scope:

PREFACE

This book begins an Atlas Series on the Development of the Human Central Nervous System. We start with a volume devoted exclusively to the spinal cord for two reasons: (1) The spinal cord has similar structures throughout its length so a complete picture of its development at any specifi c age requires only a few different levels-and in early stages, only a single level is needed. (2) A transverse cutting plane is all that is necessary to show spinal cord structures. Thus, a single volume can feature all the stages of spinal cord development. In contrast, many different levels of the brain must be shown in a variety of cutting planes to get an overview of its structure at each stage of development. Four additional volumes (in preparation) in the Atlas Series will present a comprehensive picture of the developing human brain cut in frontal, sagittal, and horizontal planes. The second volume will feature the third trimester; the third volume will feature the second trimester; and fi nally, volumes 4 and 5 will feature the late and early parts of the fi rst trimester, respectively.

Our recent work on the developing spinal cord (Altman, J., and S. A. Bayer, Development of the Human Spinal Cord. An Interpretation Based on Experimental Studies in Animals, Oxford University Press, 2001) and this Atlas are our fi rst publications linking developmental studies of the central nervous system in animals to humans. From the early 1960s to the present, our work has featured the development of the rat nervous system at the cell and tissue levels. We use 3H-thymidine autoradiography to gather a large database that maps the stem cell mosaic in the neuroepithelium, the times of origin of neuronal populations within the nervous system, and the migratory pathways of neuronal populations as they settle in the maturing nervous system. Our initial foray into human central nervous system development began in the late 1980s with a request from Serge Duckett to contribute a chapter on human central nervous system development (Bayer, S.A., J. Altman, R. J. Russo, and X. Zhang, Embryology, In: Pediatric Neuropathology, S. Duckett, Ed., Lea & Febiger, Philadelphia, PA, 1995, pp. 54-107). It was while we were working on that chapter that we realized how little is known about the development of the human central nervous system. An up-to-date interpretation simply does not exist.

The best work was published in German (Hochstetter, F.: Beiträge zur Entwicklungsgeschichte des menschlichen Gehirns. I. Teil. Vienna and Leipzig, Franz Deuticke, 1919) and has never been translated into English. There is a striking similarity between the developing human and rat central nervous systems. But Hochstetter did his pioneering work on development when even the mature anatomy of the nervous system was not well understood. A re-examination of human central nervous system development is needed to link what has been learned from animal studies to normal development in humans. Medical textbooks of human embryology are in need of an update on central nervous system development. In addition, the many disorders resulting from abnormal neural development can be better understood if normal development is better known.

We decided to apply our knowledge of rat nervous system development to humans by directly examining histological sections of normal human specimens. The National Museum of Health and Medicine in the Armed Forces Institute of Pathology (AFIP), Walter Reed Hospital, Washington D.C. has the Carnegie Collection of human embryos and the Yakovlev Collection of human fetuses; while we were there we accidentally found the excellent Minot Collection of human embryos. Unfortunately, attempts to obtain funding for research at the AFIP were unsuccessful, so we decided to fi nance the research ourselves. During several trips to Washington D.C. in 1996 and mostly during an extended stay in the summer of 1997, we examined and took over 10,000 photographs of the best preserved normal specimens in the Yakovlev, Carnegie, and Minot collections. All of the fi lm developing was done by us. Technicians were employed to scan each negative and create high resolution digitized fi les. These fi les are the sources for a comprehensive database on the development of the human central nervous system at the cell and tissue levels. This material is being analyzed in our Laboratory of Developmental Neurobiology that is equipped with photographic instruments, high resolution scanners, printers, and Apple computers capable of handling large fi les and running 3-dimensional reconstruction software. This Atlas Series is the result of our efforts.

It is our hope that this fi rst volume will update knowledge of the developing human spinal cord and provide new insights into the steps involved in the morphogenesis of the mature spinal cord. All Companion Plates in this Atlas are presented in a "user friendly" format so that the reader can view unlabeled and labeled photographs on facing pages. The fi rst Companion Plate (2A-2B) shows the human spinal cord at the 4th gestational week at its most primitive stage consisting mainly of neuroepithelial stem cells (NEP cells). The last set of Companion Plates (104A-104B to 117A-117B) shows a spinal cord nearing maturity in the 4th postnatal month. What lies in between these Companion Plates are "freeze frames" of the developmental record. The large sections on 3-dimensional reconstructions provide holistic views of the developing spinal cord at critical stages when maturing structures fi rst appear during the fi rst and early second trimesters (Figures 2 through 32). A Glossary at the end of the book gives a brief defi nition of each structure that is labeled in the Plates and Figures.