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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

24 days after fertilisation Three dimensional embryo Bilateral symmetry, 2.5-4.4mm long Neural tube-neuroectoderm Rudimentary gut tube-endoderm Rudimentary heart- mesoderm Coelom Somites-musculoskeletal system Stalk linking to growing placenta

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