Download Reproduction4

Mature ovum (ovulated secondary oocyte)
corona radiata – follicle cell layer surrounding
secondary oocyte
zona pellucida – glycoprotein layer surrounding ovum
(only one sperm gets through)
Process of Fertilization
• Sperm swim from vagina, up cervix, up
endometrial lining and into fallopian tube
• If female is near the middle of her menstrual
cycle, then a secondary oocyte may be present
• Typical location for fertilization is in the fallopian
tube; many sperm work to get through corona
radiata
1. Sperm that make it to zona pellucida compete
to get into ovum (only one gets in) by secreting
enzymes held in their acrosomes
2. First sperm to get through has its plasma
membrane fused with ovum’s plasma
membrane (this initiates cortical reaction)
Cortical Reaction
• In cytoplasm of ovum (all around
plasma membrane) are small vesicles
called cortical granules
• When the 2 gametes fuse their
plasma membranes, the cortical
granules fuse with the ovum’s
membrane and secrete enzymes to
the outside
• The enzymes cause a chemical
change in zona pellucida making it
impermeable to sperm
Note: this happens immediately after first sperm enters to ensure that only
one sperm fertilizes egg
Pregnancy
I. Early Embryonic Development
•
first mitotic division ~24 hrs after fertilization
•
by the time the fertilized egg (zygote)
reaches the uterus and is ready for
implantation in the endometrium it is ~100
cells (~7 days later)
embryo at this stage is
a blastocyst
- outer cells become
the placenta
- inner cells will
become the
embryo
II. Early Pregnancy
• If implantation occurs then embryo will secrete
its own hormone, HCG (human chorionic
gonadotrophin)
• The HCG gets into mom’s bloodstream and
targets the corpus luteum; it stimulates the
corpus luteum to continue secreting
progesterone so the endometrium stays in tact
Positive pregnancy test
indicated by presence
of HCG (this is only
made by embryo)
III. Role of the Placenta
• After implantation, the blastocyst (outer layer)
and mom’s endometrium form the placenta
• Growing embryo requires nutrients and
therefore a connection between mom and fetus
requires placenta and umbilical cord
• Later in pregnancy placenta secretes hormones
• Umbilical cord contains 3 blood vessels
– 2 vessels carry blood from fetus to placenta (these
are deoxygenated and contain waste products)
– 1 vessel carries blood from placenta to fetus
(oxygenated with nutrients)
Materials from fetus to
mom via placenta
Materials from mom to
fetus via placenta
Carbon dioxide
Oxygen
Urea
Nutrients (e.g., glucose,
amino acids)
Water
Water
Hormones (e.g., HCG)
Hormones
Vitamins, minerals
Alcohol (why we don’t
drink when pregnant)
Some viruses (measles,
HIV)
Note: At no time
does the blood of
the fetus and the
blood of the mother
actually mix…all
molecular
exchanges occur
via active/passive
transport in
placenta
Note: Placenta also takes over job of
secreting estrogen and progesterone to keep
endometrium/blood supply available to fetus
(corpus luteum is now gone)
• Some tissue of developing
embryo is used to make
amniotic sac
• Within the sac is amniotic fluid;
the fetus floats in this fluid
• Functions of amniotic fluid:
– Providing cushioning in case of
blunt force trauma to abdomen
– Allows fetus free movement for
muscle and skeletal
development
– Thermal stability
Note: this is the fluid sampled during amniocentesis
The Process of Birth
1. Progesterone levels drop
2. Oxytocin secreted from pituitary gland; levels
increases after each contraction which causes
contractions to get more frequent and more
intense (this is known as positive feedback and it only stops once the uterus
has nothing to contract on) (Pitocin – used to induce labor)
• Cervix dilates to 10cm
• Babies typically born head first, face down
(otherwise called breech)
• After baby, mom’s push out afterbirth (placenta)
• Lactation (breast milk production) begins soon
after birth