Part II Flashcards
The generation of specialized cell types is called
Differentiation
Last, overt stage in a series of events that commit a particular blastomere to become a particular cell type.
Changes in cellular biochemistry and function (differentiation) are preceded by a process resulting in the commitment of the cell to a certain fate.
Process of commitment has two stages:
- Specification – capable of differentiating autonomously when placed in a neutral environment with respsect to the developmental pathway. Reversible.
- Determination – capable of differentiating autonomously even when placed into another region of the embryo (non-neutral environment). Assumed irreversible.
Three major strategies of specification:
Embryos of different species use different combinations of these strategies. All three are based on the apportioning of certain sets of TFs to different cells.
- Autonomous
- Conditional
- Syncytial
Always either autonomous or conditional?
Often some cells inherits a set of TFs from the egg cytoplasm causing them to autonomously start developing, but the TFs also activate genes for paracrine and juxtacrine factors that are then secreted and conditionally specify the cells around them.
A morphogen is
a diffusible biochemical molecule that can determine the fate of a cell by its concentration, fx TFs (within cell) or paracrine factors (affecting other cells).
The range of a paracrine factor (and thus the shape of its morphogen gradient) depends on
several aspects of that factor’s synthesis, transport and degradation.
Cell surface molecules in extracellular matrix can stabilize and aid its diffusion or enhance degradation and retard diffusion; affecting the gradient.
Conditional or autonomous specification?
Conditional
Conditional or autonomous specification?
Autonomous
Conditional specification of cell fate is determined by?
Neighboring cells, but also by morphogens, i. e. diffusable
biochemical molecules that can determine the fate of a cell by their concentration.
Autonomous specification details:
The blastomere inherits a set of transcription factors from the egg cytoplasm. These regulate transcription, directing the cell into a particular path of development.
Egg cytoplasm is non-homogenous => different parts contain different morphogenetic determinants (TFs or their mRNAs). Each cell receives its instructions independently of any interaction with other cells.
Conditional specification details
Cells achieve their respective fates by interacting with other cells, largely specified by paracrine factors secreted by its neighbours and morphogen gradients.
Syncytial specification
Uses elements of both conditional and autonomous.
Early insect embryos use this.
Nuclei divide within the egg, but the cytoplasm does not => many nuclei within one common cytoplasm (syncytium).
Syncytium isn’t uniform. Nuclei in different parts are exposed to different TFs. When the nuclei are eventually incorporated into cells, these will (already) have their general fate specified (specific fate by autonomous or conditional).
