Lecture 1 Overview of early mammalian development Tools for studying mammalian development Fertilisation and parthenogenesis Mosaic vs regulated development You should understand Non-equivalence of maternal and paternal genomes Mammalian development is highly regulated

Embryogenesis in mammals occurs in utero - difficult to observe. Important to study because of direct relevance for understanding and treating disease. Isolation of tissue culture models, e.g embryonic stem cells, is relatively easy. Also highly advantageous for genetic manipulation, knock-out, knock-in etc - Functional genomics studies - Disease models for basic science and pharmacology. Mouse is preferred model; Good genetics (inbred lines etc), short generation time. Mammalian Development

Where am I?Who am I? Anterior (Head) Posterior (Tail) Ventral (Back) Dorsal (Front) Left Right An anthropomorphic view of development

In utero development in mouse occurs over days

Preimplantation Development Trophectoderm Primitive (primary) endoderm Inner cell mass Cleavage stages Zona pelucida Blastocoel cavity Activation of embryonic genome Blastomere days

Early Post-implantation Development

Gastrulation and Beyond

Extraembryonic tissues

Experimental Tools for studying mouse embryos Embryological approaches; Histological analysis and conventional microscopy Cell fate mapping (dyes and now tagged loci) In vitro culture of preimplantation stages and in some cases postimplantation stages.

In situ hybridization Immunohistochemistry Eed + Nanog Oct4 + Eed SectionsWholemount Embryological approaches; Gene expression profiling of embryos, dissected fragments, derivative tissue culture cell lines and single cells.

Chimera formation and embryo aggregation. Cell culture models e.g. tetraploid chimeras for testing gene function in extraembryonic vs embryonic lineages. Embryological approaches; Embryonic stem (ES) cells

Genetic approaches; Classical mouse mutants Brachyury mouse with short tail is dominant mutation in gene for transcription factor required for mesoderm formation. Genetic screens Wild-type and Nodal (d/d) mutant embryos with staining for markers of primitive streak (brown) and ectoderm (dark blue). Chemical (ENU) mutagenesis – requires lengthy genetic mapping and cloning to identify mutated locus Insertional or ‘gene trap’ mutagenesis in ES cells – can go directly to gene of interest SA SD Antibiotic resistance marker Reporter gene IRES PolyA signal

Production of transgenic mice - Gene construct injected into male pronucleus of 1-cell embryo - DNA integrates randomly into the genome - Usually at single site but in multiple copies - Resulting mice can be bred and then maintained by monitoring continued presence of the transgene using PCR etc. - Gene construct can be assembled in plasmid (up to 25kb) or bacterial artificial chromosome (BAC) vectors ( kb). Genetic approaches;

Transgene constructs; 100kb - Intact gene in BAC complete with tissue specific regulatory sequences enhancerpromoter - Engineered BAC with heterologous regulatory sequences, eg tetracycline inducible - Plasmid with tissue specific regulatory sequences and heterologous gene eg GFP reporter. Genetic approaches; Drawback; high copy number gives non-physiological expression levels

Gene targeting in embryonic stem (ES) cells Genetic approaches; X Homzygous mutants, double mutants etc Homzygous/double mutant ES cells

Conventional gene knockout strategy (replacement vector) Potential drawbacks are redundancy and lethality X X Positive selectable Marker gene Negative selectable Marker gene Knock-out X GFP Orf X Knock-in Genetic approaches;

Conditional gene knockout strategy; Bacterial site specific recombinases (Cre-loxP or Flp-Frt) Genetic approaches;

Positive selectable Marker gene Negative selectable Marker gene X X + site specific recombinase + Recombinase recognition sequence Conditional gene knockout strategy; Genetic approaches;

Homozygous conditional allele Transgenic mouse expressing site specific recombinase in tissue specific pattern X Analyse phenotype in F1 embryos or adults Examples of recombinase driver transgenics; - Cre recombinase driven by Nanog promoter - Estrogen receptor-Cre recombinase fusion driven by constitutive promoter. Addition of Tamoxifen to drinking water triggers nuclear translocation of recombinase giving temporal control of gene deletion. Conditional gene knockout strategy; Genetic approaches;

Fertilisation Penetration of cumulus cells Acrosomal reaction penetrates zona pellucida made up of glycoproteins Sperm and egg plasma membranes fuse and sperm nucleus enters egg. Fertilization triggers dramatic release of calcium in the egg, setting in train completion of female meiosis etc.

Pronuclear Maturation Replication initiation M-phase hr post fertilization 0 Second polar body Zona pelucida Maternal and paternal genome remain separate (pronuclei) unitil first metaphase. Male pronucleus. Female pronucleus. Syngamy

Parthenogenesis Limited viability suggests either that sperm/fertilization confers essential properties for development or that maternal genome alone is incapable of supporting development Parthenogenetic activation - Genetic background - In vitro manipulation - Pronase/hyalouronidase - Heat shock - Ethanol - Strontium chloride Oocytes can be activated in the absence of fertilization, leading to parthenogenetic development Parthenogenetic embryos have limited viability, contrasting with other model organisms

Non-equivalent contribution of maternal and paternal genomes ? Recipient zygote Donor zygote Barton, Surani, Norris (1984) Nature 311, p374-6 McGrath and Solter, (1984) Cell 37, p Gynogenetic embryos have retarded growth/development of extraembryonic tissues Androgenetic embryos have retarded growth/development of embryonic tissues

Epigenesis vs Preformation

Roux (1888) shows ‘mosaic development’ of frog embryo following ablation of one cell in two-cell embryo – formation of ‘half’ embryo. Driesch (1895) finds opposite is true for sea urchin, normal albeit smaller embryo develops from one of two cells – ‘regulated development’. Mosaic and Regulated development

Tarkowski, (1959) Nature 184, p cell embryo Donor Recipient Regulated development in mouse embryos

Chimeras from aggregaton of 8-cell stage embryos 8-cell embryos Remove zona pellucida Aggregate in dish Culture in vitro Chimeric blastocyst Transfer to foster mother Chimeric progeny Tarkowski (1961) Nature 190,

Chimeras from transfer of ICM cells Gardner later showed fate of TE and PE is determined by blastocyst stage Gardner (1968), Nature 220, p596-7

End lecture 1