The studies described here were carried out in the Neuroregul ation Group, Department of Physiology, University of Leiden, the Netherlands. Over the last decade, this group, in close collaboration with the Department of Neurosurgery of the Academic Hospital of Leiden, has studied the development of the central nervous system from a neuroanatomical as well as a clinical perspective. During this period, the expression of several morphore gulators in the developing rat spinal cord was extensively investigated. Parallel studies focused on the development of the spinal cord fiber systems, which was studied by means of the intrauterine use of neuronal tracers. The main goal of these studies was to extend our knowledge about the (normal) generation of the spinal cord and to contribute to the under standing of clinical problems related to regeneration and degeneration in the mammalian central nervous system. The studies on morphoregulators, in particular, appeared to benefit two different scientific areas. Firstly, the correlation between morphoregulator expression patterns and known anatomy contributed to our knowledge about spinal cord development. Secondly, the correlation between morpho regulator expression patterns and known developmental processes may help to understand their precise function(s). This volume of Advances in Anatomy, Embryology and Cell Biology presents these particular studies on the development of the rat spinal cord performed over the last decade. As well as integrating the results of the tracer studies, this volume also provides an update on the development of the rat spinal cord.

1 Introduction 1.1 Hemiballism Hemiballism or hemichorea is a rare neurological disorder, but the crucial invol- ment of the subthalamic nucleus (STN) in its pathophysiology has been app- ciated for decades (Jakob 1923; Martin 1927; Glees and Wall 1946; Whittier and Mettler 1949; Carpenter and Carpenter 1951; Crossman 1987). Only recently have serious doubts come forward. Postuma and Lang (2003) have described the STN as being involved in only a minority of cases, and indicated unrecognized causes such as non-ketotic hyperosmolar hyperglycaemia and complications of human immunodeficiency virus (HIV) infections. Moreover, the crucial involvement of a lesion of the STN is in doubt (Guridi and Obeso 2001; Postuma and Lang 2003). On the other hand, idiopathic Parkinson's disease (Battistin et al. 1996; Usunoff et al. 2002) is a common neurodegenerative disorder, but the key role of the STN in the pathophysiological origin of the parkinsonian state has become evident only recently (Miller and DeLong 1987; Mitchell et al. 1989; Bergman et al. 1990, 1994; Hollerman and Grace 1992; Guridi et al. 1993; Parent and Hazrati 1995b; Hassani et al. 1996; Levy et al. 1997, 2002; Blandini et al. 2000; Hirsch et al. 2000; Ni et al.