1. 1. Understand the molecular mechanisms underlying early embryonic development in vertebrates. 2. Explain, in general, how organizers function to pattern the forming axes of the early embryo. 3. Appreciate the conservation of molecular mechanisms controlling body plan development in different organisms: the case of homeotic genes. 4. Colinearity of the homeotic genes in man. Learning Outcomes

2. Developmental processes occurring during vertebrate development Axes formation -Signalling centres Left right asymmetry Anterior-posterior axis formation Outline

3. Animals must be specified in three dimensions

4. The germ layers are created during gastrulation Lecture E01

5. The germ layers form different tissues

6. Basic morphogenic processes are similar between animals Gastrulation in a fly FlyBase

7. Development in vertebrates is based on cell-cell interactions: groups of cells called organizing centres emit instructive signals that induce and pattern surrounding tissues. The concentration gradient of the (signal) morphogen induces multiple cell choices. (E05)

8. Organisers are involved in body axis formation in vertebrates Signalling centres instruct surrounding cells to form tissues Node graft

9. Two headed cow...

10. Genetic determinants involved in body axis formation in mammals The major signalling centre in vertebrates is the node Node ChickenHuman

11. Question: How does the node pattern?

12. Genetic determinants involved in body axis formation in mammals Organisers ‘pattern’ surrounding cells and tissues by secreting signaling molecules (proteins) Node cells secretes nodal and noggin and FGF FGF Nodal

13. Cells signalling through transmembrane receptors SHC Grb2 SOS RAS RAF MEK MAPK FGF FGFR Extracellular Intracellular P

14. Genetic determinants involved in body axis formation in mammals: Neural tissue Signalling centres instruct surrounding cells to form tissues Overlying tissues form a neural tube Node or FGF protein

15. Gradients of secreted proteins produce the different germ layers

16. Left-right asymmetry of internal organs Lungs Heart Gut looping Liver http://mekhala.blogspot.com/2007_11_25_archive.html

17. Left-right patterning asymmetric signalling from the node The expression of genes on the left side of the embryo leads to a cascade of gene expression and morphogenic changes Nodal Pitx2 Nodal chick Gut looping, heart looping

18. In situ hybridisations of left-right asymmetry genes

19. Node and cilia How to break symmetry

20. anterior posterior RL The node spins

21. Loss of left-right asymmetry leads to disease Situs inversus

22. Named for mutations that revealed existence Bithorax – part of haltere on 3 rd thoracic segment is transformed into part of a wing Antennapedia – dominant mutations transform antennae into legs Homeotic mutation is the transformation of one segment into another related one

23. Homeotic genes Colinearity: location on the chromosome corresponds to the spatial expression pattern 3’ 5’

24. Temporal and spatial colinearity: order of Hox genes on the chromosome follows the antero-posterior body axis.

25. Veraksa, Del Campo & McGinnis. 2000. Mol. Genet. Metab., 69, 85-100. How do we get anterior-posterior axis: the HOX Genes!!

26. Combinations of Hox genes specify the development of the anterior-posterior axis

27. Hox gene expression follows the somite bondaries Embryonic structuresAdult organs

28. Film of somitogenesis

29. When Something Goes Wrong… Lumbar vertebra Thoracic vertebra Extra rib The function of Homeotic genes in mammals is similar to in flies: the KO of hoxc8 in mouse causes an homeotic transformation: the first lumbar vertebra forms a rib. *

30. Summary: patterning of the vertebrate axial body plan the four Hox gene complexes are expressed along the antero-posterior axis Hox gene expression establishes positional identity for mesoderm, endoderm, and ectoderm gastrulation and organizer activity mesoderm develops into notochord, somites, and lateral plate mesoderm somite develops into sclerotome and dermomyotome mesoderm induces neural plate from ectoderm notochord patterns neural tube

31. Polydactyly Diseases associated with Hox gene mutations 1.Hand-foot-genital syndrome (Hox A11-13 deletion) 2.Synpolydactyly (HoxD13 deletion) 3.Cleft palate 4.Brain abnormalities 5.Leukemia (Hox D4) 6.Retinoic acid, which causes birth defects, affects Hox genes Teratology Lecture

32. Hox genes and vertebrate segment identity Hox gene mutations lead to subtle phenotypes Why?? Hox genes are used over and over again in the developing embryo >>>Multiple phenotypes, multiple cancers Reference book: Developmental Biology, Gilbert