Dendritic excrescences seem to characterize hippocampal CA3 pyramidal neurons in humans.

Excrescences are unique dendritic postsynaptic structures of the hippocampal formation. Only CA3 pyramidal neurones and hilar mossy cells possess these complex dendritic structures. Dendritic excrescences have so far only been investigated in rabbit, rat and rhesus monkey. Applying a Golgi impregnation method optimized for human brain tissue, we describe the detailed morphology of excrescences of CA3 pyramidal neurons of man. Human thorny excrescences possess a thin and single spine neck and multiple spine heads (4 on average, sometimes more than 10). Human cluster excrescences sit upon the dendrite with a broad stem, and exhibit a "papilloma-like" surface. Some human CA3 pyramidal neurons seem to possess markedly longer spine necks and larger spine heads compared to human neocortical pyramid cells; they were named long-neck spines. Thorny excrescences, cluster excrescences and the newly described long-neck spines can also be found on the dendritic main stem of human CA3 pyramidal neurons.CA2 pyramidal neurons neither possess these long neck spines nor thorny or cluster excrescences. Thus, the unique excrescences of CA3 pyramidal neurones seem to be another criterion for a demarcation between the CA3- and CA2 region of the human hippocampus.