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

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

Animals must be specified in three dimensions

The germ layers are created during gastrulation Lecture E01

The germ layers form different tissues

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

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)

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

Two headed cow...

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

Question: How does the node pattern?

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

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

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

Gradients of secreted proteins produce the different germ layers

Left-right asymmetry of internal organs Lungs Heart Gut looping Liver

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

In situ hybridisations of left-right asymmetry genes

Node and cilia How to break symmetry

anterior posterior RL The node spins

Loss of left-right asymmetry leads to disease Situs inversus

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

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

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

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

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

Hox gene expression follows the somite bondaries Embryonic structuresAdult organs

Film of somitogenesis

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. *

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

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

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