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Biology 12
Unit 2: Reproduction and
Development
Pregnancy
Fertilization
• Fertilization takes place in the fallopian tube
• When the sperm enters the egg, the ovum undergoes
meiosis II
• if an ovum is fertilized, the levels of estrogen and
progesterone must remain high to maintain the
endometrium and prevent further secretions of FSH
• as the fertilized ovum develops it secretes chorionic
gonadotropin (HCG) which maintains the corpus luteum
until the placenta begins to secrete its own progesterone as
it develops
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Early Development
• The fertilized ovum is called a zygote
• The zygote travels down the fallopian tube, it
continues to divide, becoming a hollow ball of
cells (blastocyst) by the time it reaches the uterus.
• Prior to implantation, the blastocyst undergoes
gastrulation.
• Gastrulation and cell migration set up the three
germ layers - endoderm, ectoderm, and mesoderm
• Implantation is when the zygote attaches to the
endometrium.
• As the embryo continues to grow by mitosis, part of the
outer layer of cells (chorionic villi of the chorion)
contribute the fetal portion of the placenta
• The endometrium forms the maternal side of the placenta
• A layer of tissue inside the chorion forms a fluid
filled membrane called the amnion.
• A third membrane called the allantois provides the
blood vessels to the placenta.
• The embryo is attached to the placenta by the
umbilical cord.
Early Development
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Early Human Development
Two weeks
after
conception.
Three weeks after
conception.
Five weeks after
conception.
Eight weeks
after
conception.
(Human?)
Twelve weeks
after
conception.
First Trimester
– Zygote becomes embryo when implanted
– Amnion, chorion, and allantois develop early
– Embryo grows to about 5.7 cm by end of
trimester.
– Heart, limbs , and brain have
begun to develop
– By nine weeks the embryo is
called a fetus
– The sucking reflex is present
Second Trimester
– The fetus will begin to kick
– All organ systems will develop and continue to
mature
– Cartilage is replaced with bone
– Hair and eyelids form
– With modern medicine, some fetuses can
survive after only 22 weeks.
– The longer the pregnancy the greater the
chances for survival.
Third Trimester
– The last trimester is marked by rapid growth
and final maturation of the organs.
– The fetus will grow to over 50 cm and up to 4
kg.
– 9 months, 38 weeks, or approximately 266 days
after implantation labor begins
Time of Birth.
Birth
– Progesterone levels drop near the end of
pregnancy. This may trigger labor.
– Relaxin which is produced by the placenta
causes the ligaments of the pelvis to relax.
– Oxytocin produced by the anterior pituitary
causes strong uterine contractions.
– Oxytocin is controlled by a positive feedback
loop.
Lactation
– Lower levels of progesterone after labor allow prolactin
to be released from the anterior pituitary.
– Prolactin stimulates mammary glands to produce breast
milk.
– To let down the milk, oxytocin must be released by the
pituitary
– During the first few days colostrum is let down instead
of milk. Colostrum does not contain fat
– Milk contains all substances the growing infant needs
including antibodies