NEUROGLIA

For the past 160 or so years the cells of the nervous system have been divided into two main categories: neurons and glia (Kettenmann and Verkhratsky, 2008). Prior to this, ever since the first image of a neuron was published in 1836 by Gabriel Valentin, the nerve cell had been in a class of its own (Lopez-Munoz et al., 2006). Some 20 years later in 1856 the term neuroglia was introduced by the German physician Rudolph Virchow. Virchow, also known as the “Pope of pathology” (Kettenmann and Ransom, 2005; Magner, 2002), described a “connective substance … in which nervous system elements are embedded” and referred to it as “nervenkitt” (or nerve putty). This description led to the use of the term “neuroglia,” which derives from archaic Greek, meaning something sticky or clammy. The notion that neuroglia were there merely as neural putty was treated with the reverence usually reserved for a bona fide papal encyclical and as such neuroglia remained sidelined for decades to come. Even though Virchow was responsible for the term neuroglia coming into use, at this stage he did not recognize that it was made up of cells rather than an acellular connective tissue.

It is now over 200 years since Theodore Schwann first described the cell which bears his name. Such early descriptions of nervous system components were done without the powerful microscopes we have today, yet Schwann and Ramon Y. Cajal made foundation observations which still stand. Cajal's papers, especially, show the power of careful observation, an essential element of good science.

The Schwann cell has been historically underrated and poorly understood. In particular, the myelin-forming Schwann cells or their myelin are still often referred to as a simple ‘sheath’ for the neuron. However, Schwann cells in all their complexity form essential partnerships with neurons, and muscles. This is of particular relevance in the case of the myelin-forming Schwann cell, an enormous cell that expresses unique molecules and complex relationships related to maintenance of the compact and non-compact myelin regions of its plasma membrane. Schwann cells have other complex interactions, not least of which are found where nerve terminals and muscle fibres form the tripartite synapse in association with the perisynaptic Schwann cells. There are also the poorly understood satellite cells that surround the dorsal root ganglion nerve cell bodies, and of course the complexity of non-myelinated Schwann cells and their axonal associations.

It may be that the histopathological prominence of abnormalities of compact myelin has focussed research on this region of the Schwann cell.

THEODOR SCHWANN 1810–1882

The Schwann cell is named in honour of the German physiologist Theodor Schwann (1810–1882, Figure 1.1) who is now acknowledged as the founder of modern histology. In addition to describing the Schwann cell, he made numerous contributions to the fields of biology, physiology and histology – not least as one of the instigators and main advocates of cell theory. The cell theory defined the cell as the base unit of all living organisms, and had great influence on the study of both plants and animals. The cell theory was radical for the time and irrevocably discredited Vitalism, the mainstream belief that life was attributed to a vital force. Among other things, Schwann is known for recognising that the crystals seen during fermentation, first reported by Leeuwenhoek in 1680, were in fact living organisms; although it was not until Pasteur in 1878 wrote to Schwann acknowledging this observation that Schwann's finding was accepted. In fact, Pasteur's germ theory stems from Schwann's work in which he showed that microorganisms were required for the putrefaction of meat.

Schwann spent his undergraduate years at the University of Bonn and then the equivalent of postgraduate study in Wuerzburg and Berlin. Schwann was appointed Professor of Anatomy at Louvain in 1839. In 1848 he moved to the Chair of Anatomy in Liege. In a biography of Schwann (Causey 1960), Causey reported that he avoided the strife of scientific controversy and appears to have risen above petty jealousies.

Introduction

Marsupials come in many shapes and sizes (Strahan 1995) and so do their brains. Some of the extinct forms such as Diprotodon were as large as small elephants while others such as the living planigales are among the smallest mammals. The smallest marsupials though are still over twice the weight of the smallest eutherians – a bat and a shrew both of which as adults weigh 2 g. The nervous systems of marsupials all have the same basic mammalian pattern (Johnson 1977). Large or small, that most complex of all computers, the mammalian central nervous system, is wired up to the rest of the body via the cranial, peripheral and autonomic nerves.

The central nervous system (CNS) is composed of the brain and spinal cord. The peripheral nervous system (PNS) carries information into and out of the brain or spinal cord. The PNS includes some of the cranial (head) nerves, the spinal nerves and the various ganglia in which the cell bodies of many of these peripheral nerves are located.

Cells of the nervous system

The nervous system is composed of two major types of cells, the neurons or nerve cells and the glial cells. Neurons are made up of a nerve cell body and its processes. In the CNS the neurons have short branching processes or dendrites which carry bioelectrical impulses to the cell body, and long processes called axons which carry bioelectrical impulses from the cell body. There are about 10,000 different types of neurons.

Introduction

While the mammalian body may be said to be a hotel for millions of potential pathogens, it is the warm, moist tissues of all mammals that can provide the breeding ground. To combat the effects of invading, replicating pathogens, evolution has provided the immunolymphatic system. This system is essential for survival in the day-to-day life of marsupials.

Because the lymphatic arm and the immune arm of the system cannot be easily seen and because these are difficult to dissect, particularly in marsupials, this system has been rather neglected in the past. Studies of the marsupial immunolymphatic system have concentrated on few species and general statements are made based on these. Such a selective approach is, however, the way in which most biological systems are studied. The marsupial immune system is now the focus of a number of intensive studies worldwide. Because the neonatal marsupial is immature at birth, i.e. altricial, and because organogenesis occurs in the pouch, the pouch young provide a unique and accessible model for studies of the development of the mammalian immunolymphatic system. Old and Deane (2000) have reviewed the development of immunological protection in pouch young, and make the point that the pouch provides an environment which is particularly challenging.

The immunolymphatic system encompasses lymphatic vessels and immune system tissues. These tissue include the tonsils, adenoids, lymph nodes, thymus gland, Peyer's patches and spleen. As well, there are tissues collectively known as the mucosa-associated lymphoid tissues (MALT).