•  Fertilization
•  Implantation
•  Placentation
Fertilization
Oviduct is site of
fertilization
Normally occurs in
upper third of oviduct
(ampulla)
Must occur within 24
hours after ovulation
Sperm usually survive
about 48 hours; can
survive up to 5 days in
female tract
Sperm travel through cervical canal, uterus, and to upper third of oviduct
Female reproductive tract aids in migration
• Contractions of myometrium
• Upward contractions of oviduct smooth muscle
• Allurin released by mature eggs
• Thin cervical mucous
Fertilization
Millions of sperm in ejaculate
Only few hundred sperm make it to
uterine tubes
Fates of ejaculated sperm:
–  Leak out of the vagina immediately after
deposition
–  Destroyed by the acidic vaginal environment
–  Fail to make it through the cervix
–  Dispersed in the uterine cavity or destroyed by
phagocytic leukocytes
–  Reach the uterine tubes
•  Sperm must undergo capacitation before
they can penetrate the oocyte
Activation of spermatozoa
•  Capacitation
•  Spontaneous or triggered by agents produced
by ovum
•  Release hyaluronidase
•  An ovulated oocyte is encapsulated by:
–  The corona radiata and zona pellucida
–  Extracellular matrix
•  Digest matrix
•  Sperm moves through zona pellucida
Block to polyspermy
•  Only one sperm is allowed to penetrate the oocyte
•  Two mechanisms ensure monospermy
–  Fast block to polyspermy – membrane
depolarization prevents sperm from fusing with the
oocyte membrane. Cortical reaction.
–  Slow block to polyspermy – zonal inhibiting
proteins (ZIPs):
•  Detach sperm
Fertilization
First sperm to reach ovum
–  Fuses with plasma membrane of ovum
–  Head of fused sperm gradually pulled into ovum’s cytoplasm
–  Fusion triggers second meiotic division of oocyte
–  Within hour, sperm and egg nuclei fuse
–  Fertilized ovum now called a zygote
Embryonic Development
Fertilized ovum divides mitotically
–  Forms morula
–  Cell cleavage (no increase in overall size)
–  Totipotent up to 32 cell stage
–  When cells are totipotent, division of the morula
results in identical twins
–  Fraternal (non-identical) twins result from
fertilization of 2 oocytes released during one cycle
Embryonic Development
Within week zygote grows and differentiates into blastocyst
capable of implantation
Outer cell layer: trophoblast
•  will become fetal placenta
Inner cell mass
•  will become embryo
Fluid-filled cavity: blastocoele
Early Stages of Development from
Fertilization to Implantation
Implantation
•  Begins six to seven days after ovulation when the
trophoblasts adhere to a receptive endometrium
•  Trophoblasts proliferate to form two distinct layers
–  Cytotrophoblast – cells of the inner layer that
retain their cell boundaries
–  Syncytiotrophoblast – cells in the outer layer that
lose their plasma membranes and invade the
endometrium
•  The implanted blastocyst is covered over by
endometrial cells
•  Implantation is completed by the fourteenth day after
ovulation
Implantation
Implantation of blastocyst
Implantation
Blastocyst implants in endometrial
lining by means of enzymes
released by trophoblasts
– 6-7 days post-fertilization
– Enzymes digest endometrial
tissue:
Carve hole in endometrium for
implantation of blastocyst
Release nutrients from
endometrial cells for use by
developing embryo
Stem-villus stage
Placentation
•  Formation of the placenta from:
–  Embryonic trophoblastic tissues
–  Maternal endometrial tissues
Placentation
•  The chorion:
•  Membrane derived from trophoblast and extraembryonic tissue
•  Develops fingerlike villi, which:
–  Become vascularized
–  Extend to the embryo as umbilical arteries and veins
–  Lie immersed in maternal blood (erosion of spiral artery)
•  Embryonic placental barriers include:
–  The chorionic villi
–  The endothelium of embryonic capillaries
Placentation
Placentation
Placenta
• 
Organ of exchange between maternal and fetal blood
•  Acts as transient, complex endocrine organ that
secretes essential pregnancy hormones
– Human chorionic gonadotropin
» Maintains corpus luteum until placenta takes
over function in last two trimesters
– Oestrogen
» Essential for maintaining normal pregnancy
– Progesterone
» Essential for maintaining normal pregnancy
Secretion Rates of Placental Hormones
Effects of Oestrogens and Progesterone
Late embryonic and fetal development
After implantation, chorion and amniotic cavity develop
By 5 weeks, placenta functioning, heart beating
Embryo termed fetus after 8 weeks
Gestation
About 38 weeks from conception
Ovulation prevented by negative feedback effect of
progesterone - prevents LH surge
Progesterone levels rise from 30-150nmol/L to about
500nmol/L
Basis for pregnancy test is rise in hCG secretion
hCG may also cause “morning sickness”
Maternal adaptations
Physical changes within mother meet demands of pregnancy
•  Uterine enlargement (weight x 20 excluding contents)
•  Breasts enlarge and develop ability to produce milk
•  Volume of blood increases 30%
–  Red cell mass and plasma volume increases
–  Total body water increases due to
» Increased oestrogens
» Increased renin production - aldosterone
•  Cardiac output rises: increased HR and SV
•  Renal and skin blood flow increases
•  Weight gain
•  Respiratory activity increases by about 20%
–  Increased tidal volume and minute ventilation
•  Urinary output increases
–  Renal blood flow and GFR increase
•  Kidneys excrete additional wastes from fetus
•  Nutritional requirements increase