Download Fertilization and Development

Fertilization and Development
Fertilization
Fertilization of the Ovum occurs in the
oviduct
– Only one sperm allowed
• Electrical block
• Surface of ovum altered
• Binding sites for sperm
altered
Development
Nuclei of sperm and ovum fuse to form the
zygote
– 1st division (mitosis) Occurs in oviduct 30 hrs
after fertilization
Development
Cell division continues as mass moves
through the oviduct to the uterus
– a central hollow forms and fills with fluid
– Now called a blastocyst
• Cells on inside develop into embryo
• Outer cells develop into
supportive cells
– Identical Twins
Implantation
On 6th day blastocyst breaks free of zona
Implantation blastocyst in uterine wall on
day 7- 8
Implantation
Implantation
 Endometrium is maintained
– Human corionic gonadotropic hormone (HCG)
sectreted by outer cells (Chorion) of blastocyst to
maintain corpus luteum
• LH levels remain high
• HCG used in pregnancy tests
– Corpus luteum maintains
levels of estrogen and
progesterone to maintain
endometrium
The Embryo
During implantation, the outer layer of
cells of the blastocyst produces two
membranes that surround, protect, and
nourish the embryo
– Chorion forms the placenta
– Amnion forms amniotic sac
Development
Considered an embryo after implantation is
complete
– Approx. 2 weeks from fertilization
Chorion
Amnion
Forms a fluid filled sac that insulates the
embryo and fetus that protects from
–
–
–
–
Infection
Dehydration
Impact
Temperature changes
Umbilical cord
Contains one vein and two arteries
– vein carries nutrient and oxygen rich blood
to the fetus.
– two arteries return blood containing waste
from fetal circulation back to the placenta.
• It takes about 30 seconds for a blood cell to
make the round trip through the cord
– cord can be as long as 4 feet at birth.
Prenatal Development
The in’s and then outs
Prenatal development
 Pregnancy usually last for approximately nine
months
 Divided into three trimesters
 First Trimester
– Fertilization to the end of the 3rd month
– by second week – germ layers form
• Ectoderm, endoderm, and mesoderm
– At one month – size is only 7mm
Prenatal development
First Trimester (Cont.)
– Heart and brain have formed
• heart rate can be heard at 10 weeks with a special
instrument called a Doppler
–
–
–
–
Limb buds with fingers and toes present
Placenta begins developing
Considered fetus after 8th week
Movement occurs around 9th week
• Sucking reflex
st
1
Trimester
4 wks
8 wks
12 wks
nd
2
Trimester
Fetus is 57 mm – 350 mm long
recognizable body form
– Arms, hands, fingers, legs, feet and toes are
fully formed
• Bone begins to replace cartilage
most of organs and tissues developed
– Eyes are almost fully developed
nd
2
Trimester
16 wks
20 wks
nd
2
Trimester
 At the end of 16 wks:
– Fetus is developing reflexes such as sucking and
swallowing
– Sex is identifiable
– Skin is bright pink, transparent and covered with soft,
downy hair
 At the end of 20 wks:
– Mother begins to feel fetal movement
– Internal organs are maturing
– Eyebrows, eyelids and eyelashes appear
nd
2
Trimester
At the end of 24 weeks:
– Eyelids begin to part and eyes open
occasionally for short periods of time
– Skin is covered with protective coating called
vernix
– Fetus is able to hiccup
rd
3
Trimester
Rapid growth
– Fetus is 350 mm – 530 mm long
– Body mass increases from 0.68 kg to 3.4 kg
– Organs are maturing
rd
3
Trimester
At the end of 7 months:
– If born at this time, he/she will be considered a
premature baby and require special care
– Taste buds have developed
– Fat layers are forming
– Skin is still wrinkled and red
rd
3
Trimester
rd
3
Trimester
At the end of 8 months:
– Tremendous brain growth occurs at this time
– Most body organs are now developed with the
exception of the lungs
• Movements or "kicks" are strong enough to be
visible from the outside
• Kidneys are mature
• Skin is less wrinkled
• Fingernails now extend beyond fingertips
rd
3
Trimester
At the end of 9 months:
– The lungs are mature
– Baby is now fully developed and can survive
outside the mother's body
• Skin is pink and smooth
• Baby settles down lower in the abdomen in
preparation for birth and may seem less active
Parturition (Birth)
 Uterine contractions signal beginning of labor
– Cervix thins and begins to dilate (max. 10 cm)
– Labor starts when contractions are 10-15 min
 Amniotic membrane is forced into birth canal
– Often bursts and amniotic fluid lubricates the canal
(water breaking)
– Usually ruptures during delivery but may occur before
 Contractions start moving baby through birth
canal
Parturition
Hormones involved in birth
Relaxin
– Produced by placenta causes ligaments in
pelvis to loosen
Oxytocin
– positive feedback loop enhances strength of
contractions
– Prostaglandins also involved
Hormones involved in birth
 Milk Production
– Prolactin
• Levels increase dramatically after birth occurs
• estrogen & progesterone levels drop
• Stimulates glands in the breasts to produce fluids 1.5 L/day
– Oxytocin
• Stimulates contractions that move milk into ducts
• Also causes contractions of sm. Muscle in uterus to return to
normal size