Organization of the corticotectal projection in hamsters with neonatally elevated levels of serotonin in the superior colliculus

Development of the commissure of the superior colliculus in the hamster

The development of the corpus callosum (CC) and the anterior commissure (CA) is well known in a wide variety of species. No study, however, has described the development of the commissure of the superior colliculus (CSC) from embryonic state to adulthood in mammals. In this study, by using the lipophylic tracer DiI, we investigated the ontogeny of this mesencephalic commissure in the hamster at various ages. The development of axonal terminals, growth cone morphologies, and axons branching were described for the superior colliculus (SC) contralateral to the tracer injection. The first CSC axons cross the midline at embryonic day 11 (E-11) and grow further into the intermediate layers of the contralateral SC between E-12 and E-14. There is little axon growth therein between E-14 and the day of birth (P-0). Growth cones at the tip of these axons adopt complex morphologies at E-12 and progressively simplify until P-0. Pioneer axons are clearly visible between E-14 and P-1. These are followed by other axons progressively more numerous between P-0 and P-5. Axons do not show any branching until P-2. Between P-3 and P-9, the axons progressively arborize in the intermediate layers. Some axons reach the superficial layers at P-5, and they become more numerous around P-11, and only a few axons remain therein by P-21. Myelinated axons appear at P11 and are very dense at P-21. Our results indicate that the CSC follows developmental schemes similar to those of the CC and the AC but that initial axon midline crossing occurs earlier.

Regional analysis of neurofilament protein immunoreactivity in the hamster's cortex

The laminar distribution of several distinct populations of neurofilament protein containing neurons has been used as a criterion for the delineation of cortical areas in hamsters. SMI-32 is a monoclonal antibody that recognizes a non-phosphorylated epitope on the medium- and high-molecular weight subunits of neurofilament proteins. As in carnivores and primates, SMI-32 immunoreactivity in the hamster neocortex was present in cell bodies, proximal dendrites and axons of some medium and large pyramidal neurons located in cortical layers III, V and VI. A small population of labeled multipolar cells was also found in layer IV. Neurofilament protein immunoreactive neurons were found throughout isocortical areas. Very few labeled cells were encountered in supplemental motor area, insular cortex, medial portion of associative visual cortex and in parietal association cortex. Our data indicate that SMI-32 immunoreactive cells can be efficiently used to trace boundaries between neocortical areas in the hamster's brain. The regional distribution SMI-32 immunoreactivity in the hamster cortex corresponds quite closely with cortical areas as defined by their cytoarchitecture and myeloarchitecture. The primary sensory cortical areas contain the most intense of SMI-32 immunoreactivity and are also those with the highest density of myelinated axons. Very low SMI-32 immunoreactivity was found in orbital, insular, perirhinal, cingulate and infralimbic cortices, which are also poor in myelinated axons. This supports the association between SMI-32 immunoreactivity and myelin contents.