1Introduction to Glia

There are two major classes of cells in the brain – neurones and glia (Figure 1.1).The fundamental difference between these lies in their electrical excitability –neurones are electrically excitable cells whereas glia represent nonexcitable neuralcells. Neurones are able to respond to external stimulation by generation of aplasmalemmal ‘all-or-none’ action potential, capable of propagating through theneuronal network, although not all neurones generate action potentials. Glia areunable to generate an action potential in their plasma membrane (although they areable to express voltage-gated channels). Glial cells are populous (as they accountfor ∼90 per cent of all cells in the human brain) and diverse. In the centralnervous system (CNS) they are represented by three types of cells of neural (i.e.ectodermal) origin, often referred to as ‘macroglial cells’ (which may also beproperly called ‘neuroglial cells’). These are the astrocytes, the oligodendrocytesand the ependymal cells. The ependymal cells form the walls of the ventricles inthe brain and the central canal in the spinal cord. Ependymal cells are involvedin production and movement of cerebrospinal fluid (CSF), in forming a sepa-rating layer between the CSF and CNS cellular compartments, and in exchange ofsubstances between the two compartments. In addition to neuroglia, the umbrellaterm glia covers microglia, which are of non-neuronal (mesodermal) origin andoriginate from macrophages that invade the brain during early development andsettle throughout the CNS. In the peripheral nervous system (PNS), the main classof glia is represented by Schwann cells, which enwrap and myelinate peripheralaxons; other types of peripheral glia are satellite cells of sensory and sympatheticganglia and glial cells of the enteric nervous system (ENS) of the gastrointestinaltract.

1.1 Founders of glial research: from Gabriel Valentin toKarl-Ludwig Schleich

The idea of the co-existence of active (excitable) and passive (non-excitable)elements in the brain was first promulgated in 1836 by the Swiss professor of

Glial Neurobiology: A Textbook Alexei Verkhratsky and Arthur Butt© 2007 John Wiley & Sons, Ltd ISBN 978-0-470-01564-3 (HB); 978-0-470-51740-6 (PB)

COPYRIG

HTED M

ATERIAL

4 CH01 INTRODUCTION TO GLIA

Neurones(~10%)

Glia(~90%)

Macroglia(~85–90%)

CNS PNS

Microglia(~10–15%)

Schwanncells

Astrocytes(~80%)

Oligodendrocytes(~5%)

Ependymalcells (~5%)

Neural Cells

Figure 1.1 Neural cell types

anatomy and physiology Gabriel Gustav Valentin (1810–1883), in the book Überden Verlauf und die letzten Enden der Nerven. The concept and term ‘glia’ wascoined in 1858 by Rudolf Ludwig Karl Virchow (1821–1902, Figure 1.2), inhis own commentary to the earlier paper ‘Über das granulierte Ansehen derWandungen der Gehirnventrikel’ (published in the journal Allgemeine Zeitshriftfur Psychiatrie; Vol. 3, pp. 242–250), and elaborated in detail in his book, DieCellularpathologie in ihrer Begründung auf physiologische und pathologischeGewebelehre. Virchow was one of the most influential pathologists of the 19thcentury – he was one of the originators of the cellular theory (‘Omnis cellula ecellula’) and of cellular pathology.

Virchow derived the term ‘glia’ from the Greek ‘����’ for something slimyand of sticky appearance (the root appeared in a form ��o�o� in writings ofSemonides where it referred to ‘oily sediment’ used for taking baths; in worksof Herodotus, for whom it meant ‘gum’; and in plays of Aristophanes, who usedit in a sense of ‘slippery or knavish’. In Modern Greek, the root remains inthe word ‘��o���’, which means filthy and morally debased person.) Virchowcontemplated glia as a ‘nerve putty’ in 1858 when he held a chair of pathologicalanatomy at Berlin University. He initially defined glia as a ‘connective substance,which forms in the brain, in the spinal cord, and in the higher sensory nerves a sortof nervenkitt (neuroglia), in which the nervous system elements are embedded’;where ‘nervenkitt’ means ‘neural putty’. For Virchow, glia was a true connectivetissue, completely devoid of any cellular elements.

The first image of a neuroglial cell, the radial cell of the retina, was obtainedby Heinrich Müller in 1851 – these are now known as retinal Müller cells. Severalyears later, these cells were also described in great detail by Max Schultze. Inthe beginning of 1860, Otto Deiters described stellate cells in white and greymatter, these cells closely resembling what we now know as astrocytes. Slightly

Figu

re1.

