Membrane specializations in the developmentally transient perireticular nucleus of the rat

The perireticular nucleus lies among the fibres of the internal capsule and in the rat is relatively large at birth. Like the subplate of the neocortex, it is dramatically reduced in the adult (Mitrofanis, 1992). On embryonic day 17 (E17), cells of the perireticular nucleus project to the cortex (Adams and Baker, 1995); at birth there is also a projection to the thalamus (Mitrofanis and Baker, 1993). When corticofugal axons reach the perireticular nucleus, they separate into the descending tracts and the corticothalamic pathway. The axons then enter the internal capsule on their way to the thalamus, defasciculate and contribute to a complex zone of interweaving fibres seen in this region (Adams and Guillery, 1994). We have found that the cells of the perireticular nucleus are immunopositive for an antibody (3A10) to a phosphorylated neurofilament protein (Yamada et al., 1991), and we have shown that these 3A10-positive perireticular cells extend their dendrites across the path of cortical fibres at the earliest age studied (E17). We have confirmed this dendritic orientation by Golgi staining at a later stage (P3). Electron micrographs show that axo-somatic and axo-dendritic synapses first appear in the perireticular nucleus at about the day of birth. Prenatally, between E14 and E19, the major specialized membrane interrelationships seen are omega formations between adjacent profiles that often contain vesicles. Omega formations are rarely seen at later stages. Puncta adhaerentia typified by opposing membrane densities with no associated vesicles were seen at all ages studied. We suggest that early membrane contacts (omega formations) may represent transient organisational or guidance influences since they occur during a period when axonal pathways are being defined.