Changes

===Anterior - Posterior Patterning===

===Anterior - Posterior Patterning===

*Important in producing the correct number of digits of the correct morphology.

Anterior - posterior positioning is important in producing the correct number of digits 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)that 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 and posterior digits have more cells than anterior digits. The more cells, the more morphogen produced. The more morphogen, the more posterior the digits. '''Polydactly''' is common in animals, and is due to defects in the ZPA.

*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 defects in the ZPA.

===Dorsal - Ventral Patterning===

===Dorsal - Ventral Patterning===