The Journal of Neuroscience, May 15, 1999, 19(10):3900-3917
Genesis, Neurotrophin Responsiveness, and Apoptosis of a
Pronounced Direct Connection between the Two Eyes of the Chick Embryo:
A Natural Error or a Meaningful Developmental Event?
Solon
Thanos
Department of Experimental Ophthalmology, School of Medicine,
University of Münster, D-48149 Münster, Germany
Unilateral intraocular injections of either of two fluorescent
carbocyanine dyes into the embryonic chick eye resulted in both
retrograde staining of ganglion cells (GCs) in the eye contralateral to
site of injection and anterograde labeling of axons whose cell bodies
were located within the injected eye. This prominent retino-retinal projection formed by thousands of GCs having a nasal origin and temporal termination appeared at embryonic day 6 (E6), attained its
maximum intensity at E13-E14, and gradually disappeared until E18. The
axonal growth cones ended superficially and never penetrated deeper
layers of the retina. Treatment of the projection with BDNF resulted in
massive terminal branching of the axons within deeper layers of the
target retina. Double injection into the eye and the isthmo-optic
nucleus showed a concomitant ingrowth of axons in the contralateral
retina. Individual GCs died between E9 and E13, but massive apoptotic
cell death was mainly monitored at E14 and later. Disintegrated cells
showed typical images of apoptosis. Because degenerating cells were
prelabeled with the membranophilic fluorescent carbocyanine dye, their
death allowed the concomitant visualization of phagocytosing cells,
too. Radial Müller glia were the only class of cells observed to
become phagocytotic between E9 and E16. These cells became replaced
exclusively with microglial cells from E17 on. The results suggest that
the topologically restricted retino-retinal projection may have some
developmental significance rather than representing a massive erroneous
projection. Most likely, the projection may serve as a "template"
to guide centrifugal isthmo-optic axons into the retina.
Key words:
retinal GCs; retino-retinal projection; chick
development; programmed cell death; fluorescent dyes; vision
Copyright © 1999 Society for Neuroscience 0270-6474/99/19103900-18$05.00/0