Ultrastructural signs of regenerative-degenerative processes in long-term dentate fascia grafts.

Abstract

An ultrastructural investigation of embryonic (E20) dentate fascia grafts transplanted into an acute cavity in the somatosensory neocortex of adult rats revealed a continuous dynamic state of the tissue nine months postgrafting. The grafts consisted mainly of typical granular cells with some admixture of hippocampal pyramidal neurons and polymorph hilar cells with a normal, mature ultrastructure. Many features of the transplanted tissue suggested continuing development and growth. Dendritic branches with growth tips, axonal growth cones, synaptic boutons with growth vesicles, immature myelin sheaths and myelin-producing cells were observed. In contrast, ultrastructural signs of degeneration were present in some axons, and, less often, in dendrites. These processes, as well as some of the terminal synapses, contained various amounts of lysosomes and lipofuscine granules. In many such terminals the signs of degenerative change were combined with the presence of multiple mitochondria, polymorph vesicles and tubular reticulum, indicating simultaneous reparative processes. It is suggested that continuous recycling of neuronal processes occurs in longterm dentate grafts. This morphological instability nay depend on the paucity of synaptic targets within the dentate tissue transplanted with a minimal quantity of hippocampal pyramidal cells and on the limitation of the afferent input. However, the observed features of the grafted dentate tissue are not qualitatively different from those observed in normal dentate with its protracted development and active compensatory reorganization.