Gastrulation and Germ Layer Formation in Embryology: An Overview

1. 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

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

6. Notochord • Notochordal plate • Detaches from endoderm • Notochord----vertebral column-midline axis • Induces neural plate----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

8. Allantois • Outgrowth of the yolk sac • Early bladder development

9. Trophoblast • Primary villus • Secondary villus • Tertiary villus

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

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

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

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

15. Lateral folding • Ectoderm=outside • Mesoderm=middle layer • Endoderm= inside • Ventral wall closes

16. 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

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