Formation of alternating tiers in the optic chiasm of the chick embryo

When the fibers of the two optic nerves of the chick cross to the contralateral side at the prospective chiasmatic region, they segregate into clearly defined bundles. These bundles form horizontally oriented tiers which alternate between the right and the left optic nerve.

We have analyzed the development of these tiers qualitatively and quantitatively using light and electron microscopy between embryonic days (E) 4 and E19.

The formation of the chiasm begins on E4. In the course of E4, tiers become visible for the first time. Their number increases rapidly until E7. Then the increase is slowed down and the final value (32 +/- 1) is approximated by E18/19. Growing axons allow one to distinguish three different segments: the growth cone, the distal, and the proximal segment. The latter originates in the perikaryon. Growth cones and distal segments are found predominantly in the ventralmost tiers. Their frequency decreases from ventral to dorsal. Proximal segments which indicate the presence of older axons appear first in the dorsal tiers and later also in more ventrally located tiers.

Based on these criteria it is concluded that newly formed axons contribute primarily but not exclusively to the ventral tiers. There is a gradient of maturity of axons from ventral to dorsal whose slope becomes steeper with age until the last growth cones have arrived by E18. Thus, the formation of the chiasm corresponds to the spatiotemporal pattern of ganglion cell formation in the retina. The process of cell death of retinal ganglion cells is also seen in the chiasm but probably does not lead to a transitory diminution in the number of tiers.