the first three weeks of human embryogenesis department of histology, cytology and embryology...
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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
Fertilization (positive rheotaxis,gyno-androgamones,capacitation, acrosomal reaction) = zygote
Cleavage2 cells stage
3-5 cells stage Morula Blastula
ImplantationDay 6 - 8
uterusuterine tube.
.
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. GastrulationGastrulation 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
Embryonic disc:EctodermEndoderm
Ectoblast
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
Trophoblast
ExtraembryonicMesoderm
Chorion
Yolk Sac
Ectoderm
Endoderm
Amnion
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)
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
• Notochord is formed from invaginated ectoderm and primitive streak
Notochord formation (longitudinal section)
Development of the Neural Tubeby 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 CrestNeural 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)
Somite
Notochord
Neural tube
Formation of the Body Axis
Left – Right Sidedness
FGF8 N
odal L
efty
PITX2
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
Yolk Sac
Oral Plate
CloacalPlate
foregut
midgut
hindgut
AllantoisHeart
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
What should we study by heart ?
EXTRAEMBRYONIC ORGANS
• Allantois
• FUTURE PLACENTA
Amnion
Yolk sac
chorion
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 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
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)
Yolk Sac
Oral PlateStomatodeum
CloacalPlate
foregut
midgut
hindgut
Allantois
Heart
Mesoderm - formation of the first blood vessels in the wall of yolk sac and
allantois
blood vessels
Mesoderm
NotochordEndoderm
Ectoderm
Yolk Sac
Amniotic Cavity (Somite)
Intermediate mesoderm(nephrotome)
Lateral plate mesoderm(somatopleuric,splanchnopleuricmesoderm)
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
Chorionic
Cavity
Amniotic cavity
Yolk Sac and allantois participate in the formation of ambilical cord
Chorionic plate participates in the formation of placenta
UterineWall
Late embryonic stages
• Histogenesis
• Organogenesis