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| ===Proximal - Distal Patterning=== | | ===Proximal - Distal Patterning=== |
| + | *Governed by the PZ. |
| + | *The first cells to leave the PZ have a proximal identity. |
| + | *These cells have been in the PZ for a shorter amount of time than those that leave the PZ last, which have a distal identity. |
| + | *"Identity" is achieved by the HOX - A gene expression. |
| + | **Cells that leave the PZ early have fewer HOX - A genes expressed than those which leave later. |
| + | **Less HOX - A gene expression results in a more proximal cell appearance. |
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| ===Anterior - Posterior Patterning=== | | ===Anterior - Posterior Patterning=== |
| + | *Important in producing the correct number of digits that are of the correct morphology. |
| + | *An area of mesenchyme exists adjacent to the AER, in the posterior of the distal limb called the '''zone of polarising activity (ZPA)'''. |
| + | *The ZPA produces a '''morphogen'''(substance that induces other cells to change). |
| + | **This morphogen works in a ''dose - dependant'' manner. |
| + | **The morphogen diffuses to form a concentration gradient and has different effects on cells depending on it's concentration. |
| + | **Higher concentrations of this morphogen produces posterior digits. |
| + | ***Posterior digits have more cells than anterior cells. |
| + | ***The more cells, the more morphogen produced. |
| + | ***The more morphogen, the more posterior the digits. |
| + | *Polydactly is common in animals, and is due to |
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| ===Dorsal - Ventral Patterning=== | | ===Dorsal - Ventral Patterning=== |