Title: Mechanism of light energy transport in the avian retina,
Abstract: We studied intermediate filaments (IFs) in the retina of the Pied flycatcher
(Ficedula hypoleuca) in the foveolar zone. Single IFs span Müller cells (MC)
lengthwise; cylindrical bundles of IFs (IFBs) appear inside the cone inner
segment (CIS) at the outer limiting membrane (OLM) level. IFBs adjoin the cone
cytoplasmatic membrane, following lengthwise regularly spaced, forming a
skeleton of the CIS, located above the OLM. IFBs follow along the cone outer
segment (COS), with single IFs separating from the IFB, touching and entering
in-between the light-sensitive disks of the cone membrane. We propose a
mechanism of exciton transfer from the inner retinal surface to the visual
pigments in the photoreceptor cells. This includes excitation transfer in
donor-acceptor systems, from the IF donors to the rhodopsin acceptors, with
theoretic efficiency over 80%. This explains high image contrast in fovea and
foveola in daylight, while the classical mechanism that describes Müller
cells as optical lightguides operates in night vision, with loss of resolution
traded for sensitivity. Our theory receives strong confirmation in morphology
and function of the cones and pigment cells. In daylight the lateral surface of
the photosensor disks is blocked from the (scattered or oblique) light by the
pigment cells. Thus the light energy can only get to the cone via intermediate
filaments that absorb photons in the Müller cell endfeet and conduct excitons
to the cone. Thus, the disks are consumed at their lateral surfaces, moving to
the apex of the cone, with new disks produced below. An alternative hypothesis
of direct light passing through the cone with its organelles and hitting the
lowest disk contradicts morphological evidence, as thus all of the other disks
would have no useful function in daylight vision.