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Health Sciences 2711A/B Chapter Notes -Erythropoiesis, Microcephaly, Embryonic Disc

Health Sciences
Course Code
HS 2711A/B
Treena Orchard

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HS 2700A
September 28, 2009 Readings
Berk pp. 60-77
ovum is a tiny sphere, measuring 1/175 inch in diameter
although it is invisible to the naked eye, it is a giant to the microscopic world.
sperm is 1/500 inch
if the ovum is not contacted by the sperm, pregnancy does not occur and it is
discarded 2 weeks later by menstruation.
sperm is produced 300 million a day in the testes
sperm live up to 6 days and can lie and wait for the ovum
corpus luteum: the spot on the ovary from which the ovum was released
secretes hormones that prepare the lining of the uterus to receive a fertilized ovum.
ovum only survives for 1 day after being released into the fallopian tube
38 weeks of pregnancy divided into three stages:
lasts about 2 weeks (from fertilization in the fallopian tube until the cell is released
into the uterus)
by the 4th day, 60-70 cells fill the hollow, fluid-filled ball called a blastocyst
inside the blastocyst, are embryonic disks, which are the cells inside
the outer ring of cells are called the trophoblast
Implantation: between 7th and 9th days after fertilization, this is when the
blastocyst buries itself deep into the uterus lining.
trophoblast multiples and forms a membrane called the amnion, that closes the
organism in amniotic fluid (helps keep temperature and provides cushion)
yolk sac emerges and produces red blood cells until the liver, spleen, and the
bone marrow are mature enough to take over this function.
30% of zygotes do not survive these first 2 weeks
The Placenta and the Umbilical Cord:
trophoblast forms another membrane called the chorion, where tiny blood
vessels (villi) emerge.
as the villi buries itself into the uterine wall, the placenta forms.
placenta permits food and oxygen to reach the organism and waste products to
be carried away
placenta is connected to the organism by the umbilical cord (contains one large
vein, and two arteries)
vein: delivers blood with nutrients
arteries: remove waste products
last from implantation to the 8th week of pregnancy (in a span of 6 weeks)
groundwork of all body structures and organs are done
The Last Half of the First Month:

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in the first week of this period (3rd week of the month) the embryonic disk forms
three layers of cells:
ectoderm --> nervous system and skin
mesoderm --> muscles, skeleton, circulatory system, and other internal
endoderm --> digestive system, lungs, urinary tract, and glands
ectoderm folds over to become the neural tube --> spinal cord and brain
neurons form, heart begins to pump blood, and the muscles, digestive tract,
backbone, and ribs appear
embryo is only 1/4 inch long at this point
The Second Month:
the eyes, ears, nose, jaw and neck form
intestines grow, heart develops separate chambers
liver and spleen take over the production of making red blood cells
embryo can now move, but still too tiny to be sensed by the mother
9th week to the end of the pregnancy
The Third Month:
brain starts to signal, fetus starts to kick, bend its arms, etc
tiny lungs start to work
by 12th week, external genitals begin to form, sex of fetus can be determined
heartbeat can now be heard
1st trimester of the pregnancy is complete at the end of the 3rd month
The Second Trimester:
between 17-20 weeks
vernix forms - protects skin from chapping during the long months spend
bathing in the amniotic fluid.
lanugo forms - white, downy hair covering the entire body, helping the vernix
stick to the skin
brains neurons are in place
organs are well-developed
glial cells are still forming (still continues to even after birth)
brain growth = new behavioural capacities
The Third Trimester:
age of viability - the point at which fetus can first survive, between 22 and 26
cerebral cortex enlarges
greater responses to stimulation
temperament begins (fetus may get a fever = active in first year of life)
during the final two months, fetus can distinguish tone and rhythm
gains 5 pounds and grows 7 inches
Teratogens: an environmental agent that causes damage to the baby during the
prenatal period
harm of the teratogen is dependent on dosage, heredity, age, and other negative
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