An ultrastructural morphometric study of mossy fiber endings in pigeon, rat and man.

Mossy fiber endings and their synapses have an increasing morphological complexity along the phylogenetic scale which is specially evident when the evolution from reptiles to birds is considered. Among birds and mammals, only scanty and merely qualitative differences have been noticed in the mossy fiber organization. In the present study, the mossy fiber endings organization within the neuropil of four pigeons, four rats and six men was studied at optic and ultrastructural levels using morphometric methods. No significant differences were found in the fraction of volume of the neuropil and in the number of granule cells per unit volume of granular layer in all the three species. In man, a significant decrease in the volume of mossy fiber endings per unit volume of neuropil, and significant increases in their surface-to-volume ratio, in the fraction of their neurolemma occupied by synaptic contacts and in the average length of their synaptic contact zones, were found. The total synaptic area of mossy fiber endings per unit volume of neuropil was larger in man than in other animals in spite of their more reduced volume. These results show that significant differences in mossy fiber organization can be demonstrated when man is compared with a small mammal (the rat) and a bird (the pigeon). It is suggested that differences in the stereological organization of man's terminals might be related to his capacity for making finely graded movements. Moreover, it is suggested that the phylogenetic evolution of the mossy fiber organization has progressed among the higher vertebrates as it did in the rest of the animal scale.