In high Tc materials electrons on the whole are only weakly coupled to the phonons except for a few specific local modes. The latter gives rise to the formation of polaronic charge carriers on specific molecular centers in those materials. There is considerable experimental evidence for their existence in form of localized states and we conjecture that localized bi-polaronic states lie inside the valence band of itinerant electrons. With increased doping this valence band is gradually being emptied out, eventually resulting in a situation where the chemical potential coincides with the level of the bi-polaronic state. At this point an insulator ~metal transition is triggered off and due to hybridization between the localized bi-polarons and the itinerant electrons the initially localized bi-polaronic states acquire itinerancy upon lowering the temperature ; eventually undergoing a transition to a superfluid state at a critical temperature To A characteristic feature of this scenario of an hybridized Boson-Fermion mixture is the pinning of the chemical potential at the Bosonic (bi-polaronic) level which only weakly depends on doping in the fully developed metallic regime. Tc is then essentially controlled by the number of Bosons which does depend on doping and leads to the known universal behaviour of Tc as a function of Boson concentration and mass.