A Golgi study on the early sequence of differentiation of ganglion cells in the chick embryo retina

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Abstract
An examination of retinal structure in chick embryos, impregnated with the Golgi-Stensaas method between 2 and 6 days of incubation, discloses, on the one hand, a uniform typology of the proliferating ventricular cells, the pre- and postmitotic forms of which were tentatively identified; on the other hand, postmitotic neuroblasts are evidenced in the stages of differentiation previous to the growth of their neurites. In the earliest embryos (up to 51/2 days of incubation), all cells that detach from the ventricular lining to differentiate as neurons do so while the ventricular cell precursor has an interphasic configuration. This means that, although they free themselves from the scleral attachment site, they keep for a while a vitreal attachment. The vitreally-attached endfeet subsequently transform into axonal growth cones, sprouting filopodia and lamellipodia. While the axons grow towards the optic nerve head, cell bodies and remaining scleral processes are progressively retracted inwards, leading to the appearance of typical ganglion cells. After 51/2 days of incubation, a great number of postmitotic neuroblasts detach while still in the G1 phase of the ventricular cell cycle. Those of them that show the longest leading processes become also ganglion cells, after their leading tip has acquired a growth cone configuaration and has bent into the optic fiber layer. These results on early mechanisms of ganglion cell genesis are discussed in relation to data in the literature, and a simple hypothesis is offered which explains the biphasic pattern in which presumptive ganglion cells detach from the ventricular lining of the chick retina.