Quantitative assessment of dendritic branching and spine densities of neurons of hippocampal embryonic tissue transplanted into juvenile neocortex.

Embryonic 18-day-old hippocampal tissue was transplanted into a fresh cavity in frontal neocortex in 18-day-old (juvenile) Wistar rats. The transplant was examined after 2-5 months. The rapid Golgi procedure was used to assess quantitatively the dendritic branching (intersections and branching orders) and spine numbers per neuronal region, and spine densities (per 20 microns segment) on the pyramidal neurons, granule neurons and other types. The results revealed that the graft neurons generally have significant deviations from those of normal native hippocampal neurons. The aberrations were present in both pyramidal and granule cells. There were also many neurons which could not be classified into the known types. The pyramidal neurons showed a significant deficiency in dendritic branching, more often in basal dendrites than in apical dendrites. Apical dendrites of the graft pyramidal neurons also developed spine densities relatively better than basal dendrites, and sometimes significantly higher than even normal neurons. The significant deficiency in spine numbers of basal dendrites was primarily due to deficiency in dendritic branching rather than to spine density. On the contrary, the granule cells developed significantly less of both dendritic branching and spine densities (hence spines per neuron) than normal hippocampal granule neurons. The unclassifiable neuronal types had generally high dendritic branching and spine densities relative to the other neurons of the graft. The study discusses the importance and necessity of making quantitative assessments of neurons to understand whether the apparently normal neurons of transplants are really normal, and how far they are deviant. So far, quantitative assessments have been seldom reported in this important area of research, hence, this study is the first of its kind to highlight it.