1. Inner Cell Mass (ICM) delaminates to form hypoblast and epiblast Occurs just prior to implantation & gastrulation Epiblast (green cells) is 2-layered (i.e., it is bilaminate) disc of approximately cuboidal cells & will form the embryo proper Flatter hypoblast cells lie on top of epiblast and will form yolk sac Formation of the epiblast & hypoblast(review)

2. morphogenetic movements and their significance to human embryogenesis

3. Following are just some of the types of cell movements that occur in animal embryos: Ingression: cells break away from the tissue and migrate as individuals Delamination: layers of cells separate from each others more or less as sheets of cells Intercalation: two cell layers interlace with each other Epiboly: a form of cell spreading in which cells flatten out; this allows them to cover a much larger surface area (1st detailed in frog development). Invagination (Evagination): a tissue layer folds in (out) Involution: cells move over a lip of tissue and into the interior Convergent Extension: cells reorganize to form less layers allowing the cells to extend out from a point.

4. 10/17/20154 Gastrulation sohrabi

5. Gastrulation in Human Embryo During human gastrulation, cells move over the blastodisc surface enter the primitive streak and move internally. – On the surface they move in association with other cells but once they turn the corner around the lip of the primitive streak the cells separate as individuals to migrate internally to form the mesoderm and endoderm. The moving surface cells first pile up to form a prominant bump known as the node. This occurs because the cells move along the top faster than they can separate off and move internally. – The node was discovered in mammals by Hensen and only referred to as the node in humans. – The cells that enter through the node will become the notochord – As the cells continue to move in, the primitive groove forms The cells that migrate internally first will become the endoderm which contains the presumptive notochordal tissue as well The cells move internally later will migrate over the endoderm and form the mesoderm The cells that remain on the surface will form ectoderm

6. Results of Gastrulation Gastrulation will covert the bilaminate epiblast into the three primary embryonic germ layers: 1-Ectoderm: outside; this embryonic layer more or less surrounds the other germ layers 2-Mesoderm: middle; this germ layer lies between the ectoderm and endoderm 3-Endoderm: inside; this germ layer lies at the most interior of the embryo Subsequently neurulation will form epithelial and neural ectoderm from the ectoderm

7. Role of Gastrulation

8. Formation of the primitive streak

9. Gastrulation in Human Embryo

10. Cross Section of Human Gastrula

12. Bottle Cells Bottle cells were first observed in amphibian gastrulation Occur during gastrulation in humans and many other species Also seen during neurulation and during other types of cellular rearrangements Involves a loss of cell-cell adhesion so cells can move as individuals; recent work has shown that this is due to the loss of E-cadherin Change in shape of epithelial to bottle cell is believed to be one of driving forces for event – Cytoskeletal changes lead to bottle cell shape

13. Bottle Cells Bottle cells appear at time of neurulation Change in shape of epithelial to bottle cell is believed to be one of driving forces for event Cytoskeletal changes lead to bottle cell shape Microfilaments (F-actin) at apex Microtubules down long axis of cell

15. Embryo Axes Humans, like most other living things, have a distinct organization. three axes become established during early development : – Anterior-Posterior Axis; A-P axis – ventral – dorsal Axis;V-D axis – Right-Left Axis ;R-L axis

16. Factors involved in genetic control of :early embryonic development 1- Transcription factors: Homeodomain - Hox gene family Zinc finger – a family of steroid-binding transcrition factors Basic helix-loop-helix protein – myogenic regulatory factors Winged helix – hepatocyte nuclear factor-3 2- Signalling molecules Transforming growth factor-B (TGF-B) – activate posterior Hox genes Fibroblast growth factor (FGF) – activate anrterior Hox genes Nerve growth factor (NGF) – stimulate growth of axons Hedgehog proteins – mediate early inductive interactions 3-Cell adhesion molecules Some are calcium-dependent (Cadherins) Some are calcium-independent (CAM)

17. HOX and the A-P Axis The A-P axis of all animals appears to be specified by the expression of Hox genes expressed sequentially in the same order that they are arranged in the chromosomes – The Human Hox complex (HOXA-HOXB-HOXC-HOXD) is present in four sets on four separate chromosomes in each haploid set of chromosomes. – Retinoic acid affects A-P axis formation and HOX gene expression.

