Development of the area centralis and visual streak in the grey kangaroo Macropus fuliginosus

To further our understanding of area centralis and visual streak formation we have investigated the distribution and number of the total cell population in the retinal ganglion cell layer of the western grey kangaroo Macropus fuliginosus using adults and young from 7-200 days postnatal. This species was chosen since, as we describe, the adult possesses a particularly prominent area centralis and visual streak. By studying the total cell population we were able to compare cell distributions in immature retinae, in which cell types could not be distinguished, with topography in the adult. By 57 days postnatal a weak visual streak was apparent; a pronounced area centralis and visual streak were seen by 84 days although densities across the retina, particularly in the far periphery, were still considerably higher than in the adult. Dying cells were also observed up to 160 days, but not at 188 days or in the adult. The fall of approximately one third in the total cell number as the area centralis and visual streak developed is presumably related to cell death. Furthermore, at 57 days, dying cells were seen preferentially in retinal regions outside the immature visual streak and may therefore play a part in refining density gradients to their mature form. As the area centralis and visual streak developed, the relative lengths of the dorso-ventral and naso-temporal axes remained similar, suggesting that differential radial growth cannot underlie the changing live cell density gradients.