1. Chapter 21. Development of Multicellular Organisms

2. Frog Development

3. Developmental process against the 2nd law of Thermodynamics ?
Developmental processes cause
-Increase of orderliness?
-Decrease of entropy?

4. The 2nd Law of thermodynamics

5. Information is -ΔG Maxwell’s demon
Information is -ΔG

6. The Genome is The Maxwell’s demon
Life is open system
Life has information
Life consumes –ΔG,
(Erwin Schrodinger, )

7. Selective gene expression control four processes by which the embryo is constructed
Cell proliferation
Cell specialization
Cell interaction
Cell movement

8. Similar basic anatomical features
Universal mechanisms of animal development
Similar gene usage
Similar basic anatomical features

9. Universal mechanisms of animal development-similar gene usage

10. Universal mechanisms of animal development-similar basic structure

12. Unicellular vs. Multicellular organisms
Tranmembrane proteins (e.g C. elegans genes over yeast)
-Ion channels
-Cell adhesion molecules
-Cell surface receptors
Gene regulatory proteins
(e.g. HLH gene family: 141 humans, 84 fly, 41 C. elegans, 7 in yeast)

13. How species can be different?
Different animals utilize similar collection of genes
Species identity genes (e. g 1% between human and chimpanchee )
non-coding, regulatory DNA sequences are highly differential between species

14. Different genome causes different behaviors of cells
Urchin
Urchin
Frog
Fly, Mouse
Frog
8.5

15. Cell fate and morphogenesis

16. Cell fate determination

17. Determination (결정) Differentiation (분화된 상태)
= formation of specialized cell types
Commitment (예정된 상태)
= biochemical changes in a cell that restrict its developmental fate

18. Arms vs. Legs: Differential Gene Expression
Two transcription factors:
Tbx5: Forelimbs
Tbx4: Hindlimbs
Expression dependant on anterior/posterior location
Tbx4 in leg bud
Tbx5 in wing bud

19. Cell fate commitment by genes

20. Tbx5
Tbx4

21. How cell fate is determined?
Autonomous
specification
Regulative
specification

22. Commitment Three modes of initiating commitment have been described.
Autonomous Specification
Conditional Specification
Syncytial Specification

23. Autonomous Specification
= cell fate is determined before fertilization my morphogenetic determinants in ovum.
Morphogenetic Determinants
= mRNA or proteins that cause cellular commitment
Mosaic Development
= embryo functions like a “mosaic” of independent self-differentiating parts.

24. 8.22
b-Catenin
positive cells
Asymmetric division

25. Mosaic Development
determinant
zygote

26. Mosaic Development
determinant
zygote
Dies or 1/2 Embryo Forms

27. Conditional Specification
Conditional Specification = cell fate is determined by the conditions surrounding the cell.
Morphogenetic determinants produced by cells within the embryo. (signaling among cells)
Regulative Development = cells of an embryo can change fate based on the conditions within the embryo.

28. Regulative Development
Normal Embryo Forms
zygote

29. Regulative Development

30. Syncytial Specification
Syncytial Specification = cell fate is determined by the conditions affecting nuclei in a single multinucleate cell.
Syncytium = a cytoplasm containing multiple nuclei.
Morphogens may form a gradient within the cytoplasm.

31. Basic mechanisms of cell fate determination
Inductive interaction
Morphogen
Extracellular inhibitor
Intrinsic program for time course
Lateral inhibition

32. Basic mechanisms of cell fate determination
Inductive interaction
Morphogen
Extracellular inhibitor
Intrinsic program for time course

33. Morphogen = Soluble molecule that causes cellular commitment but is secreted some distance from the target cells.
Morphogen Gradient = concentration gradient of a morphogen.

34. Morphogen Threshold Concentrations
embryo
Morpho-
gen
conc.
position
p. 63

35. Morphogen Threshold Concentrations
embryo
Morpho-
gen
conc.
position
p. 63

36. Morphogen Threshold Concentrations
embryo
Morpho-
gen
conc.
position
p. 63

37. Influence of Other Cells
Morphogen Receptor Gradient = frequency gradient of the receptors for a morphogen in target cell cell membranes.
Morphogen gradient
Morphogen receptor gradient

38. Activin Gradient
Activin = morphogen in frog blastula, morphogen gradient of activin commits cells as a type of mesoderm.
No activin = ectoderm
heart
cells
noto-
chord
muscle
blood

39. Frog Blastula (section)
ectoderm
mesoderm
endoderm
blastocoel
vegetal pole

40. Morphogenetic Field
Morphogenetic Field = a group of cells whose position and fate are specified with respect to the same set of boundaries.
Within a morphogenetic field fate is not yet specified.
The limb field will form a limb.
If divided the limb field will form two limbs.

41. Sonic Hedgehog

42. Ahn and Joyner (2004) Cell, 118,
Gli1-CRE + LacZ conditional expression
+/+
Gli2-/-
Gli3-/-

43. Basic mechanisms of cell differentiation
Inductive interaction
Morphogen
Extracellular inhibitor
Intrinsic program for time course

44. Basic mechanisms of cell differentiation
Inductive interaction
Morphogen
Extracellular inhibitor
Intrinsic program for time course
-Time keeping mechanisms
-Cell division associated
-Glial progenitor cells become oligodendrocytes after 8 divisions

45. Morphogenesis Commitment Cell shape changes. Cell movement.
Cell death.
Changes in cell membranes or secreted products.

