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What are the germ layers?
ectoderm, endoderm, mesoderm
outer: skin and nervous system
digestive and respiratory system
Middle: Circulatory system, blood, bones, gonads, osmoregulatory and urinary system
What instructs the neural ectoderm to fold and become the neural tube?
The underlying mesoderm
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
What is another name for Organogenesis and what does it entail?
- FUNCTION: further cell differentiation,
- arrange cells into tissues and organs,
- limb development
What are the stages of early development?
- 1. Fertilization
- 5. Organogensis
What is Differentiation?
- Differentiation is the generation of a specialized cell type from an undifferentiated precursor....
- There are 2 steps:
- 1. commitment
- 2. differentiation
What are the steps of commitment?
- 1. Specification (reversible and occurs 1st)
- a. conditional
- b. Autonomous
- c. Syncytial
- 2. Determination (irreversible)
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
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.
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
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
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
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
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
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)
What is a morula?
solid cluster of cells--- after a few cell divisions
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