DEVELOPMENT OF THE PATHWAY FROM THE RETICULAR AND PERIRETICULAR NUCLEI TO THE THALAMUS IN FERRETS - A DII STUDY

This study examines the connections of the thalamic reticular and peri reticular cell groups in developing ferrets. Small crystals of Dil -di octadecyl-3,3,3',3'-tetramethylindocarbocyanine perchlorate) were impl anted into either the dorsal thalamus or the cerebral cortex of aldehy de-fixed prenatal and postnatal ferret brains. A small implant of Dil into the presumptive lateral geniculate nucleus during early prenatal development [between embryonic day 23 (E23) and E25] reveals many retr ogradely labelled cells in the reticular nucleus. At E40, just before birth, the number of cells retrogradely labelled in the reticular nucl eus has become reduced compared to earlier prenatal implants, whether from small or large implants of Dil into the lateral geniculate nucleu s. By postnatal day 7, an adult-like pattern of retrograde labelling i s seen in the reticular nucleus; at this age, a small implant of Dil l imited to the lateral geniculate nucleus retrogradely labels a discret e group of cells located in the caudal regions of the reticular nucleu s. In the internal capsule, adjacent to the reticular nucleus, there a re two distinct groups of neurons. One group, called the large-celled perireticular zone (LPR), enters the internal capsule very early in de velopment (from E25; Mitrofanis, J., fur: J. Neurosci., 6, 253-263, 19 94) and is not labelled from the lateral geniculate nucleus at any dev elopmental stage. Small implants of Dil into presumptive visual and so matosensory cortices shows that the LPR lies in a distinct region of t he primordial internal capsule. Corticothalamic and thalamocortical ax ons turn sharply in the region of the LPR, whilst corticospinal and co rticobulbar axons pass straight through the LPR on towards their more caudal targets. Later, after both sets of axons have reached their tar gets, the LPR is not seen in the internal capsule. The other group of cells in the internal capsule, called the small-celled perireticular z one (SPR), forms a distinct band of cells lying midway between the ret icular nucleus and the globus pallidus. These cells enter the internal capsule much later in development, at about E40. Unlike the cells in the LPR, cells in the SPR are retrogradely labelled after an implant o f Dil into the lateral geniculate nucleus, and there are many which re main in the adult (Clemence, A. E. and Mitrofanis, J., J. Comp. Neurol ., 322, 167-181, 1992).