Organization of the mossy fiber system of the rat studied in extended hippocampi. I. Terminal area related to number of granule and pyramidal cells

Abstract

A quantitative and qualitative study of the septotemporal organization of the hippocampal mossy fiber system has been carried out in the rat. Transverse sections of isolated and extended hippocampi were stained according to Nisel's or Timm's method. At 12 equidistant levels covering the entire length of the hippocampus the following parameters were measured: the length of the granule cell layer and the mossy fiber layer, the area of the fields containing mossy fiber boutons, the number of granule cells, and the number of pyramidal cells in the hilus and in the regio inferior. Septally the pyramidal layer of the regio inferior is broad and the cells within the layer are relatively widely spaced. Here the mossy fiber boutons intrude deep into the layer. At more temporal levels the cells are tightly packed, the cell‐layer is narrower, and the boutons predominantly terminate suprapyramidally. The length of the granular layer is greatest septally, while the length of the mossy fiber layer is greatest temporally. The area of the hilus, occupied by the mossy fiber boutons, is large at most of the temporal levels and small at the septal levels, whereas in CA3 the reverse is true. The greatest number of granule cells was found at the more septal levels. Conversely, the majority of the pyramidal cells in the hilus and in the regio inferior is situated temporally. The ratio of granule cells to pyramidal cells (hilus and regio inferior together) declines linearly from approximately ten at septal levels to approximately one at temporal levels. Because the mossy fibers are distributed fairly transverse to the long axis of the hippocampus, the variation in the ratio of granule cells to pyramidal cells reflects a corresponding variation in the number of innervating to innervated cells within the mossy fiber system.