Transcript Germ disc differentiation

Third to Eighth Week:
The Embryonic Period
Jun Zhou（周俊）
School of Medicine, Zhejiang University
20131230
Review
End of the Week of Twos
•2 major cell masses
–Trophoblasts
–Inner cell mass
•Trophoblasts
–Cytotrophoblasts
–Syncytiotrophoblasts
Uteroplacental Cir.
•Embryonic Cavities
–Amniotic cavity
–Yolk sac
•Germ Disc
–Epiblasts
–Hypoblasts
“Periods” of Gestation
Pregerm - First 2 weeks
(i.e. prior to gastrulation, formation of
Endoderm, Mesoderm & Ectoderm)
 Gestation called:
 Zygote, morula, blastocyst,
bilaminar disc
Embryonic - 3 weeks to 8 weeks
 Gestation called:
 Embryo
 Morphogenesis
 Organogenesis
Fetal - 9 weeks to 9 months
 Gestation called:
 Fetus
 Growth and maturation
Term Gestation
 38 weeks from conception
 40 weeks from last menstrual
period
Formation of trilaminar disc
•Early of three weeks
•Primitive streak: caudal end, a
midline indentation on the
epiblast surface
•Primitive node/knot
•Primitive pit
•Proliferation & Invagination of
epiblasts
Intra-embryonic mesoderm
Gastrulation
•Migrating epiblasts will form:
–Endoderm: migrated cells displace
hypoblast cells
–Mesoderm: cells btw the endo and
ecto
–Ectoderm
(Note - epiblasts=>all layers)
•Mesoderm cranial and lateral
extension
Distribution of intra-embryonic
mesoderm
-spread laterally and cranially
-Fused membranes
 Oral: oropharyngeal
membrane
 Cloacal: cloacal membrane
Formation of notochord
- Invaginate in through primitive
pit/knot
- a solid rod of mesoderm
- Basis of the axial skeleton in the
midline
Review/Background
•First Week
•Second Week
•Third- 8th week
–Gastrulation
•Primitive pit
•Primitive node
•Primitive streak
•Notochord
Expansion and Elongation
•Migration of Epiblasts to
form
–Endoderm
–Mesoderm
–Ectoderm
•Expansion of the disk
•Notochord:
-midline rod of mesoderm
-orientation the germ disc
Differentiation of trilaminar germ disc
3rd –8th weeks (Embryo)
embryonic period: the major body form and
organ structure take shape
Bilaminar disc: flat and roundelongated with a
broad cephalic and a narrow caudal end
Trilaminar disc: a pear-shaped appearance
Ectoderm - Neuralation
•Overlying notochord
Thickens – Neural Plate
•Edges - Neural Folds
•Fuse at future neck region
•Zippers shut
–Neural Tube
•Ant/Post Neuropores
Ectoderm - Neuralation
•Overlying notochord
Thickens – Neural Plate
•Edges - Neural Folds
•Fuse at neck region
•Zippers shut
–Neural Tube
•Ant Neuropore: 25th days
•Post Neuropore: 27th days
Ectoderm – Neuralation
•Failure of Neurotube
closure
–Anencephaly – Ant
–Meningocele – post
Ectoderm – Neural Crest
•As neural tube closes
-Neural crest
•Detach and migrate
–Melanocytes – skin
–Dorsal root ganglia
Differentiation of the Ectoderm :
-the central nervous system (CNS)
-Neural crest cells (melanocyte, PNS)
-Sensory epi. of ear, nose, eye
-epidermis
In general terms it may be stated that the ectoderm
gives rise to those organs and structures that maintain
contact with the outside world.
Mesoderm
•Loose – condenses day 17
•3 columns
–Paraxial mesoderm
–Intermediate mesoderm
kidneys & Gonads
–Lateral Plate mesoderm
body wall, limbs
Paraxial Mesoderm
•Arranged around a small cavity
•Cells of Ventral, medial walls migrate
toward notochord-----sclerotome
•Dorsal cells extend in a ventral direction---myotome
•Cells under the overlying ectoderm--dermatome
Paraxial Mesoderm
•Segmentation
•Blocks of tissue along axis
•Somite:20th days, 3 pairs/ per day,42-44
pairs by the end of 5th weeks
-Sclerotome: ventral, vertebra
-Myotome: musculature of the region
-Dermatome: dorsal, dermis of the
overlying skin
Intermediate Mesoderm
•Kidney and genital system “Urogenital tissue”
Lateral Plate Mesoderm
•Intra-embryonic coelom: →body
cavity
1.Outer layer: parietal mesoderm
→muscle, CT of the body wall
→parietal layer of cavities
2.Inner layer: visceral mesoderm
→muscle, CT of the gut
→Visceral layer of cavities
3.Cavity: peritoneal, pleural and
pericardial cavities
mesenchyme: →CT, cardiovascular and lymph system
Endoderm
•From the Yolk sac
•Forms Primitive gut
•Besides the GI system
-Foregut: thymus,thyroid,lung,liver,
pancreas, etc.
-Midgut: none
-Hindgut: bladder,urethra
•Surrounded by visceral lateral
plate mesoderm
Embryonic Folding
•Craniocaudal folding--CNS
•Lateral folding—somites
•Endoderm-lined cavity is
incorporated into the embryo
properprimitive gut
•Yolk sacvitelline duct
•Oropharyngeal membrane
•Cloacal membrane
•Flat discpear, round
3-8th week
OBJECTIVES
1.Bilaminar and trilaminar germ discs
2.Understand the origin of the three embryonic
“germ” layers that make up the trilaminar disk.
3.Describe the formation and derivatives of
ectoderm, mesoderm and endoderm.