An immunocytochemical study of the distribution of some plasma proteins within the developing forebrain of the pig with special reference to the neocortex.

The distributions of 4 plasma proteins (fetuin, transferrin, alpha-fetoprotein and albumin) have been studied by means of immunocytochemistry in the forebrain of the pig from 20 to 109 days of gestation, term being at 114 days. These proteins are present in plasma, cerebrospinal fluid, meninges, some epithelial cells of choroid plexus and some neural cells and processes. The most striking observation is that these proteins are excluded from the brain to a large extent even at the earliest ages. Fetuin is present early in gestation in a few cells bordering the ventricle. Fetuin-positive cells are subsequently seen principally in the newly formed cortical plate and later in the subplate zone. This suggests that fetuin may be acting as a signal for the early migrating cells of the cortex and that its presence is necessary to these cells, particularly as there appears to be an increase in the amount of fetuin in these same cells in mid-gestation at a time when many fibres are arriving in this region to make cortical connections. Late in gestation after connectivity, it has been established that there are few fetuin-positive neurons remaining. However, at this late stage many neurons are strongly positive for transferrin, for which they have been negative earlier. This appearance may be related to the onset of activity. Alpha-fetoprotein has a distribution similar to that of fetuin. Albumin, although present transiently in the cerebrospinal fluid in the middle of gestation, is not seen in neural cells within the cortex. There has been some controversy as to whether these proteins which have been reported to be present in the brains of other species have been synthesized by neural cells or been taken up by specific mechanism. This study suggests that during the course of brain development, both processes may play their part.