Callosal connections of the posterior neocortex in normal‐eyed, congenitally anophthalmic, and neonatally enucleated mice

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Abstract
Following multiple injections of horseradish peroxidase into the posterior neocortex of one hemisphere, we examined the distribution of retrogradely labeled cells and anterogradely labeled terminations in tangential and coronal sections through contralateral areas 17 and 18 in throe groups of adult mice: normal‐eyed (ZRDCT‐n and C57Bl/6J strains), congenitally anophthalmic (ZRDCT‐an strain), and neonatally enucleated (ZRDCT‐n strain). In agreement with previous studies, we observed that the pattern of callosal connections in areas 17 and 18 of normal‐eyed mice contains the following features: (1) a dense band of callosal cells and terminations separating the interiors of areas 17 and 18, which have relatively few callosal connections, (2) a ring‐like configuration anterolateral to area 17, (3) a region of dense labeling lateral to area 18, (4) a narrow band of labeling bridging the posterior portion of area 18, and (5) a region of labeling anteromedial to area 17. We find that all these features of the normal callosal pattern are recognizable in congenitally anophthalmic mice. Their presence in mice that never had eyes supports the hypothesis that central visual pathways can develop many aspects of their connectivity in the absence of input from the periphery. However, we also find that the details of certain features of the callosal pattern in congenitally eyeless mice often differ from those of the same features in normal‐eyed mice, and that the between‐animal variability in the appearance of these features is higher in eyeless mice. These latter findings indicate that the eyes are needed during normal development to fine‐tune the pattern of callosal connections. Our results also reveal that the callosal pattern in neonatally enucleated mice does not differ significantly from that in congenitally anophthalmic mice, indicating that the period in which the eyes guide callosal development extends into postnatal life. While the present data do not delineate the time course of this period, the finding of similarly abnormal callosal patterns in congentially anophthalmic and neonatally enucleated mice suggests that the eyes exert little if any influence prenatally. Finally, examination of coronal sections indicates that the laminar distribution of callosal connections develops normally in both groups of eyeless mice.