Photoluminescence (PL) image size and angular resolved spectra were studied in a GaAs/AlAs planar microcavity (MC), with an embedded GaAs quantum well (QW) that contains a photogenerated, variable density, two-dimensional electron gas (2DEG). The PL was measured at T = 7 K, under nonresonant, cw excitation. The in-plane polariton migration from the photoexcitation spot (whose radius is ∼30 μm) is deduced from the PL image size. It is observed to be anisotropic in the MC plane, and for negative detuning energy values (δ = EC - EX) it extends over in-plane distances of ∼300 μm. Introducing a 2DEG into the QW while maintaining the strong coupling between the excitons and the MC photon mode, reduces the migration range. On the other hand, the angular resolved PL spectra show that the emission from the k∥ ∼ 0 polariton states is enhanced as the 2DEG density is increased. From these observations we propose that the polaritons migrate while they occupy the long-lived states above the relaxation bottleneck in the lower polariton (LP) branch. The efficient electron-polariton scattering processes transfer the polaritons through the bottleneck into the radiative trap (k ∥ ∼ 0) in the LP branch. These scattering processes occur at the point of photoexcitation in the MC plane, and therefore they decrease the migration range.