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Lecture 2 Overview of preimplantation development Specification of the trophectoderm

Lecture 2 Overview of preimplantation development Specification of the trophectoderm Specification of primitive endoderm Stem cell lines from early mouse embryos. You should understand Key transcription factors and signalling pathways in preimplantation embryos

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Lecture 2 Overview of preimplantation development Specification of the trophectoderm

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  1. Lecture 2 • Overview of preimplantation development • Specification of the trophectoderm • Specification of primitive endoderm • Stem cell lines from early mouse embryos • You should understand • Key transcription factors and signalling pathways in preimplantation embryos • Mechanisms governing specification of the trophectoderm lineage • Mechanisms governing specification of the primitive endoderm lineage • Stem cell lines from early mouse embryos and their relationship to early • lineages.

  2. Mosaic and Regulated development • 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’.

  3. Regulated development in mouse embryos Donor Recipient 2-cell embryo Tarkowski, (1959) Nature 184, p1286-7

  4. 8-cell embryos Remove zona pellucida Aggregate in dish Culture in vitro Transfer to foster mother Chimeric blastocyst Chimeric progeny Chimeras from aggregaton of 8-cell stage embryos Tarkowski (1961) Nature 190, 857-860

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

  6. Preimplantation Development Cleavage Morula Blastocyst Day 3.0 Day 3.5 Day 4.0 Primitive ectoderm (PrEct) Inner cell mass (ICM) Blastomere Zona pelucida Primitive endoderm (PE) Blastocoel cavity Trophectoderm (TE)

  7. Four master transcription factors for early lineage determination in preimplantation development 1. Oct4/Pou5f1; uniformly expressed in cleavage stages. Switched off in trophectoderm of blastocyst. Knockout fails to develop ICM. Cleavage Morula Blastocyst Day 3.0 Day 3.5 Day 4.0 2. Cdx2; stochastically expressed from 8-cell stage. Progressively restricted to outer TE cells of blastocyst. Knockout fails to develop trophectoderm. Primitive ectoderm (PrEct) Inner cell mass (ICM) 3. Nanog; stochastically expressed from 8-cell stage. Switched off in TE. Expressed in salt and pepper pattern in ICM eventually restricted to primitive ectoderm at d4. Knockout fails to develop ICM. 4. Gata6 (+Gata4); stochastically expressed from 8-cell stage. Switched off in TE. Expressed in salt and pepper pattern in ICM eventually restricted to primitive endoderm at d4. Double knockout fails to develop PE. Blastomere Zona pelucida Primitive endoderm (PE) Blastocoel cavity Trophectoderm (TE)

  8. Inside-Outside Hypothesis 16-cell compacted morula 8-cell embryo Outside cell Inside cell Tarkowski and Wroblewska, (1967) J Embryol Exp Morphol. 18, p155-80

  9. Testing the inside outside hypothesis 4-cell embryo Hillman, Sherman, Graham (1972) J. Embryol. Exp. Morphol. 28, 263-278

  10. The role of compaction and the cell polarity model • Compaction; at 8-cell stage cells flatten along basolateral surfaces (those with cell-cell contacts). • Apical (outside facing) surfaces develop distinct features, eg microvilli. • Cell polarity model posits that divisions at 8-cell stage produce 2 polar or 1 polar and one apolar • cell, depending on the plane of division (stochastic). Johnson and Ziomek (1981), Cell 21, p935-942

  11. Cell polarity at compaction discriminates outer and inner cells of the morula 8-cell compaction 16-cell morula Apical determinants Basolateral determinants Non-polar Inside cell Polar outside cell • Only outside cells express apical determinants – provides potential mechanism for • the differentiated fate decision.

  12. Molecular mechanism linking polarity to TE specification? • Proteins of the apical-basal polarity pathway localise assymetrically in the morula

  13. Inhibition of Hippo signalling in polarised cells induces Cdx2 • Tead4, the downstream effector of Hippo pathway is required for Cdx2 expression in outer cells. • Tead4 co-activator, dephosphorylated YAP is present in the nucleus only in inner cells of 32 cell morula. Nishioka et al (2009) Dev Cell 16, p398-410

  14. Maintenance of TE/ICM specification • Double negative feedback loop with Oct4/Nanog confines Cdx2 expression to TE cells.

  15. Specification of primitive endoderm lineage Day 3.0 Day 3.5 Day 4.0 Primitive ectoderm (PrEct) Inner cell mass (ICM) Primitive endoderm (PE) Blastocoel cavity Trophectoderm (TE) High Nanog Low GATA6 Low Nanog High GATA6 • Reciprocal salt and pepper pattern of Nanog and GATA6 in ICM cells of mid-stage blastocysts Chazaud et al (2006) Dev Cell 10 p615-24.

