Transcript 9fertilizationanddevelopment08

Fertilization, Development, and Birth
adapted by S. Jackson ‘08
What do
you see in
this
picture?
Let’s take a
closer look.
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Outline
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Fertilization
Embryonic Development (before 8 weeks =
56 days)
Miscarriages
Fetal Development (9 weeks to 40 weeks –
birth)
Birth
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Fertilization
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An ovum (egg cell) can be fertilized for
about 24 hours after ovulation
Sperm remain viable for up to 48 hours
within the female reproductive tract
This gives a three day “window” for
intercourse to result in fertilization: two days
before to one day after ovulation
Fertilization usually takes place in the
oviduct.
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Fertilization
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Steps of fertilization (Conception).
– Several sperm penetrate corona radiata.
– Several sperm attempt to penetrate zona
pellucida.
– One sperm enters egg and nuclei fuse,
producing a zygote.
 Egg’s plasma membrane and zona
pellucida change to prevent
polyspermy.
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Fertilization
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Human Development before Implantation
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Processes of development
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Zygote undergoes rapid mitotic cell division,
but these do not increase the size of the
zygote – called cleavage divisions
Cleavage produces a solid sphere of cells,
still surrounded by zona pellucida – now
called a morula.
At 4.5 to 5 days, cells have developed into a
hollow ball of cells – blastocyst.
It is at this stage that it enters the uterus.
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Implantation
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At about 6 days after ovulation blastocyst
implants into endometrium of the uterus.
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The blastocyst starts to secrete HCG human chorionic gonatotropin
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Stimulates estrogen and progesterone to
prevent menstrual flow
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Causes "morning sickness" in some
women...
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Pregnancy test measures the amount of this
hormone
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After Implantation
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Days 7-10: Gastrulation: major cellular
reorganization into 2 or 3 tissue (germ) layers:
-Ectoderm: skin, nervous system
-Endoderm: lining of gut and internal organs
-Mesoderm: muscles, bones, heart
Gastrula: Early embryo with 3 tissue layers. All
cells have the same DNA; however, different cells
now begin to "turn on" (or "express") different
genes to become different organs. (cells begin to
differentiate)
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Gastrulation : The formation of germ layers
from a (1) blastula to a (2) gastrula. Some of
the ectoderm cells (orange) move inward
forming the endoderm (red).
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Early Developmental Stages
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If the embryo makes it this far...it is usually
off to a good start!
The first 3 weeks are the most hazardous
periods in your life. Roughly one third to one
half of all fertilized zygotes never make it
beyond this point. Sadly, many women will
have at least one miscarriage in their
childbearing years. Some of the critical
errors that can cause a miscarriage are :
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Inheritance of a defective set of
chromosomes. Errors in meiosis (called
nondisjunctions) can produce an egg or
sperm that has an abnormal number of
chromosomes or broken chromosomes.
This is almost always lethal. About half of
the early miscarriages in humans are
afflicted with this kind of random
chromosomal defect.
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Errors in mitosis after fertilization. A
nondisjunction in a dividing blastula may
produce one abnormal cell -- but since the
blastula has so few cells, that means a
significant fraction of the embryo is
defective, preventing further development.
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Implantation errors. Human embryos have
to nestle down in a good home, in the
uterus. If the mother's hormones are not just
right, that can prevent implantation, and the
otherwise healthy blastocyst may be
sloughed away.
In addition, 0.5 - 1% of all pregnancies are
ectopic: the blastocyst tries to implant in the
wrong place, most often in the fallopian
tubes. This is always fatal for the embryo,
and has the potential to be fatal for the
mother.
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Week
How
you
1 month past
Week 4:4:
How
you looked
at 1looked
month past at
conception....
conception....
Placenta is forming by the 4th week
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Week 4-8 is when all the major organ systems of
the body are formed and when most teratogens
(substances that can cause birth defects) have
their greatest effect
By day 35 (5 weeks) the heart is beating, and eye
and limb buds are present.
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Week 9-40 it is now a fetus.
After 12 weeks or so, the baby's development is
largely "finished" - except brain and lung
development . Gender can be determined during the
3rd month.
The fetus just spends much of the 2nd and 3rd
trimesters just growing (and doing various flip-turns
and kicks inside the amniotic fluid)
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Three-to-Four Month-Old Fetus
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Fifth through Seventh Months
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Mother begins to feel fetal movement.
Wrinkled skin covered by fine hair – lanugo
and is covered by a greasy substance vernix caseosa.
Lungs lack surfactant so if baby is born
prematurely it will have to be on a respirator
(respiratory distress syndrome).
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Six-Month-Old-Fetus
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Eighth and Ninth Months
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Fetus usually rotates so head is pointed
down toward cervix.
Fetus is now about 20 in. in length and
weighs about 6.5 lbs.
Full-term babies have the best chance of
survival.
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Birth
True labor is marked by uterine contractions that
occur regularly every 15-20 minutes and last for
40 seconds or more.
Labor (parturition)
Stage 1.
• The period from the onset of true labor
contractions until the cervix is completely dilated at
10 cm.
• The uterine contractions cause the cervix to dilate,
and the amniotic sac may rupture.
• Usually lasts 6 – 24 hours depending on the
number of previous deliveries.
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Stage 2.
Period from maximal cervical dilation until
the birth of the baby
Lasts minutes to an hour
Contractions become more intense and
frequent.
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Stage 3.
Placenta delivered.
Called the afterbirth.
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