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