1. FERTILIZATION AND DEVELOPMENT
A. Keywords
1. Acrosomal Reaction – the reaction that occurs in the acrosome (cap-like modified
lysosome at the tip/head of the sperm cell) when the sperm approaches the cell; The
acrosome bursts open and releases zona-digesting enzymes to break down the jelly-like
coating of the end.
2. Blastocyst – an early stage of human development (50-100 cells) found in early pregnancy
before implantation
3. Cleavage – mitotic cell divisions (converting zygote to blastula… eventually to multicellular
organism)
4. Conception – the fusion of gametes to produce a new organism, occurs when blastocyst
implants itself in the wall of the uterus (no implantation = no pregnancy)
5. Cortical Reaction – the exocytosis of cortical granules (Enzymes and protease in the
cortical granules diffuse into the zona pellucida and alters its structure); induces zona
reaction
6. Embryo – an early stage of development between the zygote (fertilized egg) and the fetus
stage (more than 8-10 weeks)
7. Endometrium – the inner membrane of the mammalian uterus
8. Fast Block to Polyspermy – caused by depolarization due to opening of Na+ gates in egg
plasma membranes; this depolarization prevents other sperm from fusing with the
membrane (a tent of a second of fusion and -70mV resting potential)
9. Fertilization – the initiation of biological reproduction by insemination or pollination,
creation by physical union of male + female gametes
10. Fertilization Membrane – resistant membranous layer in eggs formed after fertilization by
the thickening + separation of the vitelline membrane from the cell surface, prevents
multiple fertilization
11. Fetus – a developing mammal or viviparous vertebrate after embryonic stage and before
birth (a child in the mother’s womb/ unborn offspring after 8-9 weeks of conception),
more pronounced body parts
12. Gestation – the carrying of an embryo or fetus inside a female viviparous (production of
born alive offspring) animal
13. Human Chrorionic Gonadotropin – naturally occurring peptide hormone produced by the
embryo (and later, the trophoblast) which keeps progesterone levels up and makes the
uterine lining (corpus luteum) ready/thick enough for pregnancy
14. Labor – also called childbirth or delivery;
15. Lactation – secretion of milk from the mammary glands of the breast’s fatty tissue
(essential hormones: prolactin, which controls amount of milk produced and oxytocin,
which stimulates uterine contraction and milk production)

2. 16. Organogenesis – the process by which ectoderm, endoderm and mesoderm develop into
internal organs
17. Parturition – labor/childbirth; latent (early cervix softening), dilation, expulsion and
placenta delivery
18. Placenta – temporary organ which joins the mother and the fetus which is roughly disk-
shaped and rich in blood vessels (functions: allows nutrients + oxygen uptake from the
mother, waste elimination and gas exchange)
19. Slow block to Polyspermy – Inositol Triphosphate (IP3) causes the egg to release Ca ions.
Ca causes the fusion of the cortical vescicles and plasma membrane of the egg. This raises
the vitelline membrane with inactive receptors, and further sperm that bind to the
receptors do not fertilize the egg. (10 seconds of fusion)
20. Trimester – “three months” (First Trimester: conception – week 14; Second Trimester:
weeks 14 – 28; Third Trimester: week 28 – delivery/labor/average week 40)
21. Trophoblast – cells that form the outer layer of a blastocyst which provide nutrients to the
embryo and develop into a large part of the placenta; formed during the first stage of
pregnancy and are one of the first cells to differentiate from the fertilized egg
22. Umbilical Cord – lifeline made of 3 blood vessels (2 smaller arteries plus a larger vein that
returns blood to fetus) hat attaches the fetus to the placenta
23. Zona Pellucida – the glycoprotein membrane surrounding an oocyte (ex. Ovum as it
develops in the ovary) This remains in place during the travel to the fallopian tubes. For
fertilization, the sperm must penetrate the thinning membrane, which disappears upon
implantation in the uterus.
24. Zygote – the cell that results from fertilization or the union of two gametes
B. Describe the events that occur during fertilization.
a. Sperm migrates through the coat of follicle cells of the egg and binds to receptor
molecules in the zona pellucida of the egg
b. Binding induces acrosomal reaction, where the sprem releases digestive enzymes into
the zona pellucida
c. The hydrolytic enzymes help the sperm reach the plasma membrane of the egg,
membrane proteins of the sperm bind to the receptors of the egg membrane
d. Plasma membranes fuse, allowing the contents of the sperm cell to enter the egg
e. Binding of the sperm to the egg depolarizes the egg membrane, functions as a fast
block to polyspermy
f. Cortical reaction of the egg causes the granules in its cortex to be released through
exocytosis
g. Enzymes released during the egg's cortical reaction harden the zona pellucida,
functions as a slow block to polyspermy

