Cytodifferentiation of quail tectal primordium transplanted homotopically into the chick embryo

Abstract

The development of the retinotectal system in the quail embryo starts earlier and evolves faster than in the chick embryo. In order to establish whether the mesencephalic alar plate (i.e., the primordium of the optic tectum) of a quail embryo maintains its own rate of cytodifferentiation after transplantation into a chick embryo or whether this rate could be influenced by the host, we performed homotopic transplantations of the tectal primordium between the two species on day 2 of incubation (E2) by removing the mesencephalic alar plate in the chick and replacing it with that of the quail embryo. Graft extension was evaluated by means of the well-known quail nucleolar marker, and cytodifferentiation of both operated and unoperated tecta was analyzed from E3 to E12. It was found that: (1) in most cases, the operated tectum is a chimera formed by a large dorsal territory consisting solely of grafted quail cells and a smaller ventral territory almost entirely made up of host chick cells. A clear boundary exists at the interface between these two territories. (2) In the host, the temporal sequence of appearance of the various laminae, following a well-established rostroventral-caudodorsal developmental gradient, is comparable in both the operated tectum and the host territory of the chimeric tectum to that of a control chick tectum. (3) In the graft, the migration of postmitotic cells starts earlier than in the host. However, in the former there is about a 12-h delay with respect to a control quail tectum. (4) Proliferation and migration of cells take place in the graft much faster than in the host. Thus, the formation of the 8 deepest layers occurs according to the normal quail schedule, indicating that the early delay is quickly recuperated. This process of lamination follows the normal quail rostroventral-caudodorsal developmental gradient. (5) The postmitotic neurons originating in the grafted neuroepithelium follow a normal radial migration. Nevertheless, a few grafted cells occupy the host tectal territory far from the host/graft interface. These cells have been observed in both the stratum griseum centralis and the uppermost tectal layers, indicating that some tectal neurons are able to displace themselves tangentially. (6) Contrary to what happens in the 8 deepest layers, which in the graft follow the normal quail cytodifferentiation schedule independently of the host, cytodifferentiation in the upper tectal layers is partially influenced by the host. The stratum opticum, the layer through which optic fibers are distributed to the entire surface of the tectum, extends over the chimeric tectum according to a normal chick schedule; that is, the surface of the graft is not entirely covered until E12, which represents a 48-h delay with respect to that of a normal quail tectum. The segregation of the cellular and plexiform layers of the upper tectum, the region where the optic fibers terminate, appears in the graft to adapt to the host rate during a 48-h period (between E8 and E10). However, this adaptation is not maintained, and at E12 (the last age analyzed) the graft shows a normal quail cytodifferentiation rate. It is concluded that the mesencephalic alar plate of the quail maintains a precise temporal sequence of cell proliferation and migration which is not modified after transplantation into the chick neural tube. The momentary retardation observed in the segregation of the uppermost layers of the grafted tectum could be explained by the inductive effect of optic axons developing asynchronously to the grafted neurons.