The time of origin of neurons in Ammon's horn and the associated retrohippocampal fields

Summary

The time of origin of the neurons in the hilus of the dentate gyrus, in the regio superior and regio inferior of Ammon's horn, and in the following retrohippocampal fields — the subiculum, presubiculum, parasubiculum, medial and lateral entorhinal areas and the perirhinal cortex, has been determined in the rat, by the technique of ³H-thymidine autoradiography. In each field the cells are generated over a limited period of about five days, with the majority being formed in a 24–48 h period. As in the neocortex, the cells in the various hippocampal fields are generated in a distinct “inside-out” sequence with respect to the ependymal zone in which they arise. In addition there are two distinct gradients along the transverse, or dentato-rhinal axis, of the formation. Thus the neurons in the hilar region of the dentate gyrus (including field CA₄) tend to arise earlier than the pyramidal cells in the regio inferior, and these in turn, are on average, generated earlier than those in the regio superior or in the subiculum. In the retrohippocampal region there is a comparable gradient extending medially from the perirhinal cortex to the presubiculum, the cells in the lateral entorhinal area being formed, on average, earlier than those in the medial entorhinal cortex, and these, in turn, are generated earlier than those in the para- or presubiculum. There is no evidence for a dorso-ventral (or septotemporal gradient) in any part of the hippocampal formation like that found in the dentate gyrus. Cell counts indicate that there are over 320,000 pyramidal cells in the regio superior (field CA₁) and about 150,000 in the regio inferior (fields CA₂ and CA₃).