1. PRINCIPLES OF EMBRYONIC DEVELOPMENT © 2012 Pearson Education, Inc.

2. Developmental Stages in an Amphibian Where do tissues/organ derive during embryonic development? What regulates cell differentiation and organ formation? How can new body plans arise? "It is not birth, marriage, or death, but gastrulation, which is truly the most important time in your life.” Lewis Wolpert (1986)

3. Most animals proceed through these stages during development: 1.Zygote 2.Early cleavage stages Blastula Establish polarity and body axes 3.Gastrulation Establish germ layers 4.Body plan (segmentation) In vertebrates, this involves neurulation 5.Morphogenesis (organogenesis)

4. LE 21-4 Animal development Zygote (fertilized egg) Eight cellsBlastula (cross section) Gastrula (cross section) Adult animal (sea star) Cell movement Gut Cell division Morphogenesis Observable cell differentiation Seed leaves Shoot apical meristem Root apical meristem Plant Embryo inside seed Two cells Zygote (fertilized egg) Plant development

5. Fertilization results in a zygote and triggers embryonic development  Embryonic development begins with fertilization, –the union of sperm and egg, forming diploid zygote  Sperm contributes 1/2 genetic material (23 chromosomes in humans)  Egg contributes: –Cytoplasm and all organelles –Nourishment –mRNAs for first proteins synthesized –Regulatory factors that guide early development (cytoplasmic determinants) © 2012 Pearson Education, Inc.

6. Cleavage produces a multicellular embryo and begins to organize body axes  Cleavage is a rapid series of cell divisions that produces –more cells, –smaller cells, and –a fluid-filled embryo called a blastula. © 2012 Pearson Education, Inc. Video: Sea Urchin Embryonic Development

7. LE 47-7 Fertilized egg Four-cell stage Morula Blastula

8. Starfish development, unfertilized egg. 2 blastomeres. Starfish development, nonmotile blastula. 4 blastomeres. 16 blastomeres.32 blastomeres. morula

9. Cleavage produces a multicellular embryo and begins to organize body axes  Cleavage is a rapid series of cell divisions that produces –more cells, –smaller cells, and –a fluid-filled embryo called a blastula. © 2012 Pearson Education, Inc. Video: Sea Urchin Embryonic Development

10. Sperm Molecules of a cytoplasmic determinant Fertilization Nucleus Molecules of another cytoplasmic determinant Unfertilized egg cell Zygote (fertilized egg) Mitotic cell division Two-celled embryo Cleavage Defines Body Axes Cytoplasmic determinants become asymmetrically distributed. Most of these cytoplasmic determinants are maternal factors (mRNA or proteins) inherited from egg cytoplasm Maternal factors generally code for or are transcription factors that regulate gene expression These cells now have different fates.

11. Bicoid Protein Provides Positional Information Bicoid is an example of cytoplasmic determinant (maternal factor) Animation: Development of Head-Tail Axis in Fruit Flies

12. LE 21-14a Head Tail Wild-type larva Mutant larva (bicoid) Drosophila larvae with wild-type and bicoid mutant phenotypes

13. Gastrulation produces 3 primary tissue (germ) layers in embryo  During gastrulation –cells migrate to new locations, –a rudimentary digestive cavity forms, and –the basic body plan of three layers is established with –ectoderm outside—becomes skin and nervous systems, –endoderm inside—becomes digestive tract, –mesoderm in the middle—becomes muscle and bone. © 2012 Pearson Education, Inc.

14. Most animals proceed through these stages during development: 1.Zygote 2.Early cleavage stages Blastula Establish polarity and body axes 3.Gastrulation Establish germ layers 4.Body plan (segmentation) In vertebrates, this involves neurulation 5.Morphogenesis (organogenesis)

15. Figure 27.11_s3 Blastula (end of cleavage) Animal pole Blastocoel Vegetal pole Gastrulation (cell migration) Formation of a simple digestive cavity Blastopore Blastocoel shrinking Gastrula (end of gastrulation) Simple digestive cavity Endoderm Mesoderm Ectoderm

16. Starfish development, gastrula during invagination. Starfish development, mid-gastrula. LM X75. Starfish, late bipinnaria.Starfish, young adult.

17. Table 27.11

18. 27.12 Organs start to form after gastrulation  Organs develop from the three embryonic layers.  Neurulation is formation of nervous system –notochord forms from mesoderm –later replaced by the vertebral column in most chordates. –The neural tube develops above the notochord from ectoderm –Will become brain and spinal cord.  The nervous system is the first organ system to develop  Once the nervous system is in place, basic body pattern and dorsal/ventral axis established. © 2012 Pearson Education, Inc. Video: Frog Embryo Development

19. Neural folds Endoderm Mesoderm Ectoderm Notochord Neural fold Neural plate Neural fold Neural plate Neural tube Outer layer of ectoderm

20. Neural tube Notochord Coelom Digestive cavity Somite SomitesTail bud Eye Somites are derived from mesoderm and form vertebrae By end of neurulation, body has been segmented and basic body plan established

21. Body Pattern Formtion  Pattern formation, –Shaping of animals major body structures in proper positions within each body segment © 2012 Pearson Education, Inc. Fruit fly embryo (10 hours) Adult fruit fly

22. Homeotic Genes Control Pattern Formation  Homeotic genes (Hox genes) –Code for protein transcription factors and regulate timing and expression of body pattern genes –contain common nucleotide/amino acid sequence (homeoboxes), that allows Hox protein to bind DNA –occur in diverse groups such as –prokaryotes, –yeast, –plants, and –animals. –Homeotic genes reveal the shared evolutionary history of life. © 2012 Pearson Education, Inc.

23. Hox Genes in Drosophila Determine Segment Identity

24. Hox Genes Control Body Segmentation

25. Figure 27.14B Fly chromosomeMouse chromosomes Fruit fly embryo (10 hours) Mouse embryo (12 days) Adult mouse Adult fruit fly

26. Multiple processes guide development  Induction –Signals from neighboring cells direct gene expression and development  Cell migrations around developing embryo  Apoptosis (cell death)  Regulated gene expression –Gene expression cascades –Homeotic Genes © 2012 Pearson Education, Inc.

27. Figure 27.13A

28. Cell signaling and cascades of gene expression direct animal development 1.Head-to-tail, top-to-bottom, and side-to-side axes are determined by proteins and mRNA in the egg from mother. 2.Cleavage results in asymmetrical distribution of these proteins within embryo - regulating expression of genes specific to the cell’s fate. 3.Leads to a cascade of gene expression - one protein acts as transcription factor for another, inducing pattern formation (anatomy of body parts and development of segments) Example: hox genes © 2012 Pearson Education, Inc.

29. Follicle cells “Head” mRNA Embryo Adult fly Expression of homeotic genes and cascades of gene expression Body segments Cascades of gene expression Gene expression Growth of egg cell Localization of “head” mRNA Egg cell Egg cell and follicle cells signaling each other 2 1 3 4