MICROSCOPY RESEARCH AND TECHNIQUE 44:1 (1999) Introduction to Histology of the Hypothalamic-Pituitary Axis CELESTE R. WIRSIG-WIECHMANN, PHD. Department of Cell Biology, University of Oklahoma Health Science Center, Oklahoma City, Oklahoma The hypothalamic-pituitary axis represents an important pathway in the control of the body’s endocrine systems. As classically described, specialized groups of neurons within the hypothalamus control the release of a variety of hormones from the adenohypophysis (anterior pituitary gland), or release hormones directly into the blood within the neurohypophysis (posterior pituitary gland). These hormones regulate the major metabolic and reproductive functions of the body. Since the discovery of the dual origins of the pituitary from diencephalon and oral ectoderm by Martin Heinrich Rathke in the early 1800s, much has been learned concerning the anatomy and neurochemistry of the neuronal relationships with the pituitary. However, the very complex and dynamic nature of pituitary secretion warrants a much more extensive examination of these neural control systems. Because this topic is very broad and new information is being generated at a fast pace, it is not possible to cover all the hypothalamic systems in detail. Rather the goal of the topical issue ‘‘Histology of the Hypothalamic-Pituitary Axis’’ is to provide examples of the modern anatomical approaches that are being used to provide new information about specific (immunocytochemically-identified) hypothalamic-pituitary systems from a comparative and functional perspective. The first two papers deal with the gonadotropinreleasing hormone (GnRH) system. These neurons are unique in that they are born within the nasal cavity and migrate into the forebrain during development. Those that reach the preoptic-hypothalamic area are responsible for controlling the secretion of luteinizing hor- r 1999 WILEY-LISS, INC. mone and follicle stimulating hormone from the pituitary gland. The neural activity of these neurons is coordinated so that they all fire simultaneously to elicit hormonal secretion. The first two papers address: (1) what molecular guidance mechanisms subserve the migration of GnRH neurons to the appropriate brain areas during development (Schwanzel-Fukuda), and (2) what structural mechanisms are involved in GnRH neuron coordinated activity (Witkin). The second set of papers covers a variety of hypothalamic neural systems related to the anterior and posterior pituitary, including those containing vasopressin, oxytocin, gonadotropin-releasing hormone, and corticotropin-releasing hormone, among others. These papers cover aging and comparative issues and specifically address: (3) what age-related changes occur in hypothalamic neurons (Zhou and Swaab), and (4) how the teleost adenohypophysis receives direct innervation from hypothalamic neurons (Batten, Moons, and Vandesande). Far from being comprehensive, this topical issue on the ‘‘Histology of the Hypothalamic-Pituitary Axis’’ serves to highlight some recent advances in our knowledge regarding the complex relationship between the brain and the master gland of the body. It also serves to illustrate the wealth of information on this system still awaiting discovery. I wish to thank all the authors for their contributions, and Dr. John E. Johnson, Jr., Editor-in-Chief of Microscopy Research and Technique, for his guidance and support during the process of compiling this series of topical papers.