Lesson 5: Fertilization and Fetal Development Flashcards
(14 cards)
Internal and External Fertilization
In aquatic species fertilization occurs externally. Both the male and female gametes
are released into the water where fertilization takes place.
In reptiles, birds and mammals fertilization occurs internally. Sperm cell are
transferred into the female body during copulation.
Internal fertilization has two main advantages:
Increases the chance that gametes will meet so these species have to produce
fewer gametes.
The fertilized ovum can be enclosed in a protective covering such as a shell or
remain in the female’s body during development.
Human Fertilization
Fertilization is the moment when one
sperm cell fuses with the secondary
oocyte to form a zygote.
Fertilization occurs in the oviducts of
the female.
The process of fertilization in human
is designed to prevent polyspermy,
the entry of more than one sperm
cell nucleus.
Steps of Fertilization
A large number of sperm are ejaculated into the vagina by the male during sexual intercourse.
Only a small number of them make it through the cervix and uterus.
The Acrosome Reaction
The secondary oocyte is surrounded by the zona pellucida, a glycoprotein coat.
The head of the sperm contains a sac of enzymes called the acrosome.
When the sperm binds to the zona pellucida, the contents of the acrosome are released.
The enzymes digest the zona pellucida.
Penetration of the Egg Membrane
The acrosome reaction exposes an area of the membrane on the sperm that has proteins
that can bind to the egg membrane.
The first sperm that gets through the zona pellucida binds and the membranes of sperm and
egg fuse together.
The sperm nucleus enters the egg cell.
The Cortical Reaction
The entry of the sperm nucleus activates the egg.
The contents of the cortical granules are released from the egg. This causes the binding
proteins on the egg membrane to be digested so that no further sperm can bind.
This also result in the hardening of the zona pellucida.
The Blastocyst
After fertilization the zygote
travels down the oviduct
towards the uterus.
As the zygote travels, it begins the process of mitosis, with a series of divisions. By the time it
reaches the uterus, 24 – 36
hours after fertilization, it is an almost hollow ball of cells called a blastocyst.
The outer cells of the blastocyst will become the placenta while the inner cell mass will become the embryo.
Implantation
6 – 10 days after fertilization the blastocyst attaches itself to the endometrium lining in a
process called implantation.
Once implanted the blastocyst begins secreting human chorionic gonadotropin (HCG).
HCG travels in the blood stream and affects the ovary. HCG signals the ovary to maintain the
corpus luteum, which produces progesterone and estrogen through the first trimester.
At this stage it is called an embryo.
Embryo Development
In the blastocyst stage, the cells are all similar to each other. However, in the second week of pregnancy the cells begin to specialize. This
process produces a gastrula and is called gastrulation.
During gastrulation the cells in the embryo become arranged in 3 layers called germ layers.
The cells move to specific positions to produce the layers called the;
Ectoderm – forms the skin and nervous system
Mesoderm – forms the kidneys, skeleton, muscles, blood vessels and gonads
Endoderm – forms the lungs and lining of the digestive tractBetween the tenth and
fourteenth day of pregnancy
the outer portions of the
embryo form four important
parts;
- Yolk Sac – supplies
nutrients for the first 2
months of development - Amnion – fluid filled sac
that protects the embryo - Allantois – removes
waste - Chorion – surrounds
everything
The Placenta
After the first three months the placenta is fully formed and begins producing placental estrogen and
progesterone to maintain the pregnancy.
The placenta is also responsible for
delivering nutrients and removing waste for the developing fetus.
The placenta is made up of the maternal endometrium and small projections from the chorion surrounding the embryo, called chorionic villi.
Chorionic Villi
Grow out of the chorion into the
endometrium and are rich in
capillaries.
Provide a large surface area for
the exchange of nutrients and
waste between the mother’s
blood and that of the fetus.
Fetal blood remains in the
capillaries of the chorionic
villi which are surrounded by
maternal blood flowing into
the endometrial spaces.
This allows the mother’s
blood to come in as close
contact as possible without
the two mixing.
The Umbilical Cord
The embryo is attached to the
placenta by the umbilical cord.
Fetal blood is carried to the
placenta by two umbilical
arteries and carried back to the
fetus by a single umbilical vein.
The umbilical arteries
divide into capillaries in
the chorionic villi.
Nutrients and oxygen
diffuse form the mother’s
blood into that of the fetus
while carbon dioxide and
waste diffuse from the
fetus to the mother.
Gestation
The human gestation
period is about 40 weeks. It
is divided into 3 blocks of
time called trimesters
Each trimester is ~ 3
months long, with major
development changes
occurring in every
trimester.
First Trimester
(1-12 weeks)
At 6 weeks limbs, eyes and spine begin to form.
At 9 weeks the first bone cells form and the embryo is called a fetus.
At 12 weeks all major organs have begun to develop (liver, brain, stomach, heart). The fetus is 100mm long, has a noticeable head and limbs, sex can be identified using ultrasound.
Second Trimester
(13-25 weeks)
At 16 weeks the skeleton begins to form, the brain grows rapidly and the nervous system begins to form. The mother
begins to feel movements as the fetus uses its new muscles.
At 24 weeks the fetus is 300mm long. The fetus becomes more active and all organs are formed but not fully developed.
Third Trimester
(26-40 weeks)
There is a rapid increase in
size during the third
trimester.
Vital brain tissue is built,
and the nervous system
develops.
Risk Factors During Development
The fetus receives all of its nutrients and oxygen from the mother’s blood.
Anything that the mother eats, drinks or inhales from her environment end up in
her blood.
The first trimester is the critical period for the development of the embryo.
Cigarette smoke constricts fetal blood vessels preventing it from getting
enough oxygen
Alcohol affects the functioning of the fetus’ brain and central nervous system
as well as physical development (FAS).
Another risk factor for the embryo is the age of the mother;
Women under the age of 20 have an increased risk of delivering a premature
baby.
Women over 45 have a 1 in 20 chance of having a baby with chromosomal
abnormalities.
Other factors such as radiation, pollutants such as PCB’s and mercury can lead to genetic mutations.
