Sexual Reproduction in HumansPregnancy
STOP: Did you take the Gametogenesis Quia Quiz? Do not proceed until you have taken it.
decrease size
decrease size
external image 031010_0757_AnatomyandP1.gif?9d7bd4


For fertilization to occur, sperm must be deposited in the vagina within a few (5) days before or on the day of ovulation. During ejaculation, the male releases up to 280 million sperm. (Males do release sperm even if they do not ejaculate). During this process the sperm, stored in the epididymis travel through the vas deferens. As the sperm travel through the vas deference fluid is added to the sperm by the seminal vesicles, Cowper's glands, and the prostate gland. [View] These fluids provide
  • a source of energy (fructose)
  • an alkaline (ph below 7) environment to activate the sperm, and
  • other compounds to protect the sperm from the female immune system (which tries to attack and destroy the sperm.)
The mixture of sperm and accessory fluids is called semen. It passes through the urethra and is expelled into the vagina.

external image 250px-Scheme_female_reproductive_system-en.svg.pngOnce deposited within the vagina, the sperm proceed on their journey into and through the uterus and on up into the fallopian tubes. It is here that fertilization may occur if an "egg" is present (strictly speaking, it is still a secondary oocyte until after completion of meiosis II).

Although sperm can swim several millimeters each second, their trip to and through the fallopian tubes may be assisted by muscular contraction of the walls of the uterus and the tubes. The trip is fraught with heavy mortality. An average human ejaculate contains over one hundred million sperm, but only a few dozen complete the journey, arriving within 15 minutes of ejaculation. And of these, only one will succeed in fertilizing the egg.

external image sperm.jpgSperm swim towards the egg by sensing chemicals and hormones released by the cells surrounding the eggs. When the first sperm reaches the egg proteins on the surface of the head of the sperm will bind to proteins on the surface of the egg (zona pellucida). The acrosome at the tip of the sperm head releases enzymes that digest a path through the zona and enable the sperm head to bind to the plasma membrane of the egg. Fusion of their respective membranes allows the entire contents of the sperm to be drawn into the cytoplasm of the egg. (Even though the sperm's mitochondria enter the egg, they are almost always destroyed and do not contribute their genes to the embryo. So human mitochondrial DNA is almost always inherited from mothers only.) The sperm's flagella will remain outside the egg.

Within moments after the sperm's nucleus enters the following steps occur
  1. enzymes released from the egg cytoplasm act on the zona making it impermeable to the other sperm that arrive. This prevents fertilization by multiple sperm (polyspermy.)
  2. the egg (secondary oocyte) completes meiosis II forming a second polar body.
  3. The male and female nuclei move toward each other while duplicating their DNA in S phase. Their nuclear envelopes disintegrate.The homologous chromosomes pair together. The fertilized egg or zygote is now ready for its first mitosis. When this is done, 2 cells — each with a diploid set of chromosomes — are formed.
external image 400px-Acrosome_reaction_diagram_en.svg.png

The first month

The first two weeks are a very busy time for the embryo, even though the mother does not yet know that she is pregnant! During the first two weeks the following occur
  1. Fertilization - formation of a zygote
  2. Cellular cleavage - during which the embryo carries out cell division
  3. Gastrulation - during which the cells of the early embryo begin to take on different roles
  4. Implantation - during which the embryo implants into the mother's uterus
  5. formation of extraembryonic membranes - which will provide the support structures and the embryo's blood cells.

Pregnancy: Click here to watch a short video of the events that occur during the first week.

Development begins while the fertilized egg is still within the fallopian tube. Approximately 30 hours after fertilization the zygote begins to divide. During cellular cleavage, the embryo experiences repeated mitotic divisions as it travels through the fallopian tubes. After several rounds of cell division the embryo becomes a solid ball of cells called a morula. Further mitosis and some movement of cells converts this into a hollow ball of cells called the blastocyst. Approximately one week after fertilization, the blastocyst embeds itself in the thickened wall of the uterus, a process called implantation, and pregnancy is established.

How does a pregnancy test work? Click here to find out.

external image blastocyst.gifThe blastocyst produces two major collections of cells:
  • Three or four blastocyst cells develop into the inner cell mass, which will form
    • 3 extraembryonic membranes: amnion, yolk sac, and (a vestigial) allantois and

    • in about 2 months, become the fetus and, ultimately, the baby.

  • The remaining 100 or so cells form the trophoblast, which will develop into the **chorion** that will go on to make up most of the placenta. All the extraembryonic membranes play vital roles during development but will be discarded at the time of birth.


Shortly after forming a blastocyst, the cells of the embryo begin a process called gastrulation. During this process the blastocyst becomes a gastrula. The gastrula contains three distinct cell layers. See the picture below


Extra-embryonic Membranes

The Chorion will become the placenta. The placenta will exchange food, oxygen and waste between mother and child.

The Amnion will become the amniotic sac and amniotic fluid. These provide protection.

The Allantois (not shown in the picture) becomes the umbilical cord, connecting the baby to the placenta.

The Yolk Sac will provide blood cells until the baby can make its own. This will eventually dissolve away.

Formation of the Nervous System: click here to watch a short animation
STOP: Take the Human Development- First Month Quia Quiz? Do not proceed until you have taken it.

Take a break from the computer... grab a partner and complete the First Month Storyboard. See Mrs. Elinow for the assignment and supplies.

How do Twins Form?: click here to watch a short animation

Human pregnancy lasts for 9 months, see the chart below to learn about lengths of gestation for other animals:
external image go-figure-animals-gestation-compared-infographic-110922a-02.jpg?1316718928


The placenta: click here to watch an animation

Formation of the Heart: click here to view a short animation

End of Month 2: click here to visit the visible embryo website

Amniotic Fluid: click here to view a short animation

The second trimester begins, 12 weeks: click here to visit the visible embryo website

Boy or Girl? click here to watch a short animation

Weeks 18-19: click here to visit the visible embryo website

Weeks 24-25: click here to visit the visible embryo website

Week 26: click here to visit the visible embryo website

Week 28: click here to visit the visible embryo website

Week 32: click here to visit the visible embryo website

Week 38: click here to visit the visible embryo website

Three stages of labor: click here to visit WebMD

Infertility and Assisted Reproductive Technology: click here to visit MedicineNet

IVF is on this page:

STOP: Take the Human Development -9 months- Quia Quiz. Do not proceed until you have taken it.