Neurogenesis and identification of developing layers in the visual cortex of the wallaby (Macropus eugenii)

Birthdates of the neurons that comprise the layers of the mature visual cortex in the wallaby (Macropus eugenii) have been determined with the aid of tritiated thymidine autoradiography. The laminar positions of cells, identified by their birthdates, have then been followed at early stages during development and compared with previously published data on the distribution of thalamocortical afferents and corticothalamic projecting cells (Sheng et al. [1991] J. Comp. Neurol. 307:17-38). Neurons are born in a deep to superficial sequence typical of other mammals. The loosely packed zone of cells, which develops at the base of the thin compact zone of cells at the superficial margin of the cortical plate early in development, was identified as being part of the cortical plate. Afferents did not wait below this zone but grew into the developing cortical layers immediately after the cells that form these layers began accumulating in the loosely packed zone, starting with layer 6 on postnatal day 22 (P22). The genesis of layer 4 did not begin until P32, and these cells reached the superficial cortical plate at P54 and entered the loosely packed zone by P65. Cells of layers 5 and 6 formed the initial projection to the thalamus. Despite the protracted development of the wallaby and the large discrepancy between the time of thalamic ingrowth and genesis of layer 4, there was no extended waiting period for afferents in the subplate.