Download Fertilization and Development

Fertilization and Development
• Life’s Greatest Miracle
• http://video.pbs.org/video/1841157252/
Prenatal Stages of Human Life
• Prenatal (fertilization - birth)
• Embryo - (fertilization - 8 weeks of gestational
phase)
– Zygote - the point of conception, fertilization
– Blastula (blastocyst) - the period between conception
and embryonic stage
– Embryo - the embryonic period starts at three weeks
and continues until the end of the 8th week of
pregnancy
• Fetus (8 weeks of gestational phase - birth)
Fertilization
• In animals, fertilization is the fusion of gametes (sperm
and egg).
• Mammals rely on internal fertilization through copulation
(sexual intercourse) to deliver the sperm to the female.
• The male inserts his penis into the opening of the
vagina, the passage into the female's other sexual
organs. Once the male ejaculates, a large number of
sperm cells move to the upper vagina (via contractions
from the vagina) through the cervix and across the
length of the uterus toward the ovum— a considerable
distance compared to the size of the sperm cell
• After finding the egg, the sperm binds to
the egg, causing an acrosome reaction to
release the enzyme hyaluronidase, which
digests the outer layer of the egg.
• The sperm’s nucleus and flagellum enter
the egg, but not the mitochondria. The egg
“activates" once it fuses with a single
sperm cell (its cell membrane changes to
prevent fusion with other sperm).
• This protective coat is called a fertilization
membrane.
• When the sperm and egg unite, it makes a
diploid zygote.
• In medicine, the beginning of pregnancy is when
a sperm enters an ovum and forms a zygote.
• Now, in western medicine, pregnancy is defined
as beginning when a zygote becomes implanted
in a woman's uterus. This occurs when the
zygote becomes embedded into the
endometrium where it makes a placenta.
• * important for sake of medical research (stem
cells, etc.)
Growth of Organism once
fertilization has happened
• The zygote undergoes rapid mitotic
divisions with no significant growth (a
process known as cleavage).
• The resulting daughter cells are called
blastomeres.
• About 5-6 days after fertilization the zygote
transforms into a mass of cells called a
blastocyst (blastula).
• The blastocyst (blastula) becomes
attached to the endometrium, a process
referred to as implantation (this usually
happens 6-10 days after fertilization)
• At this point, the blastula is a fluid-filled
ball.
• The blastula contains an inner cell mass
and that contains stem cells.
Where do stem cells come from?
• Stem cells are the body’s master cells,
they are undifferentiated and can become
any cell in your body.
• Early stem cells are found in the inner cell
mass of the blastocyst. Once the inner cell
mass is removed from the blastocyst, the
stem cells are placed in a culture dish
where they grow and replicate over time. In
normal reproductive development, the
inner cell mass becomes all the tissues of
the body.
• Once implantation begins, cell division
stops temporarily. When cell division starts
again, all cells that are produced are not
alike.
• Differentiation is the process by which
cells develop into specialized tissues and
organs.
• At around 16 days after fertilization,
gastrulation happens and this is when the
blastula folds in on itself (blastula can be called
gastrula)
• The inner cell mass inside the blastocyst
rearranges itself into 3 distinct layers which are
responsible for the development of the organs
and accessory structures:
• Endoderm (“inside skin”)
• Ectoderm (“outside skin”)
• Mesoderm (“middle skin”)
• Endoderm turns into the inner linings of
the digestive tract, as well as the linings of
the respiratory passages. It also forms
many glands/organs, such as the liver and
pancreas.
• Ectoderm turns into the skin, nails, the
epithelium of the nose, mouth and anal
canal; the lens of the eye, the retina and
the nervous system.
• Mesoderm forms the muscles, circulatory
and excretory systems of the body.
• At the 3rd week of development four
membranes begin to form:
1. chorion: the protective sac around the
embryo. It makes up most of the placenta.
It contains many villi (finger-like
projections), which allow for exchange of
nutrients, gases and wastes between
mother and embryo.
2. Yolk sac: Transports nutrients to the
embryo during the second and third weeks
3. Amnion: a tough, thin, transparent
membrane that envelops the embryo. It’s
interior space, the amnionic cavity,
becomes filled with a watery, amnionic
fluid. The amnion functions to: insulate the
embryo/fetus, protect it from infection,
dehydration, impact and temperature
change, and help dilate the mother’s
cervix at the start of labour.
4. Allantois: involved in the formation of
blood cells and its blood vessels become
the vein and arteries of the umbilical cord
• Placenta (part of chorion): a thick bed of
tissues and blood vessels embedded into the
wall of the uterus that provides an indirect
connection between mother and fetus
• 3 functions of placenta:
• It transports material between mother and fetus
• It provides nutrients and energy to the fetus
• It secretes HCG hormone
• *** Sugar, water, oxygen, hormones, infectious
agents, toxic substances and drugs can cross
the placental barrier.
• Umbilical cord: a connecting tube
between the embryo and the placenta,
carrying carbon dioxide and nitrogen
waste from the embryo and oxygen and
nutrients to the embryo.
• This all happens before 20 days.
• At 20 days, the foundations of brain, spinal
cord and nervous system are established
• Day 21 – heart begins to beat
• Day 26 – arm buds appear
• Day 28 – backbone and muscles are
forming
Second month (5-8 weeks)
• the face is forming
• ears covered by thin membrane and continue
developing
• brain, liver, kidneys, bloodstream, and digestive
system are developing
• arms and legs developed
• By the end of the second month, the embryo has
become a fetus. It is about 1 and 1/8th inches
long and weighs 1/30th of an ounce
Third month (9-12 weeks)
• the fetus' eyelids are fused
• facial features are present, the nose and outer
ears are formed
• nail beds established
• movement such as head turning or sucking
begins
• teeth buds forming
• all internal organs formed
• swallows amniotic fluid
Fourth month (13-16 weeks)
• strong heart beat begins
• lanugo or fine body hair develops
• At the end of the fourth month, the fetus is
about 6 inches long and weighs four
ounces.
Fifth month (17-20 weeks)
• heartbeat can be heard with a stethoscope
• finger and toe nails formed
• sucks thumb
• responds to noise
• grows hair and eyebrows
• movements become increasingly vigorous
Sixth month (21-24 weeks)
• eyes are open
• a creamy substance called vernix covers the
skin
• skin is wrinkled and the fetus appears very thin
• At the end of the sixth month, the fetus is about
11-14 inches long and weighs 1 1/2 pounds.
• The placenta and umbilical cord are now fully
developed and continue to increase in size.
• At 38 weeks, the uterus will have reached
the breastbone and you may begin to
experience early signs of labour.
• During this period, the most dramatic
change in the fetus is its continued growth.
By the seventh month it weighs about two
and a half pounds and is approximately 15
inches in length.
• By the eighth month the fetus weighs about 5 pounds
and is 16 to 18 inches long. During the last month of
pregnancy the fetus weighs seven or eight pounds and
may be more than 20 inches long. In addition:
• skin wrinkles become less pronounced as body fat
accumulates and begins to smooth out the body
• eyes open and close
• fetus responds to light
• lanugo disappears
• fetus stores iron and builds skeleton