Development Flashcards
lectures 11 +12
What is a cleavage?
Series of mitotic divisions that (usually) divides egg cytoplasm into many smaller nucleated cells - blastula
Changes in the cell of a cleavage.
total volume remains the same, but each individual cell gets smaller.
Very fast.
The start of embryonic development.
What are the patterns of cleavageing determined by?
- amount and distribution of yolk protein
- factors in egg cytoplasm that influence the angle and timing of formation of the mitotic spindle - which determines the formation of the cleavage
- Phylogeny - animals of different phyla from cleavage in different ways.
What is the difference between a complete and incomplere cleavage?
complete = yolk sparish and evenly distributed. can be spiral or radial.
incomplete = yolk is dense and in the centre of the egg. can be superficial.
how does a spiral cleavage work?
Polycheates and snails
oblique cleavage
daughter cells don’t sit on top of each other.
few divisions to form a solid stereoblastula - solid ball of cells.
how does a radial cleavage work?
different angles of division - meridional and equatorial.
daughter cells sit directly on top of , or next to, each other.
More divisions to form 120-cell hollow blastula.
How does superficial cleavage work?
dense yolk fills most of the cell, thin cytoplasm layer under cell membrane.
Nucleas is at the centre of the cell. - divides repeatedly, but yolk prvents the cell from dividing completely = syncytium.
Nuclei migrates to cytoplasm membrane folds inwards around each nucleas - forms many cells.
What is c. gastrulation?
Blastula - position of cells is determined during cleavage
gastrulation - rearrangemnet of cells, multi-layered body plan can develop.
How do cells know what they will be? D. cell specification
- distribution of patterning molecules - morphogenic determinants. Transcription factors, activate/surpress transcription of specific genes within cells. Factors / characterisitcs inherited by cells as they divide.
*cell-cell signaling
morphogens -biochemical molecules which diffuse from cell-cell. Dtermine cell fate by [] - lowest furthest from source cell. > fate of recipient cell depends on distance from source cell.
What is autonomous specification?
cell “knows” what it will be - determinative development
blastomere inherits morphogenic determinants from the egg cytoplasm - not homogenous.
Like a snail.
What is conditional specification?
each blastomere regulates its development, but also influenced by interaction with neighbouring cells - indeterminate development.
Isolated blastomeres can develop into complete larvae.
Like echinoderms.
What is syncytial specification?
Insects
Uses elements of autonomous and conditional specification - nuclei and transcription factors interact in common cytoplasm. Each nuclei inherit TF by virtue of position and TF that act as morphogens from different parts of embryo.
Early development of insects: drosophila fate map
all ‘primordia’ on surface of embryo.
* Larval development = conversion of a gastrula
(with 2 or 3 layers of cells) into a developmental stage that looks nothing like the adult
* Metamorphosis = Developmental process that converts larva with a particular morphology into a juvenile with a different morphology
* Metamorphic competence reached when larva has developmental capacity to undergo metamorphosis
– Triggered by internal or external factors
the cycle of early life.
larval devlopment, including organogenesis, larval development.
Larval development and metamorphasis in marine invertebrates?
- small larvae
- induced internally
- juvenile structure before larval structures.
pelagic vs benthic lifestyles - ciliary band for swimming, conflict between larval and jubvenile needs.
rapid - loss of larvea specific structures while functionality of most of rest of body is maintained and performed juvenille characters become functional.
