Download Pregnancy & Development

Chapter 28
From Egg to Embryo
Fertilization
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~ 300 million sperm enter female reproductive
tract, most are lost
~2000-5000 reach egg
Fertilization occurs when a sperm fuses with an
egg to form a zygote.
Capcitation: activation of the sperm cell membrane
allows release of acrosomal enzymes.
Sperm cells bind to the ZP3 glycoprotein receptor
on the zona pellucida triggering the acrosomal
reaction & releasing acrosomal enzymes
From Egg to Embryo
Figure 28.2
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Hundreds of sperm cells must release their
acrosomal enzymes before fertilization can occur
Acrosomal enzymes cut through the zona pelucida
From Egg to Embryo
Figure 28.2
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A protein on sperm binds to membrane receptors
of oocyte activating the egg receptor to cause
fusion of the egg & sperm membranes. sperm
nucleus is pulled into the oocyte cytoplasm.
From Egg to Embryo
Figure 28.2
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Polyspermy, or fertilization by more than one
sperm cell, leads to a lethal number of
chromosomes, & must be prevented.
From Egg to Embryo
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Polyspermy must be prevented.
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fast block to polyspermy
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oocyte membrane depolarizes & prevents binding by
other sperm cells.
Calcium release from oocyte ER
The slow block to polyspermy; the destruction
of sperm receptors, & swelling of the
membrane removes other sperm cells from the
surface.
From Egg to Embryo
Figure 28.3
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After a sperm enters an oocyte, it loses its tail &
midpiece, & migrates to the center of the oocyte
while the oocyte completes meiosis II.
From Egg to Embryo
Figure 28.3
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After meiosis II is completed, male & female
pronuclei fuse & produce a zygote, which almost
immediately enters into mitosis.
Figure 28.3
Figure 28.4
From Egg to Embryo
Figure 28.4
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Preembryonic Development
 Preembryonic development begins with
fertilization & continues with the movement of
the preembryo to the uterus, where it implants
in the uterine wall.
Cleavage results in smaller cells as mitotic
divisions after fertilization occur without much
growth between divisions.
From Egg to Embryo
Figure 28.4
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Cleavage forms two identical cells,
blastomeres, which by cleavage form a morula,
(a hollow ball of cells) by 72 hours.
After 4–5 days, the blastocyst escapes from
the degrading zona pellucida to implant in the
uterine wall.
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Implantation occurs after 6–7 days; the
trophoblast adheres to the endometrium, &
produces enzymes that irritate the endometrium.
From Egg to Embryo
Figure 28.5
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Uterine capillaries become permeable & leaky, &
the trophoblast proliferates, forming the
cytotrophoblast & the syncytiotrophoblast.
From Egg to Embryo
Figure 28.5
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Trophoblast cells secrete human chorionic
gonadotropin (hCG), which has LH type
activity keeping the corpus luteum functional.
Placentation is the formation of the placenta,
by proliferation of the trophoblast.
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Forms cytotrophoblast & syncytiotrophoblast
By the end of the third month of gestation
the placenta functions as a nutritive,
respiratory, excretory, & endocrine organ
Events of Embryonic Development
Figure 28.7
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Formation & Roles of the Embryonic Membranes
While implantation is occurring, the blastocyst is being
converted into a gastrula, in which three primary germ
layers form & embryonic membranes develop.
Events of Embryonic Development
Figure 28.7
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The amnion forms the transparent sac ultimately containing
the embryo, & provides a buoyant environment that
protects the embryo from physical trauma.
Events of Embryonic Development
Figure 28.7
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The yolk sac forms part of the gut, produces the
earliest blood cells & blood vessels, & is the source
of germ cells that migrate into the embryo to seed
the gonads.
Events of Embryonic Development
Figure 28.7
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The allantois is the structural base for the umbilical
cord that links the embryo to the placenta, &
becomes part of the urinary bladder.
Events of Embryonic Development
Figure 28.7
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The chorion helps to form the placenta, & encloses
the embryonic body & all other membranes.
Embryonic Disc
Figure 28.8a-e
Folding
Figure 28.8a-e
Figure 28.10a-d
Gastrulation
Figure 28.8a-e
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Gastrulation: Germ Layer Formation
Gastrulation; the process of transforming the
two-layered embryonic disc into a threelayered embryo.
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ectoderm, mesoderm, & endoderm
Gastrulation
Fig. 28.8
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Figure 28.8f-h
Gastrulation begins with the appearance of the
primitive streak, which establishes the long
axis of the embryo.
Cells migrate into the streak to form endoderm
then mesoderm
Organogenesis
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Organogenesis: Differentiation of the Germ Layers
Organogenesis is the formation of organs &
organ systems; by the end of the embryonic
period, all organ systems are recognizable.
Organogenesis
Figure 28.8f-h
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The ectoderm gives rise
to structures of the
nervous system & the
epidermis.
Neurulation, the
formation of the brain &
spinal cord, is the first
event of organogenesis.
Figure 28.9a-d
Organogenesis
Figure 28.12a-c
Figure 28.8f-h
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Fig. 28.8
The mesoderm gives rise to all types of tissues not formed by
ectoderm or endoderm, such as muscle tissue.
Mesodermal specialization forms the notochord, & gives rise
to the dermis, parietal serosa, bones, muscles, cardiovascular
structures, & connective tissues.
Organogenesis
Figure 28.12a-c
Figure 28.8f-h
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Fig. 28.8
The endoderm gives rise to epithelial linings of the
gut, respiratory, & urogenital systems, &
associated glands.
