1. Carolyn Joy Rodgers MD FACOG
Embryology (2)
Carolyn Joy Rodgers MD FACOG

2. Learning Outcomes: lecture two
Describe simply the process of gastrulation and the establishment of three germ layers
Describe simply the establishment of body axes
Know simply which germ layer forms the basic building blocks of the main body systems

3. The third week Gastrulation Establishes ectoderm, mesoderm, endoderm
Begins with primitive streak
Further development of trophoblast
Beginning of specialisation-neurulation

4. Day 15-16 Primitive streak-narrow groove Primitive node Primitive pit
Invagination of epiblast
Displace hypoblast-endoderm
Between epiblast and endoderm-mesoderm
Remaining epiblast-ectoderm
The bilaminar disc has elongated and widened at the front or anterior end destined to be the cranial end. The streak appears toward the back or posterior end where it is narrower.
At first it is just a faint groove then more distinct and cells at the cranial end mound up to form primitive node surrounding a depression the primitive pit.
Cells of epiblast proliferate and migrate toward streak.
They invaginate into the streak and a new layer of cells emerges beneath between the epiblast now designated ectoderm and the hypoblast now designated endoderm. This new layer which spreads laterally and cranially is the mesoderm. We now have the trilaminar disc.
So--- epiblast is the source of all the germ layers and the cells of these three germ layers will give rise to all cells and tissues.
Some of the first cells to migrate thru the streak will move forward to form the prechordal plate important for establishing site of forebrain development. And again marking the beginning of body axis.
This migration is controlled by cells in the streak secreting growth factor. Left/right sideness also seems to be established by growth factor gene expression by cells of the streak.
Epiblast is source of all the germ layers and cells in these layers give rise to all cells and tissues Some of the first cells to migrate thru and move cranially form the pre-chordal plate

5. Body axes and Fate Map Cranial/caudal;Left/right;Ventral/dorsal
Cranial- prechordal plate and notochord
Lateral edge of node and cranial end of streak- paraxial mesoderm
Midstreak- intermediate mesoderm
Caudal streak- lateral plate mesoderm
Further growth of the embryonic disc is mainly at the cephalic end
Before and during gastrulation the basic body plan is being established by gene expression of growth factors
Growth of the disc at the cephalic end is by continuous migration of cells from the streak in a cephalic direction and the process of invagination and migration forward and lateral continues until end of 4th week
Thus gastrulation continues in the caudal segments whereas in the cephalic sections differentiation begins by middle of third week
The mesoderm spreads laterally and cranially and migrates beyond the edge of the disc to make contact with the EEM of the yolk-sac and amniotic cavity
Some of the mesodermal cells that migrate anteriorly have formed the prechordal plate which will form the notochord the basis of our vertebral column
Mesodermal cells migrating around the node and uptoward the cranial end is designated paraxial mesoderm—skeleton/ muscle system
Cardiogenic mesoderm migrates to the cranial end beyond the prechordal plate
So cells are signalled to move to different sites depending on their ultimate fate
Mesodermal cells migrating thru the middle part of the streak are designated intermediste mesoderm– urinary system and gonads
Mesoderm at the caudal end is lateral plate which splits into parietal and visceral layers and is ultimately destined to form the body cavity walls and linings

6. Notochord Notochordal plate Detaches from endoderm
Notochord----vertebral column-midline axis
Induces neural plate----nervous system
Prenotochordal cells invaginate and move cranially in the midline. They become joined to the hypoblast so that a notochordal plate of two cell layers forms. As the hypoblast is replaced by endodermal cells the notochordal plate proliferates and becomes detached to form a solid cord of cells- the definitive notochord which will act as a basis for the axial skeleton and which will induce the overlying ectoderm to become neuroectoderm of the neural plate and the beginning of the nervous system

7. Oropharyngeal and cloacal membranes
Depressions at cranial and caudal end of ectoderm
Consists of tightly adherent ecto and endo derm
Future openings of mouth and genito-urinary tracts and GI tract
Further establishment of body axis
Sites where there is no intervening mesoderm

