Transcript Fertilization

PowerPoint® Lecture Slides prepared by Vince Austin, University of Kentucky
Pregnancy and Human
Development:
Fertilization
Human Anatomy & Physiology, Sixth Edition
Elaine N. Marieb
Copyright © 2004 Pearson Education, Inc., publishing as Benjamin Cummings
From Egg to Embryo – know definitions
 Pregnancy – events that occur from fertilization until the
infant is born
 Conceptus – the developing offspring
 Gestation period – from the last menstrual period until birth
 Preembryo – conceptus from fertilization until it is two
weeks old
 Embryo – conceptus during the third through the eighth
week
 Fetus – conceptus from the ninth week through birth
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Relative Size of Human Conceptus
Figure 28.1
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Accomplishing Fertilization - know
 The oocyte is viable for 12 to 24 hours
 Sperm is viable 24 to 72 hours
 For fertilization to occur, coitus (intercourse) must
occur no more than:
 Three days before ovulation
 24 hours after ovulation
 Fertilization – when a sperm fuses with an egg to
form a zygote
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Sperm Transport and Capacitation - know
 Fates of ejaculated sperm
 Leak out of the vagina immediately after deposition
 Destroyed by the acidic vaginal environment
 Fail to make it through the cervix
 Dispersed in the uterine cavity or destroyed by
phagocytic leukocytes
 **Reach the uterine tubes (rarest event)**
 Sperm must undergo capacitation before they can
penetrate the oocyte
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Acrosomal Reaction and Sperm Penetration - details
 An ovulated oocyte is encapsulated by:
 The corona radiata and zona pellucida
 Extracellular matrix
 Sperm binds to the zona pellucida and undergoes the
acrosomal reaction
 Enzymes are released near the oocyte
 Hundreds of acrosomes release their enzymes to
digest the zona pellucida
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Acrosomal Reaction and Sperm Penetration - details
 Once a sperm makes contact with the oocyte’s
membrane:
 Beta protein finds and binds to receptors on the
oocyte membrane
 Alpha protein causes it to insert into the membrane
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Acrosomal Reaction and Sperm Penetration illustration
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Figure 28.2a
Blocks to Polyspermy - know
 Only one sperm is allowed to penetrate the oocyte
 Two mechanisms ensure monospermy
 Fast block to polyspermy – membrane
depolarization prevents additional sperm from
fusing with the oocyte membrane
 Slow block to polyspermy – zonal inhibiting
proteins (ZIPs):
 Destroy sperm receptors
 Cause sperm already bound to receptors to
detach
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Completion of Meiosis II and Fertilization - know
 Upon entry of sperm, the secondary oocyte:
 Completes meiosis II
 Casts out the second polar body
 The ovum nucleus swells, and the two nuclei
approach each other
 When fully swollen, the two nuclei are called
pronuclei
 Fertilization – when the pronuclei come together
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Events Immediately Following Sperm Penetration illustration
Figure 28.3
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Preembryonic Development - know
 The first cleavage produces two daughter cells
called blastomeres
 Morula – the 16 or more cell stage (72 hours old)
 By the fourth or fifth day the preembryo consists of
100 or so cells (blastocyst) around a hollow center
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Preembryonic Development - details
 Blastocyst – a fluid-filled hollow sphere composed
of:
 A single layer of trophoblasts
 An inner cell mass
 Trophoblasts take part in placenta formation
 The inner cell mass becomes the embryonic disc
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Cleavage: From Zygote to Blastocyst - know
Degenerating
zona pellucida
Inner cell mass
Blastocyst cavity
Blastocyst
cavity
(a) Zygote
(fertilized egg)
Fertilization
(sperm meets
egg)
(b) 4-cell stage
2 days
(a)
(c) Morula
3 days
(d) Early blastocyst
4 days
Trophoblast
(e) Implanting
blastocyst
6 days
(b)
(c)
Ovary
Uterine tube
(d)
Oocyte
(egg)
Ovulation
(e)
Uterus
Endometrium
Cavity of
uterus
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Figure 28.4
Implantation - know
 Begins six to seven days after ovulation when the
trophoblasts adhere to a properly prepared
endometrium
 The implanted blastocyst is covered over by
endometrial cells
 Implantation is completed by the fourteenth day
after ovulation
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Implantation of the Blastocyst - illustration
Figure 28.5a
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Implantation of the Blastocyst - illustration
Figure 28.5b
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Implantation - know
 Viability of the corpus luteum is maintained by human
chorionic gonadotropin (hCG) secreted by the
trophoblasts
 hCG prompts the corpus luteum to continue to secrete
progesterone and estrogen
 “The rabbit died” anaphylaxis (1930-1960)
 Chorion – developed from trophoblasts after
implantation, continues this hormonal stimulus
 Between the second and third month, the placenta:
 Assumes the role of progesterone and estrogen
production
 Is providing nutrients and removing wastes
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Hormonal Changes During Pregnancy - know
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Placentation
 Formation of the placenta from:
 Embryonic trophoblastic tissues
 Maternal endometrial tissues
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Placentation
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Figure 28.7d
Placentation
Figure 28.7f
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Germ Layers
 The blastocyst develops into a gastrula with three
primary germ layers: ectoderm, endoderm, and
mesoderm
 Before becoming three-layered, the inner cell mass
subdivides into the upper epiblast and lower
hypoblast
 These layers form two of the four embryonic
membranes
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Embryonic Membranes
 Amnion – epiblast cells form a transparent
membrane filled with amniotic fluid
 Provides a buoyant environment that protects the
embryo
 Helps maintain a constant homeostatic temperature
 Amniotic fluid comes from maternal blood, and
later, fetal urine
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Embryonic Membranes
 Yolk sac – hypoblast cells that form a sac on the
ventral surface of the embryo
 Forms part of the digestive tube
 Produces earliest blood cells and vessels
 Is the source of primordial germ cells
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Embryonic Membranes
 Allantois – a small outpocketing at the caudal end of
the yolk sac
 Structural base for the umbilical cord
 Becomes part of the urinary bladder
 Chorion – helps form the placenta
 Encloses the embryonic body and all other
membranes
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Gastrulation
 During the 3rd week, the two-layered embryonic disc
becomes a three-layered embryo
 The primary germ layers are ectoderm, mesoderm,
and endoderm
 Primitive streak – raised dorsal groove that
establishes the longitudinal axis of the embryo
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Gastrulation
 As cells begin to migrate:
 The first cells that enter the groove form the
endoderm
 The cells that follow push laterally between the
cells forming the mesoderm
 The cells that remain on the embryo’s dorsal
surface form the ectoderm
 Notochord – rod of mesodermal cells that serves as
axial support
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Primary Germ Layers
 Serve as primitive tissues from which all body
organs will derive
 Ectoderm – forms structures of the nervous system
and skin epidermis
 Endoderm – forms epithelial linings of the digestive,
respiratory, and urogenital systems
 Mesoderm – forms all other tissues
 Endoderm and ectoderm are securely joined and are
considered epithelia
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Primary Germ Layers
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Figure 28.8a-e
Primary Germ Layers
Figure 28.8e-h
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