The development of the chick optic tectum. II. Autoradiographic studies

Abstract

The time of origin and the pattern of migration of the cells in each of the principal layers of the chick optic tectum have been studied autoradiographically using a variant of the ‘cumulative labeling’ technique. The earliest unlabeled cells ( i.e., the first cells to be formed) are the large, multipolar neurons of the stratum griseum centrale; these appear in the rostroventral portion of the tectum between the third and forth days of incubation. The last cells to arise on the ninth day form the deepest laminae of the stratum griseum et fibrosum superficiale in the caudo-dorso-medial pole of the tectum. Viewed as a whole, the layers of the tectum are formed as the result of 3 spatially distinct, but chronologically overlapping, phases of cell proliferation and migration. The first of these occurs between days 4 and 6 and gives rise to the cells in the 4 deepest strata (the stratum griseum centrale, stratum album centrale, stratum griseum periventriculare and stratum fibrosum periventriculare). The second phase results in the formation of the outer laminae of the stratum griseum et fibrosum superficiale in which the retinal afferents terminate; this phase extends from the fourth to the eighth days of incubation. In the final phase (from days 5 to 9) the deeper laminae (h-j) of the stratum griseum et fibrosum superficiale are formed. In all 3 developmental zones there is a general rostro-ventro-lateral to caudo-dorso-medial gradient, and, in addition, the cells in each zone arise in a characteristics sequence. In the inner and intermediate zones the more superficial cells are formed before the deeper ( i.e., they follow an ‘outside-in’ gradient); in the outer zone this sequence is reversed with the cells in the deeper laminae being formed between one and two days earlier than those in the outer layers (an ‘inside-out’ gradient). The cells in the 3 developmental zones migrate at apparent rates of about 1.5–1.8 μm/h, 3.5–5 μm/h, and 3.5–4μm/h, respectively.