  16. Fibroblast growth factor (FGF) signalling transduced by MAPK • Grb2 mutant embryos fail to specify primitive endoderm Chazaud et al (2006) Dev Cell 10 p615-24.

  17. Fgf4 Fgf2r Mapk Grb2 Fgf4 Nanog Gata6 Nanog Gata6 Fgf4 high Fgfr2 high Fibroblast growth factor (FGF) signalling regulates primitive endoderm to primitive ectoderm switching Primitive ectoderm (PrEct) cell Primitive endoderm (PE) cell Cell sorting • FGF4 gene is activated by Oct4 • Only Nanog expressing ICM cells seen in Grb2 knockout or with disruption of FGF signalling • Negative feedback by Gata6 on Nanog and vice versa? • Cell sorting mechanism? Chazaud et al (2006) Dev Cell 10 p615-24.

  18. Embryonic Stem (ES) Cells Stem cells and progenitors; Stem cell; unlimited capacity to self-renew and produce differentiated derivatives Progenitor cell; limited capacity to self-renew and produce differentiated derivatives Terminally differentiated cell Terminology for differentiative capacity of stem cells/progenitors; • Totipotent; capable of giving rise to all differentiated cell types of the organism, • including extraembryonic lineages e.g. morula cells • Pluripotent; capable of giving rise to cell types of the three germ layers, ectoderm, mesoderm • and endoderm eg primitive ectoderm cells of the blastocyst. • Multipotent – capable of giving rise to a limited number of differentiated cell types, e.g.adult • stem cells and progenitors

  19. ES cells • Derived from blastocyst stage embryos • Grow as ‘clumps’ or ‘colonies’ by culturing with fetal calf-serum (FCS) on layer of inactivated • primary embryonic fibroblast cells (PEFs). Alkaline phosphatase positive Core transcription factors Oct4, Nanog and Sox2. • Have stable karyotype and contribute to all three germ layers (but not trophectoderm) • when transferred to recipient blastocyst – pluripotent. • Contribute to the germ-line of chimeric animals (blastocyst injection) and can therefore be • transmitted to subsequent generations. • Can be differentiated into embryoid bodies or defined lineages in vitro Evans and Kaufman (1984) Nature 292, p154-6

  20. What is an ES cell? • Single cell transcriptomics suggest closest to primitive ectoderm cells of the blastocyst. • No self-renewing pool of embryonic precursors in ICM or epiblast – ES cells are ‘synthetic’.

  21. Signalling pathways regulating self-renewal and differentiation of mouse ES cells LIF/STAT3 (JAK/STAT) and BMP/Smad/Id FGFs Via ERK1/2 pathway GSK inhibition (wnt?) 2i - Small molecule inhibitors of ERK LIF/STAT3 and BMP/Smad/Id GSK inhibition (wnt?) • Recent evidence suggests LIF +BMP blocks autostimulation of differentiation by FGF4 Ying et al (2008) Nature 453, p:519-23

  22. Stem cell types isolated from early mouse embryos Day 5.5 Extraembryonic ectoderm Polar Trophectoderm Primitive ectoderm Polar Trophectoderm Visceral endoderm Primitive endoderm Day 4.0 ICM Day 3.5 Mural Trophectoderm Mural Trophectoderm Parietal endoderm +FGF4 -LIF + feeders +FGF4 +LIF + feeders Epiblast +LIF +BMP +FGF +Activin EpiSC TS cell XEN cell ES cell (Extraembryonic endoderm cell) (Trophoblast stem cell) (Epiblast stem cell) Germ layers Germ line Trophectoderm Primitive endoderm Germ layers Germ line Trophectoderm Primitive endoderm Germ layers Germ line Trophectoderm Primitive endoderm Germ layers Germ line Trophectoderm Primitive endoderm Chimera Contribution (-LIF/-BMP) Germ layers Germ cells Primitive endoderm (-FGF) Trophoblast giant cells) (-FGF) Parietal endoderm like (-FGF/Activin) Germ layers In vitro differentiation Tanaka et al (1998) Science 282, p2072-5; Brons et al (2007) Nature 448, p191-5; Kunath et al (2005), Development, 132, p1649-61

  23. Interconversion of embryo stem cell types +CDX2 and/or -OCT4 +GATA6 and/or +OCT4 XEN ES TS +FGF4 +LIF +FGF4 - LIF +FGF2 +Activin +serum free medium +LIF +2i Or +KLF4 EpiSC Niwa (2007) Development 134, p635-46

  24. End lecture 2

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