25.8
Determination
During embryogenesis, cells become progressively committed to different fates through a two-step process: specification followed by determination. Spe…
In human embryos, immature cells that can form any tissue become progressively committed to a certain mature cell type through a two-step process, specification and determination.
The first step of specification results from a cell's location in the embryo, like where it lies on the dorsal, ventral axis and signals produced by it's neighbors. These positional elements expose a cell to a unique combination of factors that started down a developmental pathway such as neural formation.
If such specified cells were to be removed from the embryo and introduced to a neutral environment, like a dish containing basic growth medium, they would still go on to form their committed cell type, here neurons.
However, if proteins or other factors are added, cell fate can be altered so structures specified to form neurons could be redirected to yield another tissue like skin.
The continued action of positional factors within an embryo cause specified cells to enter the second stage, determination. Contrastingly, if determined cells are removed and inserted into a non-neutral setting, like a new embryonic area with different signals, they will still form neurons. Cell fate isn't changed.
Thus cells are gradually driven to a certain tissue type through the sequential action of specification and determination before they finally differentiate to form mature cells like neurons with distinct features.
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Q1: What is the difference between specification and determination in embryonic development?
Specification and determination are two sequential steps that commit cells to specific fates. Specification occurs when a cell's location and neighboring signals expose it to factors that start a developmental pathway, like neural formation. Determination follows, where cells become irreversibly committed so that even in different environments, they maintain their fate and form their intended tissue type.
Q2: How do researchers demonstrate that a cell is specified?
Researchers demonstrate specification by removing an embryonic region and culturing it in a neutral environment with basic growth medium. If the cultured tissue generates the cell types it would normally produce in the embryo, the region is specified. This in vitro approach shows that positional information and neighbor signals have already committed cells to their developmental pathway.
Q3: What happens when determined cells are placed in a different embryonic environment?
Determined cells maintain their fate regardless of their new environment. If removed from the embryo and placed in a non-neutral setting with different signals or a different embryonic area, determined cells still form their committed tissue type. This demonstrates that cell fate has become irreversibly fixed through the sequential action of specification and determination.
Q4: What role do positional factors play in cell fate commitment?
Positional factors, including a cell's location on the dorsal-ventral axis and signals from neighboring cells, expose immature cells to unique combinations of developmental signals. These factors initiate specification by launching cells down specific developmental pathways. The continued action of positional factors within the embryo then drives specified cells into determination, progressively committing them to their mature cell type.
Q5: How do bone morphogenetic proteins and fibroblast growth factors influence neural crest specification?
Bone morphogenetic proteins and fibroblast growth factors are signaling proteins that emanate from tissues adjacent to or underlying prospective neural crest cells. These signals induce neural crest cell fate by triggering the expression of specifier proteins within target cells. These specifier proteins then launch cells into the neural crest developmental pathway, demonstrating how molecular signals translate positional information into cell fate commitment.
Q6: Can cell fate be altered after specification occurs?
Yes, cell fate can be altered during the specification stage if proteins or other factors are added to specified cells in a neutral environment. For example, cells specified to form neurons could be redirected to yield skin tissue if exposed to different factors. However, once cells enter determination, their fate becomes fixed and cannot be changed by environmental signals.
Q7: What experimental techniques help scientists understand normal cell fates in different embryonic regions?
Fate mapping is a key technique where researchers dye or label cells early in embryogenesis, then culture whole embryos to track where marked cells end up. This reveals which tissues different embryonic regions normally generate. Such techniques in chicken embryos have shown that distinct off-center regions give rise to neural crest cells, providing baseline data for studying specification and determination mechanisms during cleavage and blastulation.