2R

udol

fV

irch

ow–

fath

erof

glia

;the

fron

tispi

ece

ofhi

sbo

okD

ieC

ellu

larp

atho

logi

ein

ihre

rB

egrü

ndun

gau

fph

ysio

logi

sche

und

path

olog

isch

eG

eweb

eleh

re(B

erlin

,V

erla

gvo

nA

ugus

tH

irsc

hfel

d,18

58)

issh

own

onth

eri

ght

6 CH01 INTRODUCTION TO GLIA

Figure 1.3 Santiago Ramón y Cajal and Camillo Golgi. The bottom panel shows originalimages of glial cells drawn by Ramón y Cajal: ‘Neuroglia of the superficial layers of thecerebrum; child of two months. Method of Golgi. A, B, [C], D, neuroglial cells of the plexiformlayer; E, F, [G, H, K], R, neuroglial cells of the second and third layers; V, blood vessel; I, J,neuroglial cells with vascular [pedicles].’ This figure was reproduced as Figure 697 in Texturaand Figure 380 in Histologie. (Copyright Herederos de Santiago Ramón y Cajal)

1.1 FOUNDERS OF GLIAL RESEARCH 7

later (1869), Jakob Henle published the first image of cellular networks formedby stellate cells (i.e. astrocytes) in both grey and white matter of the spinal cord.Further discoveries in the field of the cellular origin of glial cells resulted fromthe efforts of many prominent histologists (Figures 1.3 and 1.4), in particularCamillo Golgi (1843–1926), Santiago Ramón y Cajal (1852–1934), and Pio DelRio Hortega (1882–1945). S. Ramón y Cajal was born on May 1, 1852, in Aragon,Spain. In 1883 he was appointed Professor of Descriptive and General Anatomyat Valencia; in 1887 he was assumed a chair of in University of Barcelona andin 1892 he became Professor of Histology and Pathological Anatomy in Madrid.

Figure 1.4 Morphological diversity and preponderance of glial cells in the brain as seen byGustaf Magnus Retzius (1842–1919). Retzius was Professor of Histology at the KarolinskaInstitute in Stockholm from 1877. He investigated anatomy and histology of the brain, hearingorgans and retina. The image shows a drawing from Retzius’ book Biologische Untersuchungen(Stockholm: Samson and Wallin, 1890-1914), Vol. 6 (1894), Plate ii, Figure 5, where twoneurones are marked with an arrow; the host of glial cells are stained by a silver impregnationmethod. (The image was kindly provided by Professor Helmut Kettenmann, MDC, Berlin)

8 CH01 INTRODUCTION TO GLIA

Ramón y Cajal was, and remains, one of the most prominent and influentialneurohistologists, who described fine structure of various parts of the nervoussystem. He was the most important supporter of the neuronal doctrine of brainstructure. He won the Nobel Prize in 1906 together with Camillo Golgi.

Camillo Golgi was born in Brescia on July 7, 1843. Most of his life he spent inPavia, first as a medical student, and then as Extraordinary Professor of Histology,and from 1881 he assumed a chair for General Pathology. He supported the retic-ular theory of brain organization. Using various ingenious staining and microscopictechniques, Camillo Golgi discovered a huge diversity of glial cells in the brain,and found the contacts formed between glial cells and blood vessels, as well asdescribing cells located in closely aligned groups between nerve fibres – the firstobservation of oligodendrocytes. Further advances in morphological characteriza-tion of glia appeared after Golgi developed his famous ‘black’ (or silver nitrate)technique (la reazione nera) for staining of cells and subcellular structures, andwhen Ramón y Cajal invented the gold-chloride sublimate staining technique,which significantly improved microscopic visualization of cells (and neuroglialcells in particular) in brain tissues. Using these techniques Golgi, Cajal and manyothers were able to depict images of many types of glia in the nervous system(Figures 1.3, 1.5).

In 1893, Michael von Lenhossek proposed the term astrocyte (from the Greekfor star, astro, and cell, cyte) to describe stellate glia, which gained universalacceptance within the next two decades. The name oligodendrocyte (from theGreek for few, oligo, branches, dendro, and cell, cyte) was coined slightly later,after Pio Del Rio-Hortega introduced the silver carbonate staining technique, whichselectively labelled these cells (1921). It was also Del Rio-Hortega who proposedthe term ‘microglia’ to characterize this distinct cellular population; he was oneof the first to propose that microglia are of mesodermal origin and to understandthat these cells can migrate and act as phagocytes.