8. Allantois Outgrowth of the yolk sac Early bladder development
A small diverticulum grows out from yolk sac endoderm
Early bladder development is contiguous with the allantois but eventually it obliterates altho’ some remnants may be seen in adults
More in urinary module

9. Trophoblast Primary villus Secondary villus Tertiary villus
Continues to grow and invade more of the uterine stroma to increase the uteroplacental flow into the growing embryo. The primary villus, fingers of syncyti have a core of cyto .
Mesodermal cells begin to penetrate the core to form secondary villus
Toward end of third week these mesodermal cells are beginning to differentiate into blood vessels and blood cells to form the tertiary villus which continues to branch and increase the surface area of the organ– more in REPRO

10. Neurulation Begins about day 18
Notochord induces formation of neural plate in overlying ectoderm
Wider anterior part will become brain
Narrower posterior part will form spinal cord
Lateral edges elevate to form neural folds
Fuse – neural tube
The neural tube is therefore ectodem and the CNS BRAIN CORD RETINA DEVELOP FROM IT more in Neuro module
FAILURE OF FUSION GIVES RISE TO NTDS
Spina bifida most commonly occurs in the lumbosacral area- perhaps more sensitive to factors genetic or environmental here

11. Mesoderm Mesoderm- thin sheet of loose tissue
Day 17-paraxial mesoderm and lateral plate
Somites begin to appear in occipital area day 20 then extend craniocaudally-paired- governed by gene expression
Axial skeleton / Musculature
Week 4 the cells of the somites differentiate into
Myotome-skeletal muscles
Dermatome-connective tissues
Sclerotome-vertebra
About day 17 mesodermal cells close to midline proliferate and form a thickened plate-paraxaial mesoderm- begins to be organised into blocks cephalo-caudally
Laterally mesoderm remains thin-lateral plate and is joined to paraxial by intermediate mesoderm
Paraxial mesoderm begins to form somites, blocks of mesoderm arranged in dense columns on either side of the developing nervous system
More in the musculoskeletal module

12. Lateral plate mesoderm
Splits-parietal- lines body cavities
visceral- surround organs
Coalescence of spaces-intraembryonic coelom-later becomes the body cavities
Intermediate mesoderm-excretory units of the urinary system and the gonads

13. Folds Trilaminar disc becomes three dimensional
Embryonic folding due to rapid growth
Cranial/caudal folds and lateral folds simultaneous
So we have the three layers and separate areas have been set on their pathway of development largely by different growth factors.
Now folding occurs due to rapid growth and occurs in both horizontal and median planes simultaneously and moves these designated areas of tissue to sites we might anticipate
It also gives us the more charachteristic rounded form

14. Craniocaudal folding
Cephalic end folds ventrally,Caudal end folds ventrally
Large portion of endodermal layer is incorporated into the embryo-gut tube
Diverticulum from gut tube- lung bud
Developing brain becomes cranial part
Developing heart becomes ventral
Allantois and connecting stalk become ventral
Cephalic end folds ventrally thus swinging the cardiogenic mesoderm forward and the developing brain becomes the most cranial position
The caudal end also swings up bringing the allantois and the connecting stalk ventral position
As the folding occurs a large portion of endoderm layer is incorporated into the embryo and forms the gut tube between the oropharyngeal and the cloacal membranes… more in GI

15. Lateral folding Ectoderm=outside Mesoderm=middle layer
Endoderm= inside
Ventral wall closes
Folding involves the stack of discs of cells forming a cylinder with ectoderm outside and endoderm in the inside
Ventral wall closes except for the connection to the yolk sac and connecting stalk

16. 24 days after fertilisation
Three dimensional embryo
Bilateral symmetry, mm long
Neural tube-neuroectoderm
Rudimentary gut tube-endoderm
Rudimentary heart- mesoderm
Coelom
Somites-musculoskeletal system
Stalk linking to growing placenta

17. Summary Establishment and positioning of three germ layers
Establishment of early placenta
Understand simply which layers form the primordia for the main body systems