Pigment architecture of the human telencephalic cortex. IV. Regio retrosplenialis.

Cortical lamination and parcellation of the retrosplenial region in the human brain is evaluated with the aid of frontal serial sections stained for nerve cells (15 micrometers), myelin sheaths (100 micrometers), and lipofuscin granules (800 micrometers). For the most part, the retrosplenial region is buried in the depth of the sulcus corporis callosi covering the posterior parts of the cingulate gyrus. It lies between the supracallosal derivatives of the allocortex (fascia dentata, cornu ammonis, subiculum) and the mature parietal isocortex. The region can be subdivided into five areas. The transitory periallocortical area ectosplenialis is followed by a richly differentiated proisocortical core displaying extremely externopyramidal, externoteniate, and astriate to unitostriate characteristics. The parvocellular core is averagely poor in pigment (typus clarus) and rich in myelinated fibres (typus dives). Minor structural differences allow for its subdivision into a lateral, an intermediate and a medial retrosplenial field. The accompanying area parasplenialis is adjacent to the equoteniate parietal isocortex. It is only weakly externopyramidal, externoteniate, and propebistriate. The already homotypical field shows an average pigmentation and myelin content. These structural features permit its classification as a belt area of the retrosplenial core.