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The First Three Weeks of
Human Embryogenesis
Department of Histology, Cytology
and Embryology
Kharkiv State Medical University
Week 1-3:
Early Stages:
• 1. Fertilization
• 2. Cleavage
• 3. Gastrulation
• 4. Formation of the embryo body
• Late stages:
Histogenesis, Organogenesis
Week 1
• 1.Fertilization – is the fusion of the sperm
and ovum = Zygote formation
Fertilization. Cleavage
Cleavage
2 cells stage
3-5 cells stage
Morula
Blastula
.
.
uterine tube
Fertilization
(positive rheotaxis,
gyno-androgamones,
capacitation,
acrosomal
reaction) = zygote
uterus
Implantation
Day 6 - 8
Week 1
• 2.Cleavage – is the division of the zygote in
the uterine tube = Blastula formation
(2 cell stage – 4 cell stage …16-32 cell
stage (morula) – blastocyst)
• During cleavage the germ is moved to the
uterus
At the end of the cleavage outer cells (trophoblast)
involve nutritive fluid, which forms cavity, moving
inner cells to the periphery (embryoblast). Blastula
is formed.
Inner Cell Mass
(embryoblast)
Trophoblast
Blastocoele
Week 2.
3. Gastrulation
Gastrulation by the delamination and
migration leads to formation of three
germ layers:
ectoderm, endoderm and mesoderm,
and axial organs:
notochord, neural tube, somites
Early Gastrulation.
At the beginning of
gastrulation germ sinks
into the uterine wall –
implantation.
Gastrulation begins from delamination – division of
embryoblast into two germ layers - ectoderm and
endoderm, forming embryonic disc and two sacs –
ectoblast and endoblast
Delamination
Trophoblast
Ectoblast
Embryonic disc:
Ectoderm
Endoderm
Endoblast
Migration - extraembryonic mesoderm formation
From embryonic disc cells begin to migrate which form
extraembryonic mesoderm, surrounding upper and lower
sacs,
and underlying trophoblast
As a result appear amnion, yolk sac and chorion
(extraembryonic organs)
Week 2
Extraembryonic
Mesoderm
Trophoblast
Ectoderm
Chorion
Amnion
Endoderm
Yolk Sac
Late gastrulation – migration continues
within embryonic disk
and
leads to formation of the embryonic
mesoderm (third germ layer) and axial
organs (neural tube, notochord and
somites - 43, 44 pairs)
Transverse section. Moving ectodermal
and endodermal cells form group of cells
= primitive streak
Amniotic Cavity
Embryonic mesoderm
Ectoderm
Primitive streak
Yolk Sac
Endoderm
• Notochord is formed from invaginated
ectoderm and primitive streak
Notochord formation (longitudinal section)
Development of the Neural Tube
by the invagination of ectoderm
Development of the Neural Tube
Neural groove
Development of the Neural Tube
Development of the Neural Tube
Development of the Neural Tube
Surface Ectoderm
Neural Crest
Neural Tube
Neural tube formation
Neural tube formation
• Somites locate between ectoderm and
endoderm, arise from primitive streak and
notochord and
• consist of mesoderm
Axial organs –neural tube,
notochord, somites (mesoderm)
Neural tube
Somite
Notochord
Formation of the
Body Axis
Left – Right
Sidedness
Left – Right sidedness
• Situs inversus
–20% (Kartagener syndrome)
• Dysfunctional cilia
• Respiratory problems
• Male fertility problems
4. Formation of
the embryo
body (20 days)
Body flexion, head and tail
folds formation.
Amnion accumulates fluid and
increases, yolk sac decreases.
Formation of a gut, allantois
Body flexion formation
Body flexion formation
(Gut formation. The upper part of yolk sac forms tube inside
the forming body = gut
- primitive digestive tube
gut
Body flexion formation
(Gut. Allantois
– is extention of hindgut)
Allantois
FUTURE
PLACENTA
Body formation
hindgut
foregut
midgut
Cloacal
Plate
Oral Plate
Heart
Allantois
Yolk Sac
Body formation
Chorionic plate
participates in
the formation of
placenta
Uterine
Wall
Chorionic
Cavity
Yolk Sac and allantois participate
in the formation of ambilical cord
What should we study by heart ?
EXTRAEMBRYONIC ORGANS
chorion
Amnion
• Allantois
Yolk
sac
• FUTURE
PLACENTA
Extraembryonic organs (supportive,
nutritive)
• Amnion – protective bag of water
• Yolk sac – gut, germs of gametes
first blood vessels, cells
• Allantois – urinary bladder
• Chorion – protection, hormones, placenta
• Placenta – main nutritive, protective,
hormonal
Differentiation of GERM LAYERS:
1. Differentiation of Ectoderm
A. Surface Ectoderm
B. Neural Tube
2. Differentiation of Endoderm
A. G.I. Tract
B. Respiratory Tree
C. Endocrine glands
3. Differentiation of Mesoderm
A. Somites-dermatome, myotome, sclerotome
B. Intermediate- nephrotome
C. Lateral-splanchnotome
D. Mesenchyme
Surface Ectoderm differentiates to skin, its
derivatives, oral, rectal epithelium, corneal
epithelium, tooth enamel
amnion
ectoderm
proctodeum
stomatodeum
Ectoderm forms neuroectoderm –neural
tube (neurons and neuroglia of the brain and the retina)
and neural crests (nerve ganglia, neuroglia, adrenal
medulla, melanocytes, APUD-system).---15-20 weeks
Surface Ectoderm
Neural Crest
Neural Tube
Endoderm differentiates to epithelium of
the stomach, intestine, liver, pancreas,
respiratory, endocrine systems -- 3-4 weeks - gut
endoderm
gut
Endoderm (gut, allantois)
foregut
hindgut
midgut
Cloacal
Plate
Heart
Oral Plate
Stomatodeum
Allantois
Yolk Sac
Mesoderm - formation of the first
blood vessels in the wall of yolk sac and
allantois
blood vessels
Mesoderm
Ectoderm
Amniotic Cavity
(Somite)
Intermediate mesoderm
(nephrotome)
Lateral plate mesoderm
(somatopleuric,
splanchnopleuric
mesoderm)
Endoderm
Yolk Sac
Notochord
dermatome - dermis
of skin
Somite myotome - muscles,
sclerotome skeleton, except
skull
Mesoderm
urogenital system
including kidneys, gonads,
ducts, and accessory glands
Intermediate Mesoderm.
Nephrotome
Lateral Mesoderm -
serous membranes of pleura,
pericardium and peritoneum
Mesenchyme (loose part) –
connective tissue of viscera
and limbs, blood and lymph
cells, cardiovascular and
lymphatic systems
Chorionic plate
participates in
the formation of
placenta
Uterine
Wall
Chorionic
Cavity
Yolk Sac and allantois participate
in the formation of ambilical cord
Late embryonic stages
• Histogenesis
• Organogenesis