Early Development of the Thalamo‐Pallial Stage of the Tectofugal Visual Pathway in the Chicken (Gallus gallus)

ABSTRACT The tectofugal pathway is a highly conserved visual pathway in all amniotes. In birds and mammals, retinorecipient neurons located in the midbrain roof (optic tectum/superior colliculus) are the source of ascending projections to thalamic relays (nucleus rotundus/caudal pulvinar), which in turn project to specific pallial regions (visual dorsal ventricular ridge [vDVR]/temporal cortex) organized according to a columnar recurrent arrangement of interlaminar circuits. Whether or to which extent these striking hodological correspondences arise from comparable developmental processes is at present an open question, mainly due to the scarcity of data about the ontogeny of the avian tectofugal system. Most of the previous developmental studies of this system in birds have focused on the establishment of the retino‐tecto‐thalamic connectivity, overlooking the development of the thalamo‐pallial‐intrapallial circuit. In this work, we studied the latter in chicken embryos by means of immunohistochemical assays and precise ex vivo crystalline injections of biocytin and DiI. We found that the layered organization of the vDVR as well as the system of homotopic reciprocal connections between vDVR layers were present as early as E8. A highly organized thalamo‐vDVR projection was also present at this stage. Our immunohistochemical assays suggest that both systems of projections emerge simultaneously even earlier. Combined with previous findings, these results reveal that, in striking contrast with mammals, the peripheral and central stages of the avian tectofugal pathway develop along different timelines, with a tecto‐thalamo‐intrapallial organization arising before and possibly independently of the retino‐isthmo‐tectal circuit. We studied in chicken embryos the development of the thalamo‐pallial‐intrapallial stages of the tectofugal visual system by means of immunohistochemical assays and ex vivo injections of neural tracers. We found that the layered organization of the visual DVR as well as the system of columnar reciprocal connections between vDVR layers (blue axons) were present as early as E8. A highly organized thalamo‐vDVR projection (light blue axons) was also present at this stage. Immunohistochemical assays suggest that both systems of projections emerge simultaneously even earlier. Combined with previous findings, these results reveal that, in striking contrast with mammals, the peripheral and central stages of the avian tectofugal pathway develop alo