Fertilization: Sperm and Egg Journey

• Sperm proceed through the uterus, heading towards the fallopian tubes to find the egg.
• The number of sperm is significantly reduced during the journey.
• Sperm swim towards the fallopian tubes, with only half entering the tube containing the egg.
• Millions of cilia line the fallopian tubes, creating a downward current which sperm must swim against.
• Some sperm get trapped or lose direction in the fallopian tubes.
• Enzyme inhibitors at the tips of the sperm's heads wear away, fully capacitating them for fertilization.
• Around 50 of the initial 200 million sperm reach the egg.
• The egg is surrounded by two layers of nutritive cells that have nourished it.
• Sperm release digestive enzymes to penetrate the egg's protective layers.
• Only one sperm typically fertilizes the egg.
• The egg starts to rotate due to the sperm's efforts.
• The first sperm to reach the egg's membrane is drawn inside.
• After fertilization, the egg membrane becomes impermeable to other sperm.
• The penetrating sperm loses its midpiece and tail.
• The sperm head swells and ruptures, releasing genetic material.
• The genetic material from the sperm combines with the egg's genetic material.
• Two cells join to form a single new cell.
• Within 24 hours, the new cell begins to divide.

Early Cell Division

• The fertilized egg contains two nuclei.
• The first division of the egg begins, resulting in two identical cells.
• The fertilized egg is called a zygote once it begins to divide.
• The zygote divides repeatedly, creating four cells, then eight cells.
• Each new generation of cells within the zygote becomes smaller than the previous one.

Blastocyst Formation and Implantation

• As the zygote divides, it moves towards the uterus.
• The zygote transforms into a dense cluster of cells.
• After five days, the zygote is called a blastocyst.
• The blastocyst is the same size as the original egg and filled with liquid.
• Within 10 days of fertilization, the blastocyst implants in the uterine lining.
• The mother's hormones direct changes to support the growing embryo.
• The embryo is nourished by the uterine lining, then the placenta.

Embryonic Development

• After two weeks, the embryo is elongated and measures about one-tenth of an inch.
• The top of the embryo will become the head and brain.
• The tail of the embryo is attached to the placenta.
• At four weeks, the embryo has arm buds and is curled, with the beginning of eyes.
• At five weeks, the nose starts to form.
• At six weeks, leg buds appear, and the embryo, less than half an inch, floats in the amniotic sac.
• The spine is visible at six weeks.
• At seven weeks, the embryo measures three-quarters of an inch and can move its hands, which have defined fingers.
• Internal organs and eye lenses are forming at seven weeks.
• Skull bones grow together at an angle on the crown of the head at seven weeks.
• At eight weeks, fingers and toe joints are clearly visible.

Fetal Development

• At about 10 weeks, the embryo is considered a fetus.
• The fetus can move actively and has a suggestion of an ear, measuring two inches long with a tail stump.
• At 11 weeks, the fetus is two and a half inches long.
• At 12 weeks, the fetus is three inches long.
• The umbilical cord connects the fetus to its food supply.
• By 14 weeks, the fetus can bring its hands together and suck its thumb.
• By 15 weeks, sensory organs are almost fully formed.
• By 16 weeks, the fetus is actively turning inside the mother.
• At 18 weeks, the fetus is five and a half inches long.
• The mouth and lips are fully formed, and it has nasal plugs.
• The eyes are closed, but the fetus can see.
• Fetal respiration involves breathing in amniotic fluid.
• The umbilical cord links the fetus to the mother.
• The sex organs are visible.
• Physiological systems have developed, but the fetus requires at least another eight weeks to survive outside the womb.
• Uterine contractions begin when the fetus is ready to be born.