3. h. Basal body of the sperm's flagellum divides, forms two centrosomes in the zygote;
generates mitotic spindles for cell division
C. Describe the events that occur during the first, second and third trimesters
of human pregnancy/gestation.
FIRST TRIMESTER
a. Fertilization
b. Cleavage (after 24 hours), continues until day 3 or 4 after fertilization
c. Blastocyst has formed after 1 week after fertilization
d. Blastocyst is implanted in endometrium (after 5 days from when the blastocyst has formed)
e. Start of differentiation
f. Blastocyst becomes embedded in the endometrium, endometrium grows over blastocyst
g. Embryo directly gets its nutrients from the endometrium for the first 2 - 4 weeks
h. Tissues that grow out from the embryo bind with the endometrium to form the placenta
(main period of organogenesis, embryo secretes hormones to signal its presence and control
mother's reproductive system, eg. human chroionic gonadotropin (HCG))
SECOND TRIMESTER:
a. Fetus grows until 30 cm, fetal activity may be visible through abdominal wall
b. Hormone levels stabilize as HCG declines, corpus luteum deteriorates, placenta secretes
own progesterone to maintain pregnancy
(uterus grows large enough to make the pregnancy obvious)
THIRD TRIMESTER:
a. Fetus grows to 3 - 3.5 kg in weight and 50 cm in length
b. Fetal activity decreases
c. Uterus expands to accompany the growing fetus, abdominal organs of the mother are
compressed and displaced
d. Hormones induce and regulate labor

4. 1. Estrogen levels in the mother's blood reaches its highest level, triggers formation of
oxytocin
2. Oxytocin stimulate powerful contractions through the smooth mscles of the uterus,
stimulates placenta to secrete prostaglandins to enhance contraction
3. Physical and emotional stress associated with contractions stimulate release of
more oxytocin and prostaglandins --> + feedback
D. Explain why the embryo is not rejected by the mother during pregnancy.
When the zygote lands itself on the uterus lining and becomes a blastocyst, it produces chemicals
that suppress the mother’s immune system from destroying the developing embryo. An example
may be Human Chorionic Gonadotropin, whose highly-negative charge may repel the immune cells
of the mother, hence protecting the developing embryo during the first trimester.
E. Describe what happens in parturition and lactation.
Parturition Lactation
• birth of fetus from uterus • Secretion / Formation of milk by the
• 3 stages: mammary glands
1. DILATION • During pregnancy…
2. FETAL EXPOSURE • Remarkable growth in breast
3. UMBILICAL CORD CLOSURE AND size
PLACENTAL EXPOSURE • action of estrogen, adrenal
steroids, growth hormone make
ducts and alveoli in breast grow
• prolactin, progesterone also
DILATION involved
• Start of Secretory Activity
• Thinned cervix opens up to around 3 cm • in Milk producing unit / alveoli
• May or may not require contraction • When estrogen and progesterone
• Cervix is integrated in lower part of the are withdrawn from blood,
uterus and on top of baby’s head prolactin causes secretory activity
• Full dilation for baby’s head : 10 cm • Colostrum:
• 8 hours for first-timers, 4 hours for • good for baby’s health
experienced • expected at pregnancy or
childbirth
FETAL EXPOSURE • milk secretion increases after,
usually 3rd or 4th day after birth
• Pressure on cervix increases, increased • breasts are engorged, tense,
uterine contraction tender, feel warm
• Ejection of Milk
• the head is fully engaged in the pelvis
• Depends on baby’s suction
• it has to pass through the narrowest part • Contractile mechanism –moves
of the pelvis, then the pubic arch & milk fr. alveoli to ducts
introitus by pushing • During suckling..
• labia part, the woman may feel a burning • pressure seems to rice in
sensation mother’s breasts
• nerve impulse fr. sucking of baby
• after birth of fetal head: • Path of impulse : sensory

5. 1. delivery by extension neurons in nipples 
hypothalamus in the brain 
2. head points to side pituitary gland
3. rotation to expose shoulders • Pituitary gland secretes oxytocin
which induces contraction of
UMBILICAL CORD CLOSURE myoepithelial cells of alveoli and
ducts containing milk
• Maintenance of Breastmilk
• after fetal exposure
Production
• umbilical cord is cut & would close
• What is needed?
normally if not clamped
• Prolactin
PLACENTAL EXPULSION • Sucking – for removal of milk,
release of prolactin
• Milk pressure, hence periodic
• 15-30 mins after delivery
breast feeding
• Physiological separation
• Continuous unless there is
• May be done with or without assistance
congestion, emotional
• Assistance - giving oxytocin thru disturbance
intramuscular injection followed by cord
traction
Hormones involved:
• In making cervix ready – relaxin,
prostaglandin, estrogen
• In uterine contraction – prostaglandin,
progesterone, oxytocin, spec. positive
feedback in oxytocin