On layer III pyramidal cells in the parastriate borderzone of man

The parastriate borderzone in the human occipital cortex is made conspicuous by clusters of large pyramidal cells in layer III (limen parastriatus gigantopyramidalis: von Economo and Koskinas 1925). Experiments in higher primates provide evidence that the vertical meridian is represented in this region and that these large pyramidal cells connect the parastriate borderzone of both hemispheres via callosal fibres. Golgi preparations of the human parastriate borderzone reveal that the large pyramidal cells generate a stout apical dendrite which gives rise to several side branches in layer III and terminal ramifications in layer II. An unusual feature is that the apical dendrite is almost devoid of spines and that its side branches are only sparsely spined as well. Both the basal and lateral portions of the cell body give rise to spiny dendrites having their domain within layers III and IV. In pigment-Nissl preparations, the large layer III pyramids are marked by coarse and distinct Nissl bodies and a bowel-shaped accumulation of faintly tinged lipofuscin granules located close to the nucleus. Electron micrographs show that up to 30% of the soma membrane are covered by large boutons which make multiple synaptic contacts. This contrasts to the appearance of most other types of cortical pyramydal cells which normally show only a few axo-somatic contacts.