As the embryo develops from a flat plate of cells, it
rolls into a tube & the inferior endoderm becomes
the lining of the primitive gut.
Folding
Figure 28.8f-h
Figure 28.10a-d
Organogenesis
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By 3 1/2 weeks, the
embryo has a blood
vessel system & a
pumping heart.
Vascular
modifications include
umbilical arteries &
veins, a ductus
venosus, & the
foramen ovale &
ductus arteriosus.
Figure 28.13a
Events of Fetal Development
Fig. 28.14
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The fetal period extends from weeks 9–38, &
is a time of rapid growth of body structures
established in the embryo.
During the first half of the fetal period, cells
are still differentiating into specific cell types
to form the body’s distinctive tissues.
Effects of Pregnancy on the Mother
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Anatomical Changes
Metabolic Changes
Physiological Changes
Effects of Pregnancy on the Mother
Fig. 28.15
Anatomical Changes
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Reproductive organs & breasts become more vascular.
Uterus enlarges dramatically, shifts the woman’s center of
gravity compensated for by accentuated lumbar curvature
(lordosis).
The placental hormone relaxin causes pelvic ligaments &
the pubic symphysis to soften & relax.
Normal weight gain of around 28 pounds.
Effects of Pregnancy on the Mother
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Metabolic Changes
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The placenta produces;
Human placental lactogen
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Promotes breast maturation (with estrogen &
progesterone).
Promotes the growth of the fetus, & exerts a
glucose-sparing effect on maternal metabolism.
Human chorionic thyrotropin which increases
maternal metabolic rate.
Effects of Pregnancy on the Mother
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Physiological Changes (p. 1135)
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Morning sickness may be present during the
first few months of pregnancy, until
adaptation to elevated levels of estrogen &
progesterone occurs.
Heartburn due to esophageal displacement
Constipation may result due to the decreased
motility of the digestive tract.
Effects of Pregnancy on the Mother
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Physiological Changes (p. 1135)
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Increased urine production to dispose of
additional fetal metabolic waste.
Vital capacity & respiratory rate increases
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Decrease in residual volume
Many women experience dyspnea.
Blood pressure & heart rate rise. Blood
volume increases to accommodate the needs
of the fetus.
Parturition (Birth)
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Parturition is the process of giving birth.
Usually within 15 days of the calculated due date.
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280 days from the last menstrual period
Fig. 28.16
Fig. 28.17
Initiation of Labor
Fig. 28.16
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Estrogen levels peak:
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Uterine myometrial cells increase oxytocin receptors
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Blocks the quieting effect of progesterone on uterine muscle.
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Fetal cells produce oxytocin, which promotes the release of
prostaglandins from the placenta, & further stimulates uterine
contraction.
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Increasing emotional & physical stresses activate the mother’s
hypothalamus, which signals the release of oxytocin.
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Expulsive contractions are aided by a change that occurs in an
adhesive protein, fetal fibronectin, converting it to a lubricant.
Stages of Labor
Fig. 28.17
Stage 1
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The dilation stage of labor extends from onset of labor to the time
when the cervix is fully dilated by the baby’s head, at about 10 cm
in diameter.
Stage 2
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The expulsion stage extends from full dilation until the time the
infant is delivered.
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When the baby is in the vertex, or head first, position, the skull
acts as a wedge to dilate the cervix.
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Crowning occurs when the baby’s head distends the vulva, &
once the head has been delivered, the rest of the baby follows
much more easily.
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After birth, the umbilical cord is clamped & cut.
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Stage 3
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Placental stage, uterine contractions cause detachment of the
placenta from the uterine wall, followed by delivery of the placenta
& membranes (afterbirth).
Occlusion of Special Fetal Blood Vessels
& Vascular Shunts
Fig. 28.13a
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Fig. 28.13b
After birth, the umbilical arteries & veins constrict & become
fibrosed, becoming the medial umbilical ligaments, superior
vesical arteries of the bladder, & the round ligament of the liver,
or ligamentum teres.
Occlusion of Special Fetal Blood Vessels
& Vascular Shunts
Fig. 28.13a
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Fig. 28.13b
The ductus venosus closes, & is eventually converted to the
ligamentum venosum.
A flap of tissue covers the foramen ovale, ultimately sealing it
& becoming the fossa ovalis, while the ductus arteriosus
constricts, becoming the ligamentum arteriosus.
Occlusion of Special Fetal Blood Vessels
& Vascular Shunts
Fig. 28.13a
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Fig. 28.13b
A flap of tissue covers the foramen ovale, ultimately sealing it
& becoming the fossa ovalis, while the ductus arteriosus
constricts, becoming the ligamentum arteriosus.
Lactation
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Lactation is the production of milk by the
hormone-prepared mammary glands.
Lactation
Fig. 28.18
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Rising levels of placental estrogens, progesterone, &
lactogen stimulate the hypothalamus to produce prolactinreleasing hormone (PRH), which promotes secretion of
prolactin by the anterior pituitary.
Colostrum initially secreted for the first two to three days.
Nipple stimulation sends sensory input to the
hypothalamus stimulating production of PRH & prolactin
that maintains milk production.
Oxytocin results in of milk let down from the alveoli.
Breast milk has multiple advantages.
Assisted Reproductive Technology &
Reproductive Cloning
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Hormones can be used to increase
sperm or egg production & surgery
can be used to open blocked tubes.
Assisted reproductive technology
involves surgically removing oocytes
from a woman’s ovaries, fertilizing
the eggs & returning them to the
woman’s body.
Cloning involves the placing of a
somatic cell nucleus into an oocyte.
Fig. 28.19