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The First Three Weeks of Human Embryogenesis
Department of Histology, Cytology and Embryology
Kharkiv State Medical University
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Week 1-3:
Early Stages:• 1. Fertilization• 2. Cleavage• 3. Gastrulation • 4. Formation of the embryo body • Late stages:Histogenesis, Organogenesis
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Week 1
• 1.Fertilization – is the fusion of the sperm and ovum = Zygote formation
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Fertilization. Cleavage
Fertilization (positive rheotaxis,gyno-androgamones,capacitation, acrosomal reaction) = zygote
Cleavage2 cells stage
3-5 cells stage Morula Blastula
ImplantationDay 6 - 8
uterusuterine tube.
.
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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
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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
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Week 2. 3. GastrulationGastrulation by the delamination and migration leads to formation of three germ layers:
ectoderm, endoderm and mesoderm,
and axial organs:
notochord, neural tube, somites
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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
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Delamination
Trophoblast
Embryonic disc:EctodermEndoderm
Ectoblast
Endoblast
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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)
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Week 2
Trophoblast
ExtraembryonicMesoderm
Chorion
Yolk Sac
Ectoderm
Endoderm
Amnion
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Late gastrulation – migration continues within embryonic disk
andleads to formation of the embryonic
mesoderm (third germ layer) and axial organs (neural tube, notochord and
somites - 43, 44 pairs)
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Transverse section. Moving ectodermal and endodermal cells form group of cells = primitive streak
EctodermEctoderm
EndodermEndoderm
Amniotic Cavity Amniotic Cavity Embryonic Embryonic mesodermmesoderm
Primitive streakPrimitive streak
Yolk SacYolk Sac
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• Notochord is formed from invaginated ectoderm and primitive streak
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Notochord formation (longitudinal section)
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Development of the Neural Tubeby the invagination of ectoderm
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Development of the Neural Tube Neural groove
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Development of the Neural Tube
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Development of the Neural Tube
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Development of the Neural Tube
Surface Ectoderm
Neural CrestNeural Tube
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Neural tube formation
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Neural tube formation
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• Somites locate between ectoderm and endoderm, arise from primitive streak and notochord and
• consist of mesoderm
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Axial organs –neural tube, notochord, somites (mesoderm)
Somite
Notochord
Neural tube
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Formation of the Body Axis
Left – Right Sidedness
FGF8 N
odal L
efty
PITX2
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Left – Right sidedness
• Situs inversus–20% (Kartagener syndrome)
• Dysfunctional cilia
• Respiratory problems
• Male fertility problems
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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
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Body flexion formation
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Body flexion formation(Gut formation. The upper part of yolk sac forms tube inside
the forming body = gut - primitive digestive tube
gut
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Body flexion formation(Gut. Allantois – is extention of hindgut)
Allantois
FUTURE PLACENTA
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Body formation
Yolk Sac
Oral Plate
CloacalPlate
foregut
midgut
hindgut
AllantoisHeart
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Body formation
Chorionic
Cavity
Amniotic cavity
Yolk Sac and allantois participate in the formation of ambilical cord
Chorionic plate participates in the formation of placenta
UterineWall
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What should we study by heart ?
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EXTRAEMBRYONIC ORGANS
• Allantois
• FUTURE PLACENTA
Amnion
Yolk sac
chorion
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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
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Differentiation of GERM LAYERS: 1. Differentiation of Ectoderm A. Surface Ectoderm B. Neural Tube2. Differentiation of Endoderm A. G.I. Tract
B. Respiratory Tree C. Endocrine glands3. Differentiation of Mesoderm A. Somites-dermatome, myotome, sclerotome
B. Intermediate- nephrotome C. Lateral-splanchnotome D. Mesenchyme
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Surface Ectoderm differentiates to skin, its derivatives, oral, rectal epithelium, corneal epithelium, tooth enamel
amnion ectoderm
proctodeum
stomatodeum
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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
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Endoderm differentiates to epithelium of the stomach, intestine, liver, pancreas, respiratory, endocrine systems -- 3-4 weeks - gut
endoderm gut
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Endoderm (gut, allantois)
Yolk Sac
Oral PlateStomatodeum
CloacalPlate
foregut
midgut
hindgut
Allantois
Heart
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Mesoderm - formation of the first blood vessels in the wall of yolk sac and
allantois
blood vessels
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Mesoderm
NotochordEndoderm
Ectoderm
Yolk Sac
Amniotic Cavity (Somite)
Intermediate mesoderm(nephrotome)
Lateral plate mesoderm(somatopleuric,splanchnopleuricmesoderm)
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Mesoderm
Intermediate Mesoderm.Nephrotome
urogenital systemincluding kidneys, gonads,ducts, and accessory glands
Somite
dermatome - dermis of skinmyotome - muscles,sclerotome - skeleton, except skull
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
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Chorionic
Cavity
Amniotic cavity
Yolk Sac and allantois participate in the formation of ambilical cord
Chorionic plate participates in the formation of placenta
UterineWall
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Late embryonic stages
• Histogenesis
• Organogenesis