[Interrelationships between embryonal axons and lemmoblasts during the development of rat spinal nerve roots...

In order to elucidate the earliest interrelations of growing axons and lemmoblasts and the nature of the latter, a submicroscopic investigation of the rat spinal roots at places of their outlet from the neural tube was performed on the 11th and 13th day of embryonic development. The results obtained demonstrated that the spinal roots, both anterior and posterior, are formed by several fine (growing from the marginal layer of the neural tube) cords consisting of tightly packed "naked" axons with no signs of cellular elements inside them. These cords grow from the marginal layer of the neural tube towards periphery with resulting rupture of th tube's external limiting and basal membranes and formation of perforations; neither basal nor limiting membranes pass on the axonal surface. After leaving the neural tube for mesenchyma, the axonal cords get into connections with lemmoblasts that surround them completely. These cells are morphologically identical to poorly differentiated mesenchymal cells, situated nearby, and differ ultrastructurally from glioblasts of the neural tube (their processes form the external limiting membrane). A peculiar feature of lemmoblasts is the absence of a basal membrane and collagenous fibres around them; they are also absent, at this stage of development, in thin-walled capillaries formed from mesenchymal cells. In no case could be observed migration of glioblasts from the neural tube towards periphery and their transition on the cords of growing "naked" axons. The data obtained evidence in favour of the fact that lemmoblasts, surrounding the axonal cords, are formed at the place from poorly differentiated mesenchymal cells.