46. Sequential induction makes complex patterns

47. General Cell Types
Epithelial Cells = cells connected together in sheets (attached to each other and an acellular basal lamina).
fold, elevate, expand, involute, intercalate
Mesenchymal Cells = cells unconnected together and operate independently.
ingress, migrate

48. Epithelial Fold

49. Epithelial Fold

50. Epithelial Fold

51. Cell Affinity

52. Cell Affinity

53. Cell Affinity
Selective Affinity = Disassociated cells will group together with (positive affinity) or will not group together with (negative affinity) only certain other cells.
Homotropic Aggregation = Disassociated cells of the same type group together. (positive afinity)

54. Cell Adhesion
Differential Adhesion Hypothesis = explains patterns of cell sorting based on thermodynamics of affinity between adhesion molecules.
Surface tension.
Different adhesion molecules.
Different amounts of the same type of adhesion molecules.

55. 2 different amounts of the same adhesion molecule
Cell Affinity
2 different
adhesion molecules
2 different amounts of the same adhesion molecule

56. Cell Adhesiveness Adhesion molecules = proteins in cell membrane.
Cadherins (5 classes)
calcium dependent adhesion molecules
binds to other cadherins (same type)
connected to cytoskeleton by catenins
Homophilic binding = adhesion molecules attach to the same class of adhesion molecule.

57. Cadherins
cell
membrane
Ca2+
catenins
actin (cytoskeleton)
p. 72

58. Cell Affinity
N-cadherin
E-cadherin

59. Methods for developmental biology
Descriptive embryology
Experimental embryology
Developmental genetics

60. Origins of Descriptive Embryology
Epigenesis vs. Preformationism
preformationism argued for species continuity and constancy
to some, epigenesis implied a need for a mysterious vital “life force” that was required to create life de novo
careful observations on the anatomical development of embryos eventually required acceptance of epigenetic development

61. Classical Embryology
Kaspar Wolff (1767): studies of chick embryogenesis
Where did the instructions to build the embryo come from?
Were they internal or external?
‘vital force’ [vis essentialis] needed to explain embryonic organization?

62. Classical Embryology Christian Pander (1774-1865)
studied the chick embryo and identified primary germ layers found in triploblastic embryos
ectoderm: gives rise to outer layer of embryo and nervous system
endoderm: gives rise to innermost layer and gives rise to digestive tube and associated organs
mesoderm: middle layer that gives rise to bones, connective tissues, kidney, gonads, heart and hematopoietic system
primary germ layers interact to form organs

63. Classical Embryology Karl Ernst Von Baer (1792-1896)
“enwicklungsgeshicte”: extended Pander’s observations; discovered notochord
his work on chick embryogenesis was death knell to preformationism (also discovered mammalian egg)

64. Classical Embryology Von Baer’s laws:
the general features of a large group of animals appear earlier in development than specialized features in a small group
within embryos, specialized structures develop from more generalized structures
an embryo does not “pass through” the adult stages observed in lower animals: ontogeny does not recapitulate phylogeny
early embryos share characteristics in common and become more and more divergent as development proceeds

65. Classical Embryology Wilhelm His (1831-1904)
one of the major antagonists to Haeckel
developed the microtome, allowing for serial sectioning and much better anatomical resolution and reconstruction
focused on the the mechanics of development and the importance of morphogenic movements, foldings and cellular interactions in the process of development.

66. The birth of experimental embryology
Defect = destroy part of embryo.
Isolation = remove part of embryo and observe its development in culture.
Recombination = replace part of an embryo with a part of the same embryo.
Transplantation = replace part of an embryo with a part from a different embryo.

67. Birth of Experimental Embryology
Laurent Chabry (1887) ‘Qualitative mosaic’
experiments performed by isolating specific cells in developing tunicate embryos
each blastomere was responsible for producing a particular set of larval tissues
the blastomeres were apparently developing autonomously
mosaic development: embryo constructed of individual modules capable of self-differentiation

68. Birth of Experimental Embryology
Wilhelm Roux ( ) ‘Quantitative mosaic’
student of Haeckel who performed ablation experiments in frogs
Result of fate mapping in frogs implied that the destruction of certain regions in the early blastula would preclude development of certain structures
destroyed right or left halves of frog embryos at 2 and 4 cell stages
obtained “half embryos” having a complete right or left side, arguing for a mosaic model of development

69. Birth of Experimental Embryology
Hans Driesch ( ): ‘Regulative development’
each of the blastomeres from a two cell embryo developed into a complete larvae
some of the later stage cells also developed into complete larvae
conflicts with experiments of Roux and Chabry: first example of regulative development

70. Birth of Experimental Embryology
Hans Driesch ( ): pressure plate experiment; by compressing the developing embryo between two plates, he could force a change in cleavage plane from equatorial to meridional, resulting in a different pattern of cleavage from normal. This reshuffled the position of the nuclei in the embryo…did it alter the fate map?
Embryos were normal

71. Birth of Experimental Embryology
Pressure plate experiments implied:
nuclear equivalence
cytoplasmic/nuclear interactions
Driesch left science as a result of these experiments; he could not explain these results relative to the physics of his day and came to the philosophical view that living things can not be explained solely through physical laws

72. Experimental Design Matters!
J. F. McClendon (1910) Repeated experiments in frog development using Driesch’s isolation technique relative to Roux’s ablation technique
noted regulative development NOT mosaic development
isolated frog blastomeres developed into a whole frog
ablated blastomeres were still in contact with intact blastomeres; they still were providing information for developmental programming

73. Fate Mappng
Fate maps do not necessarily imply commitment; not maps of potency or states of determination
clonal restriction does not imply determination: allocation: clonal restriction in a population regardless of state of commitment
commitment: intrinsic aspect of a cell that makes it follow a particular developmental path
‘commitment’ vs. ‘determination’?

74. The birth of developmental genetics
C. elegans, Drosophila, Frog, Mice, Plant as model systems