Chapter 18
Biology 25: Human Biology
Prof. Gonsalves
Los Angeles City College
Based on Mader’s Human Biology,7th
edition and Fox’s 8th ed
Powerpoints
Fertilization
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Ejaculation 300 million
sperm, 100 reach (uterine)
fallopian tube.
Fertilization occurs in the
uterine tubes
Acrosomal reaction:
 Acrosome of sperm
contains
hyaluronidase, an
enzyme that digests a
channel through zona
pellucida.
Sperm fuses with ovum
cell membrane.
Fertilization
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As fertilization occurs,
secondary oocyte
completes 2nd meiotic
division.
Sperm enters ovum
cytoplasm.
Ovum nuclear
membrane disappears,
zygote formed.
Blastocyct Formation
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Cleavage:
 30-36 hrs. after
fertilization the zygote
divides by mitosis.
Blastocyst develops:
Inner cell mass:
fetus.
 Surrounding
chorion:
trophoblasts form
placenta.
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Implantation
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6th day after
fertilization,
blastocyst attaches to
uterine wall.
Blastocyst secretes
enzymes that allow
blastocyst to burrow
into endometrium.
Trophoblast cells
secrete hCG.
Amnion
Envelop the
embryo.
 Amnionic fluid
contains
sloughed cells
of the fetus.
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hCG
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Human chorionic
gonadotropin.
Trophoblast cells
secrete hCG.
Signals corpus luteum
not to degenerate until
placenta secretes
adequate [hormone].
Effects similar to LH.
Basis of pregnancy
test.
Placenta
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Syncytiotrophoblast
secretes enzymes that
create blood filled cavities
in the maternal tissue.
Cytotrophoblast then
from projections (villi)
that grow into the venous
blood.
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Producing chorion
frondosum on the
side that faces the
uterine wall.
Other side of chorion
bulges into the uterine
cavity.
Placental Changes
Decidual reaction:
 Endometrial growth.
 Accumulation of glycogen.
 Decidua basalis: maternal tissue in contact
with the chorion frondosum.
 Maternal and fetal blood do not mix.
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Placenta Function
Gas exchange:
 02 and C02.
 Nutrient exchange.
 Waste exchange.
 Synthesis of proteins and enzymes.
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Placental Hormones
hCS:
 Chorionic somatomammotropin.
 GH effects.
 Diabetic-like effect:
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Glucose sparing effect.
Polyuria.
Lipolysis.
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Placental Hormones
Fetal-placental unit:
 Placenta must cooperate with the adrenal
cortex in the fetus to produce estrogen.
 Estrogen stimulates:
 Endometrial growth.
 Inhibit prolactin secretion.
 Growth of mammary ducts.
 Enlargement of mother’s uterus.
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Placental Hormones
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Progesterone:
Suppresses uterine contractions.
Stimulates uterine growth .
Suppresses LH and FSH.
Stimulate development of alveolar
tissue of the mammary gland.
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Critical Stages of Embryonic and Fetal
Development
Formation of Testes
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First 40 days after conception the gonads of males
and females are similar in appearance.
Spermatogonia and oogonia migrate from yolk sac
to developing embryonic gonads.
TDF (testis-determining factor) promotes the
conversion to testes:
 Seminiferous tubules:
 Germinal cells: sperm.
 Nongerminal cells: Sertoli cells.
 Leydig cells:
 Appear about day 65.
Formation of Testes
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Leydig cells secrete testosterone.
 Begins 8th week and peaks at 12-14th
week.
 Masculinizes embryonic structures.
Testosterone then declines to very low
levels until puberty.
Testes descend into scrotum shortly before
birth.
Formation of Ovaries
Absence of Y chromosome and TDF,
female develop ovaries.
 Ovarian follicles do not appear until 2nd
trimester.
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Development of Accessory Sex
Organs
and
Genitalia
Presence or absence of
testes determines the
accessory sex organs and
external genitalia.
Male accessory organs
derived from wolffian
ducts.
Sertoli cells secrete MIF.
Female accessory organs
derived from mullerian
ducts.
Parturition
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Estrogen in late pregnancy:
 Increases amount of oxytocin stored.
 Stimulate production of oxytocin receptors in
myometrium.
 Stimulate prostaglandin production.
Uterine contractions:
 Oxytocin.
 Prostaglandins.
Pregnancy and Human Development
 Gestation: In humans starts at conception and
lasts approximately 38 weeks (266 days).
 Due date typically 40 weeks after last
menstruation.
 First Trimester: By the end of week 13
 Sex distinguished externally
 Fetus weighs about 150 grams
 Second Trimester: By the end of week 26
 Fetus weighs about 1000 g
 May survive if born at this point.
 Third Trimester: By the end of week 38
 Fetus weighs about 3500 g (7 ½ lb.)
 Testes have descended in males.
Lactation
Hypothalamus releases PRH.
 Anterior pituitary releases prolactin:
 Stimulate milk production.
 Oxytocin needed for “milk letdown”.
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