How does a single cell become a fetus and then a baby?

Human Conception and Pregnancy

Review…Like most animals, humans reproduce

sexually. You started out as a single, fertilized egg called a zygote.

Surviving sperm use their flagella to swim through the uterus and up the fallopian tube. When they reach the egg, the sperm surround it.

They release an enzyme that breaks down the proteins in the egg’s outer covering.

As soon as a single sperm gets through that covering, a reaction occurs that keeps any other sperm from entering.

The membrane of the egg then fuses with the sperm.

Next, the sperm’s flagellum and mitochondria break down.

At that point, the sperm is only a nucleus.

This explains why all mitochondrial DNA is inherited from the mother.

When the sperm and egg nuclei fuse, fertilization is complete.

The fertilized egg then begins a 5 day journey down the fallopian tube (oviduct) toward the uterus.

During the journey, the zygote undergoes many divisions.

What type of division does it undergo?

By the time it reaches the uterus, it is an embryo that looks like a tiny ball of cells

7B Human reproduction – Fertilisation

Human reproduction begins with fertilisation.

The next step is called implantation.

In implantation, the tiny embryo becomes embedded in the lining of the mother’s uterus.

Implantation is successful only about 30 percent of the time.

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Trimesters: Human pregnancy can be divided into three-month stages.

First Trimester 0- 3This lasts from fertilization to the end of the

third month.Once the embryo is implanted in the uterus,

the placenta begins to grow.What is the function of the placenta?

The placenta supplies food and oxygen from the mother’s blood to the developing embryo.

It contains a network of blood vessels that links the embryo to the mother. Waste from embryos blood diffuses into mothers blood

By the fourth week of pregnancy, the embryo is about 2-7 millimeters long. (500 times larger than fertilized egg)

The four chambered heart has formedBrain becomes visibleLimb bud with tiny fingers and toes

By the end of the second month: cartilage is replaced by bone

Embryo is called fetusmost body parts formedArms and legs begin to move Sucking reflex can be seenWhat is the difference Between an embryo and A fetus?

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It is surrounded by a thin, protective membrane called the amnion.

Inside the amnion and surrounding the embryo, is amniotic fluid that protects the embryo from shocks.

The umbilical cord has also formed.What is the function of the umbilical

cord?

It contains arteries and veins that connect the embryo to the placenta.

It enters the embryo’s body at the umbilicus, or naval.

The blood of the mother and fetus do not mix together.

Oxygen and nutrients pass from the mother’s blood to the fetus through the placenta and umbilical cord. (smoking= bad)

Waste products like carbon dioxide pass from the fetus to the mother’s blood for removal.

Second Trimester 4-6 monthsThe 57-mm fetus moves enough for to be

felt by the mother All the organs have been formedAnd like other mammals soft hairs cover the

entire bodyIn the forth month the fetus begins to

swallow amniotic fluid and hiccup and suck its thumb

In the 6th month eyelids and eyelashes formMost cartilage in the skeleton has been

replaced by bone

Third Trimester 7-9 monthsThe organ systems that developed in the 1st and

second trimester will now function properlyAll that remains will to increase body mass

http://health.howstuffworks.com/how-a-fetus-grows3.htm#

Birth The mother goes through a series of

contractions called parturition: the process of birth.

The cervix dialates and the amniotic membrane breaks (“water”) and lubricates the canal

The fetus is pushed, headfirst, through her vagina (birth canal).

The baby is still connected to the placenta by the umbilical cord.

When the doctor ties and cuts the umbilical cord, birth is complete.

Hormones and BirthHormones play a vital role in the birthing

processRelaxin is produced by the placenta= causes

ligaments in the pelvis to loosen. This provides flexibility.

Prior to labour- the hormone oxytocin is secreted by the pituitary gland

This causes strong uterine contractions which push the baby into the birth canal

After the placenta is expelled, secretion of estrogen and proesterone stops

If the mother does not breast feed her menstrual cycle begins within a few months

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