wave a little faster to move them toward the egg.

Once a sperm reaches an egg and breaks through its outer barriers, the egg changes its physical nature and becomes impenetrable to other sperm.

Conception and egg fertilization essentially mean the same thing. Conception is when an egg and sperm successfully join, that is, an egg is fertilized by the sperm.

After conception, a fertilized egg travels through the fallopian tubes to the uterus.

Three main prenatal development stages run from conception to the development of a fetus:

1. The germinal phase (one to two weeks). This phase begins at conception when a sperm fertilizes an egg and creates a zygote. The zygote divides on day five or six and becomes a blastocyst, which travels down the fallopian tubes to implant into the uterus. Around day 10 to 12, the blastocyst becomes an embryo.
2. The embryonic phase (three to eight weeks). This phase is where the cells turn distinctly into a human. The neural tubes, brain, head, eyes, nose, ears, mouth and buds for arms and legs are formed. By week eight, the embryo has all of the basic organs (except the sex organs), the basic structures of the brain, and the basics of the central and peripheral nervous systems.
3. The fetal phase (nine weeks to birth). From week nine, the embryo is now considered a fetus. By the end of the first trimester (three months), the fetus develops all of its organs, its heart is beating regularly and the sex organs are beginning to develop, too. The second trimester leads to the development of skin, hair and nails, and the lungs have formed, too. The fetus is able to see, hear, move and sleep. In the third trimester, the lung formation is complete, the bones harden, the eyes open fully and movements are more pronounced.

Here is a glossary of the terms we've discussed:

• Blastocyst. The cluster of cells that a zygote turns into about five or six days after fertilization.
• Diploid. Cells have two complete sets of chromosomes. Most human body cells are diploid cells.
• Dizygotic twins. They are formed when two separate eggs are fertilized by two separate sperm.
• Embryo. Once the blastocyst has implanted in the womb and the amniotic sac has formed, it becomes an embryo.
• Gamete. This is a sex cell. A female gamete is an egg, and a male gamete is a sperm.
• Haploid. Cells carry a single set of chromosomes. Egg and sperm cells are haploid cells.
• Monozygotic twins. They are formed when a single zygote splits into two embryos.
• Zygote. A fertilized egg in the first few days of fertilization.