Indirect and Direct Wallerian Degeneration in the Intramedullary Root Fibres of the Hypoglossal Nerve

In the previous study (Part I) a description was given of the ultrastructural changes occurring during indirect Wallerian degeneration in the intramedullary root fibre region of the kitten hypoglossal nerve. One of the striking features of this degeneration process was the appearance at an early stage of micro­ glial cells completely covered by myelin, which apparently participated in phago­ cytosis of degenerating axoplasm and to a small extent of their own myelin covering. Evidence was obtained indicating that the numerous degenerating glial cells seen somewhat later in the degeneration process were derived from these myelin-covered microglial cells. Since glial cells of the type described in the previous study have never been implicated in the process of direct Wallerian degeneration, the possibility exists that they may in fact be characteristic for indirect Wallerian degeneration. However, this possibility cannot be adequately assessed unless our present rather scanty knowledge concerning the early glial reaction during direct Wallerian degeneration is extended. Therefore the present study has been undertaken to examine the ultrastructural changes during direct W allerian degeneration in the kitten, with particular reference to the possible occurrence of myelin-covered microglial cells and degenerating glial cells of the type described in the previous study (see Part I). Since no systematic ultra­ structural study on direct Wallerian degeneration in immature animals seem to exist, observations on changes in the myelinated nerve fibres and the different types of glial cells have been included.