Optic chiasm and infundibular decussation sites in the developing rat diencephalon are defined by glial raphes expressing p35 (lipocortin 1, annexin I).

p35, a Ca(++)-phospholipid-binding protein that serves as a substrate for the EGF receptor tyrosine kinase, is expressed by primitive glial ependymal cells to define a raphe occupying the ventral midline in the spinal cord and hindbrain of rat embryos (McKanna and Cohen, Science 243:1477-1479, 1989). p35 appears transiently in the median one-third (80 microns) of the floor plate at precisely the time and place where axons cross to form the ventral commissure. We postulated that if p35 is involved with commissure development, homologous p35 raphes might be found at decussation sites rostral to the floor plate, including the optic chiasm. The present report describes two developmentally regulated p35 raphes in the diencephalon. One raphe is present for 2-3 days at the rostral lip of the nascent infundibulum, the reported decussation site of axons running from the supraoptic nucleus to the neurohypophysis; the other raphe appears in the rostral two-thirds of the optic chiasm, the site traversed by the optic axons. p35 is never expressed in the caudal one-third of the chiasm that accommodates non-retinal axons. To the best of our knowledge, this is the first identification of a specific marker for the retinal component of the optic chiasm. Because the p35 is gone by embryonic day 18.5, it is absent during final stages of chiasm formation when axons from the temporal retina decussate. Thus, p35 also may contribute to the "barrier" perceived by fibers that remain ipsilateral. Our data suggest that the p35 raphe contributes to the midline's role in commisure morphogenesis. Putative lipocortin activities including regulating PLA2, eicosanoids, or intracellular Ca++ could be involved in altering cue specificity as decussating axon growth cones traverse the p35 compartment.