From fertilization to gastrulation

Copyright © 2010 Wolters Kluwer Health | Lippincott Williams & Wilkins
Lecture 1:
From Fertilization to
Gastrulation

Why Embryology?
• Birth defects are the leading cause of infant mortality.
• Birth defects are a major contributor to morbidity,
including physical and mental handicaps.
• All women of childbearing age are at risk of having an
infant with a birth defect. The incidence rate is 6/100
births.
• Each of you will have contact with women of
childbearing age; either as a friend, as a companion,
or as a patient. Or you are one yourself.
• MANY BIRTH DEFECTS CAN BE PREVENTED!

1st
Prenatal Visit: 8 Weeks

The First Week

From the Morula Stage to Compaction
Morula (3 Days) Compacted Embryo

Formation of the Blastocyst and Implantation
Blastocyst (4-5 Days) Implanting Blastocyst (6 Days)
Syncytiotrophoblast
Cytotrophoblast

Hydatidiform Moles
• Formed from trophoblast
• Paternal genome (genomic imprinting)
• Secrete high levels of hCG
(syncytiotrophoblast)
• May become invasive (choriocarcinomas)

First Week of Development

Sites for Ectopic Pregnancies

Tubal Pregnancy

Ectopic Pregnancy in the Rectouterine (Douglas) Pouch

Assisted Reproductive
Technologies (ART)
Fertility treatments in which both the eggs and
sperm are handled in the laboratory (i.e., in vitro
fertilization and related procedures). Eggs and
sperm are placed in a culture medium; fertilized
eggs are placed in the uterus.

ART Cycles Performed, Live-Birth Deliveries, and
Live Babies Born Using ART—US, 1996-2002
115,392 procedures reported to CDC in 2002; 45,751 infants born after ART in US (1.1% of all births)

Adverse Outcomes Potentially
Associated with ART
• Embryo effects
• Spontaneous abortions
• Multiple births
• Adverse perinatal outcomes: low birth weight,
preterm delivery (even among singleton births)
• Birth defects
• Developmental disabilities
• Childhood malignancies
• Longer term outcomes?
• Effects on puberty/future fertility
• Effects on chronic disease risk

0
20
40
60
80
100
120
LBW VLBW Preterm
%adverseperinataloutcomes
Singletons Twins Triplets +
Percentage of Adverse Perinatal Outcomes
among ART Infants by Plurality -- US, 2002

Percent LBW among ART singletons (1996-1997)
compared to all singleton births to
non-teen mothers in US (1997)
Schieve et al., N Engl J Med 346:731-7, 2002

Large Meta-Analysis Study
30-40% Increased Risk of Birth
Defects from ART
From: Hansen et al., Human Reproduction, 20: 328, 2005

Implantation with Formation of Two Layers in the Embryoblast and the
Trophoblast
Implanting Blastocyst (6 Days) Implanted Embryo (7.5 Days)

Early Trophoblast (Placental) and Embryonic Development
9 Days
12 Days
13 Days
Yolk sac
cavity
Oropharyngeal
membrane

15 days15 Days
2nd Week = Week of Two’s
Cytotrophoblast: Syncytiotrophoblast &
Cytotrophoblast
Extraembryonic Mesoderm: Visceral (splanchnic) &
Parietal (somatic) Layers
Embryonic Disc: Epiblast & Hypoblast
(Parietal layer)
Extraembryonic
mesoderm
(Visceral layer)

Formation of the Primitive Streak and Establishment of the
Cranial-Caudal Axis

BMPs: Hatched area = ventral mesoderm
(kidneys and body wall) and ectoderm
BMPs (goosecoid, brachyurea T) = dorsal
mesoderm (somites) and neural ectoderm
Anterior Visceral Endoderm (AVE) Induces the Cranial Region

Overexpression of Goosecoid in Frogs Causes Double-
Headed Tadpoles: Does It Cause This Defect in Humans?

Genetic Regulation of Laterality

Situs Inversus
Kartagener syndrome (20%)
Cilia malfunction with situs inversus
(bronchiectasis and sinusitis)
Laterality Sequences
Left or right sidedness
Asplenia (right)
Polysplenia (left)

Gastrulation: Formation of the 3 Germ Layers
14 Days 16 Days
Oropharyngeal membrane
Oropharyngeal
membrane

During Gastrulation, Epiblast Cells Move Toward the Primitive Streak,
Leave the Epiblast, and Create 2 New Layers

Formation of the Prechordal Plate and Notochord
Oropharyngeal
membrane

Formation of the Notochord
A
Cut lines for
C B
Oropharyngeal
membrane

The Three Germ Layers
Mesoderm
EndodermNotochord
(Part of the mesoderm)
Ectoderm
• Ectoderm: skin, CNS, PNS, eyes, internal
ear, neural crest cells (bones and connective
tissue of the face and part of the skull)
• Mesoderm: bones, connective tissue,
urogenital system, cardiovascular system
• Endoderm: gut and gut derivatives (liver,
pancreas, lungs, etc.)

From fertilization to gastrulation

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