lect 22 animal development (1)
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BIO1022
ANIMAL DEVELOPMENT II
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Organogenesis
Morphological changes in the germ layers to give riseto organ structures
First structures in vertebrates are neural tube andnotochord
Notochord is formed from dorsal mesoderm
> Notochord elongates and stretches embryo alonganterior-posterior axis
Dorsal ectoderm above notochord becomes neural
plate Neural plate folds inwards to become neural tube
> Will become brain and spinal chord
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Organogenesis
Strips of mesoderm lateral to notochord form blockscalled somites
These later give rise to vertebrate and musclesassociated with axial skeleton
Lateral to the somites the mesoderm splits into twoparts
Form the lining of the body cavity
Morphogenesis and cellular differentiation continue to
refine the organs
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Phylotypic Stage
After neural induction, vertebrate embryosresemble each other
Further differentiation of organs will determine
the final structure of the animal
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Extraembryonic Tissue
Reptiles, birds, and mammals have extraembryonictissue
Derived from zygote but not part of embryo
These support embryonic development
Consist of amnion, chorion, allantois and yolk sac
> Yolk sac does not contain yolk in mammals
Extraembryonic membranes of shelled eggs wereconserved as mammals diverged during evolution
> Modifications adapted to development within reproductive
tract of mother
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Bird
Mammal
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Developmental Fate
Development requires spatial and temporaldifferentiation of specific cell types
Genetic and cellular mechanisms controldifferentiation
Cytoplasmic determinants present in unfertilized eggsof many animals
Heterogeneous distribution of determinants
> Help specify body axis and influence gene expression
Regional differences in early embryos
Mammals are an exception
> Do not rely on cytoplasmic determinants
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Inductive Signals
Interactions among embryonic cells inducechanges in gene expression
Diffusible chemical signals
Cell-cell interactions
Most animals use cytoplasmic determinants(mosaic development) and induction
Mammals use inductive only
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Potential of Cells
Totipotency - ability of a cell to form all parts of themature organism
In some animals only zygote is totipotent
Up to 8-cell stage, all blastomeres of a mammalian
embryo are totipotent
Can split into 8 individual cells and get 8 offspring
> Can remove one cell and it not affect development
Can continue two 8-cell embryos to form a chimera
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Progressive restriction of potency is a feature ofdevelopment
Cells from the inner cell mass of mammals arepluripotent
Can give rise to any cell type within the embryo properbut cannot give rise to extraembryonic tissue
Stem cells are progenitors for different cell types andcan have different levels of potency
eg. embryonic stem cells have extensive potencywhereas hemopoietic stem cells are restricted to cells ofthe blood lineage
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Differentiation and Pattern Formation
Role of cytoplasmic determinantsdemonstrated in amphibian development
Blastomeres are separated at 2-cell stage
If grey crescent equally divided between thetwo blastomeres --> 2 normal embryos
if grey crescent entirely in one blastomerethen only that blastomere develops normally
> Other embryo lacks dorsal structures Grey crescent region required for normal
development
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Spemann Organizer
Cell division creates cells with different developmentpotential
Cells can influence other cells (induction)
Induction shown in studies by Hans Spemann andHilde Mangold
Using transplantation they discovered the dorsallip of blastopore plays a key role in embryonicdevelopment
Initials a chain of inductions
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Piece of dorsal lip form one embryo was grafted to theventral side of another embryo
Resulted in development of second notochord andneural tube at location of graft
Subsequently other organs and structures formed Created a nearly complete second embryo
Dorsal lip of blastopore is primary organizer region
Similar regions found in birds and mammals
Molecular basis of induction is still being studied
Other organizer regions exists at earlier and laterstages (eg organ specific organizers)
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