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What are the germ layers?
ectoderm, endoderm, mesoderm
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Ectoderm
outer: skin and nervous system
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Endoderm
digestive and respiratory system
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Mesoderm
Middle: Circulatory system, blood, bones, gonads, osmoregulatory and urinary system
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What instructs the neural ectoderm to fold and become the neural tube?
The underlying mesoderm
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What is the neural crest?
When the neural tube pinches off, some cells at the junction b/t the neural and epidermal ectoderms migrate to different regions of the embryo
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What is another name for Organogenesis and what does it entail?
Organogenesis or Morphogenesis
- FUNCTION: further cell differentiation,
- arrange cells into tissues and organs,
- limb development
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What are the stages of early development?
- 1. Fertilization
- 2.Cleavage
- 3.Gasturlation
- 4.Neurulation
- 5. Organogensis
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What is Differentiation?
- Differentiation is the generation of a specialized cell type from an undifferentiated precursor....
- There are 2 steps:
- 1. commitment
- 2. differentiation
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What are the steps of commitment?
- 1. Specification (reversible and occurs 1st)
- a. conditional
- b. Autonomous
- c. Syncytial
- 2. Determination (irreversible)
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Conditional Specification definition
- -reversible: a cell is specified to a cell type only because of its position in the embryo
- - this type of development is regulative: the cells can aquire different identities depending on the regulation by their neighboring cells
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Conditional specification examples:
- 1. Amphibian blastula: if you transplant cells from a dorsal region to a ventral region the cells will re-specify, --- if you completely remove cells normal development will occur because other cells can be re-specified to replace the old ones.
- 2. Sea Urchin: isolate cells in the 2-4 cell stage and they will develop into complete organisms.
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Autonomous Specification definition
- -irreversible: cell specification depends on the cytoplasmic components present in the egg (which is not uniform)
- - the cyto make up of the balstomeres define its specification
- - this type of development is determinative (mosaic)--- there will be NO change in cell fate even if isolated
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Autonomous specification examples:
- Patella (mollusk): in the 16 cell stage the balstomeres are auto specified. -- If a balsomere that corr to a trochoblast is removed and placed into a petri dish:
- - that cell will develop into a trochoblast and the
- larva will lack the structure (none of the
- remaining cells can change their fate)
- Tunicate: 8 cell stage blastomere---IF cells are separated from embryo only fragments will form NOT complete embryos
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Syncytial Specification definition
- -Occurs in insects
- -multiple rounds of mitosis w/out cytokinesis (first 12 rounds in Drosophila) results in a cell with many nucli (a syncytium)
- - after 13 rounds the nuclei migrate to the periphery and become separated by membranes (cellularization)
- - the other specification is determined by gradients of morphogens of maternal origin present prior to cellularization
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Syncytial specification examples:
- Egg of drosophila: BICOID
- -anterior morphogen
- - maternal gene product
- (which is necessary and sufficient for inducing the formation of anteior sturctures).
- - position dependent
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What occurs during cleavage?
- -rapid series of mitotic cell divisions
- - NO embryonic gene expression and NO cell growth
- (no G1 or G2)
- -size of embryo is unchnaged but the size of the cells are getting smaller and smaller
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Where is cell division the slowest? Fastest?
- Cell division is the SLOWEST at the vegetal hemi and FASTEST at the animal hemi
- (b/c the yolk is in the vegetal and it is more dense to divide through)
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What is a morula?
solid cluster of cells--- after a few cell divisions
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What is a blastula?
- A later stage in the where the embryo b/c a hollow sphere of cells
- - the cavity is called a blastocoel: fluid filled cavity
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