Sex Cell Development
&
Human Conception
Male Sex Cell Development
• Head
• Body
The capsule is filled with
chemicals that allow the
head to enter the egg;
contains nucleus
Energy-producing
organelle (mitochondria)
• Flagellum
A whip-like tail
propels the sperm
cell
Spermatogenesis
• Sperm
produced in
seminiferous
tubules
(testes)
Egg Production
• The eggs form and mature in two ovaries, which contain
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follicles
Each follicle contains an egg
When the egg is ready, the ovary wall ruptures and the
egg is released into the oviduct
– This is called ovulation
• After ovulation, the ruptured follicle forms into the corpus
luteum
Note:
• The ovary does not produce new egg cells throughout
adult life
• The ovary contains approximately 40 000 immature
follicles at puberty, and that number continually decreases
• Only one follicle matures every month; the others break
down and are absorbed by the ovary
Oogenesis
♀ Hormones
• FSH: Follicle stimulating hormone
– Signals the ovaries to produce mature eggs
• LH: Luteinising hormone
– Released by the pituitary gland (base of brain)
• Progesterone
– Released by corpus luteum
• Estrogen
– Reproductive hormone
– Causes appearance of ♀ secondary sexual
characteristics
Hormones & Menstruation
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Pituitary gland releases FSH
FSH stimulates follicle development
Developing follicle secretes estrogen
↑ estrogen levels stimulates thickening of endometrium
Estrogen travels to pituitary, stimulating release of LH
LH causes developing follicle to release a mature egg
LH stimulates empty follicle to develop in corpus luteum
Corpus luteum produces progesterone and estrogen
↓ progesterone increases thickening of endometrium
Progesterone causes pituitary to decrease FSH
production
↓ FSH and LH prevent more eggs from being released
until progesterone decreases again
Hormone Levels
Fertilization
• The head of the sperm breaks
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through the cell membrane of
the egg
Only one sperm enters the egg
The egg (n) and sperm (n)
nuclei combine together (2n)
This is called a zygote
The flagellum of the sperm is
pinched off
Ovulation to Implantation
• The zygote undergoes many cell divisions
• When it reaches the uterus, it has become a
blastocyst (hollow ball of cells)
– Outer cells of the blastocyst will become the placenta
– Inner cells will form the embryo
• Implantation occurs when the blastocyst attaches
to the endometrium
– This produces a hormonal signal that prevents the corpus
luteum from disintegrating
– If the corpus luteum breaks down, menstruation occurs
Ovulation to Implantation
Gastrulation
• Ectoderm – skin,
nervous system
• Mesoderm –
kidneys, skeleton,
muscles, blood
vessels, gonads
• Endoderm – lungs,
lining of digestive
tract
Conception and Fertilization
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Yolk sac: supplies nutrients
Amniom: fluid-filled sac protects the embryo from
Allantois: waste removal
Placenta: exchange of food and
Umbilical cord: connects the embryo to the mother
Pregnancy - Trimester 1
• Week 1 – 12
• 4 Weeks
– Basic heart, large brain, limb buds, eyes, spine
– Embryos have a tail
• 8 Weeks
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Cartilage replaced by bone
The embryo is now a fetus
Facial features, limbs, hands, feet, fingers, and toes are visible
The nervous system responds to stimulus
The internal organs begin to function
• High Risks
– Spontaneous abortions or miscarriages, birth defects
• Signs of Pregnancy
– Nausea/vomitting, frequent urination, no menstruation
Pregnancy - Trimester 2
• Week 12 – 24
• Development
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External sex organs
Hair grows on head and body
Blink, grasp, swallow, suck, hear, and cry
Functioning nervous system
Skeleton development
300 mm long
• Risks
– If born at this time, the fetus has a 50%
chance of survival
• Signs of Pregnancy
– The mother appears pregnant
– The mother will feel fetal movement
(quickening)
Pregnancy - Trimester 3
• Week 24 – 38
• Development
– Open eyes in 8th month
– The fetus is very active (kicking,
stretching)
– The body hair begins to disappear
– Bones begin to harden
• Head bones remain soft to help the
head fit through the birth canal
– 500 mm long
• Signs of Pregnancy
– Heartburn, constipation, hemorrhoids,
abdominal pain
– Mother’s health and nutrition is very
important
Risks during Pregnancy
Birth
• Dilation
– Contractions and oxytocin cause the cervix to dilate; the amnion
breaks and releases the amniotic fluid (2 – 20 hr)
• Expulsion
– Baby is pushed through the cervix out the birth canal (0.5 – 2 hr)
• Placental
– Placenta and umbilical cord are expelled from the uterus (10 – 15
min after birth)