18. Differentiation Factors: Fibroblast Growth Factors (FGF) : – Over 12 genes encode the FGF family of proteins – FGFs function in axon extension, angiogenesis, and mesoderm formation

19. Differentiation Factors: Hedgehog Proteins: – Vertebrates have at least three genes for hedgehog proteins: sonic hedgehog (shh) – Sonic hedgehog--has the most diverse roles of all three especially in the patterning of the embryo; functions in the patterning of the neural tube and somites, mediates formation of L-R axis of the embryo and of limbs, induction of differentiation of gut regions indian hedgehog (ihh) – Indian hedgehog--operates in gut and cartilage development desert hedgehog (dhh) – Desert hedgehog--functions in the Sertoli cells

20. Differentiation Factors: Wnt Family – consists of of at least 15 different cysteine-rich glycoproteins – They are involved in inducing dorsal cells of somite to form muscle; in establishing polarity of limb and in development of the urogenital system, etc. TGFB  family, activin family, bone morphogenetic proteins (BMPs), VG1 family and more – TGF-B  proteins regulate matrix formation and cell division – BMPs were 1st discovered as bone morphogens also regulate cell division, apoptosis, cell migration and differentiation &in the control of limb development

21. Signaling Centers in embryo development There are two signaling centres regulate gene expression in the different regions of the embryo to ensure that it develops with the relevant components in the right places at the right time: – After the endoderm has migrated internally during gastrulation, the "anterior visceral endoderm" instructs the formation of head components. the anterior visceral endoderm expresses genes called Lim-1, Hesx-1 and Otx-2 which are essential for head development – “ Hensen's node ” that appears at gastrulation at the anterior end of the primitive streak contains information that oversees the construction of the whole body form. The node expresses genes called Noggin and Chordin that are not expressed by the anterior visceral endoderm.

22. cranio-caudal axis The primitive node is the first and central structure. primitive node determines the growth of the notochord cranially (by factor HNF-3  ) and of the primitive streak caudally (by the factor nodal) and the body axis (by the factor goosecoid). primitive node is also responsible for inducing the cells to become motile and migrate to pre- determined sites (by the factor T-gene). Note also how the prochordal plate determines the cranial end of the embryo (by factor Lim-1).

23. Right-Left asymmetry This right left differentiation is determined by several laterality genes. The signalling molecule: -sonic hedgehog (Shh) is expressed bilaterally and symmetrically. -Activin is expressed only on the left, inhibits the further action of Shh, and causes a differential expression of the growth factor nodal.

24. Right-Left asymmetry genes that specifically determine development of structures only on the left side. These have been called Inversus viscerum and left-right dynein.

26. Induction of Neural Tissue by Chordamesoderm During gastrulation special region of mesoderm underlies overlying ectoderm Chordamesoderm is future notochord Signals emitted by chordamesoderm have not been identified

27. After gastrulation, three germ layers are evident: Ectoderm, mesoderm, endoderm : Late Gastrula

28. Notochord Formation the presumptive notochordal tissue will separate from the endoderm and re-organize as the notochord As notochord formation is occuring, neurulation will separate the ectoderm into the epithelial ectoderm and the neural ectoderm the neural ectoderm begins folding to form the neural tube – The future neural ectoderm overlies notochordal process (chordamesoderm) – Folding of the neural ectoderm results in the neural tube The neural tube is the precursor of the brain & spinal cord

31. 10/17/201531 Neurulation

32. Hensen ’ s Node Induces Neural Axis this special region of tissue has been shown to establish the vertebrate body plan in everything from frogs to zebra fish to mice the node is an embryonic organizer in mammals – the mammalian node is not a classic organizer because it can't act alone to induce the embryonic axis but requires other anterior germ tissues to be fully effective Thus progressive cellular interactions are more important rather than a single inductive event

35. The Events of Neurulation Neural ectoderm thickens Neural folds form producing Neural Groove Neural folds contact Cells rearrange forming Neural Tube with overlying Ectoderm

36. Hensen ’ s Node Induces Neural Axis These classic experiments were done in chicken embryos Remove HN: No neural tissue development Transplant HN to host embryo: 2nd Neural Axis induced

37. Cell Lineages At Neurula Stage

40.  Embryo consists of all primary germ layers  Development continues to form tissues & organs  As the neural tube is forming,  the somites are forming  & the embryo is elongating & flexing

41. Folding of the germinal disk (4th week)

49. The End