The main peripheral glial element, the Schwann cell, was so called by LouisAntoine Ranvier (1871), following earlier discoveries of Robert Remak, whodescribed the myelin sheath around peripheral nerve fibres (1838) and TheodorSchwann, who suggested that the myelin sheath was a product of specialized cells(1839).

At the end of 19th century several possible functional roles for glial cellswere considered. Camillo Golgi, for example, believed that glial cells are mainlyresponsible for feeding neurones, by virtue of their processes contacting bothblood vessels and nerve cells; this theory, however, was opposed by SantiagoRamón y Cajal. Another theory (proposed by Carl Weigert) considered glial cellsas mere structural elements of the brain, which filled the space not occupied byneurones. Finally, Santiago Ramón y Cajal’s brother, Pedro, considered astrocytesas insulators, which prevented undesirable spread of neuronal impulses.

The idea of active neuronal–glial interactions as a substrate for brain func-tion was first voiced in 1894 by Carl Ludwig Schleich (1859–1922) in his bookSchmerzlose Operationen (Figure 1.5). Incidentally, this happened in the same

1.1 FOUNDERS OF GLIAL RESEARCH 9

Figure 1.5 Carl Ludwig Schleich and the neuronal–glial hypothesis. Schleich was a pupilof Virchow and surgeon who introduced local anaesthesia into clinical practice. In 1894 hepublished a book Schmerzlose Operationen. Betäubung mit indifferenten Flüssigkeiten, VerlagJulius Springer, Berlin (the frontispiece of which is shown on the right upper panel). Apartfrom describing the principles of local anaesthesia, this also contained the first detailed essayon interactions in neuronal–glial networks as a substrate for brain function. Lower panels showoriginal drawings from this book depicting intimate contacts between glial cells and neurones

Figu

re1.

6T

hene

uron

aldo

ctri

nean

dits

foun

ders

.Wilh

elm

Got

tfri

edvo

nW

alde

yer

was

apr

ofes

sor

ofA

nato

my

inB

erlin

from

1883

;whe

rehe

mad

enu

mer

ous

impo

rtan

tco

ntri

butio

nsto

gene

ral

hist

olog

y(i

npa

rtic

ular

hein

trod

uced

the

term

‘chr

omos

ome’

),an

dhe

also

auth

ored

the

term

‘neu

rone

’(1

891)

.Sig

mun

dE

xner

held

aC

hair

inPh

ysio

logy

inV

ienn

aU

nive

rsity

from

1891

;and

in18

94he

publ

ishe

da

book

(Ent

wur

fzu

rph

ysio

logi

sche

nE

rklä

rung

der

psyc

hish

enE

rsch

einu

ngen

,18

94),

whi

chde

scri

bed

the

neur

onal

doct

rine

ofbr

ain

orga

niza

tion.

The

righ

tpa

nel

disp

lays

anor

igin

alsc

hem

efr

omhi

sbo

ok,w

hich

show

sne

uron

alne

twor

ksco

nnec

ted

bysy

naps

es

1.3 CHANGING CONCEPTS: GLIA EXPRESS MOLECULES OF EXCITATION 11

year as the ‘neuronal’ doctrine was promulgated by Sigmund Exner (1846–1926)in the book Entwurf zur physiologischen Erklärung der psychishen Erschein-ungen, (Berlin, 1894; Figure 1.6), and only three years after the term ‘neurone’was coined by Wilhelm Gottfried von Waldeyer (1891, Figure 1.6). Schleichbelieved that glia and neurones were equal players and both acted as active cellularelements of the brain. He thought that glial cells represented the general inhibitorymechanism of the brain. According to Schleich, neuronal excitation is trans-mitted from neurone to neurone through intercellular gaps, and these interneuronalgaps are filled with glial cells, which are the anatomical substrate for control-ling network excitation/inhibition. He postulated that the constantly changingvolume of glial cells represents the mechanism for control – swollen glial cellsinhibit neuronal communication, and impulse propagation is facilitated when gliashrink.

1.2 Beginning of the modern era

The modern era in glial physiology began with two seminal discoveries made inthe mid 1960s, when Steven Kuffler, John Nicolls and Richard Orkand (1966)demonstrated electrical coupling between glial cells, and Milton Brightman andTom Reese (1969) identified structures connecting glial networks, which we knownow as gap junctions. Nonetheless, for the following two decades, glial cells werestill regarded as passive elements of the CNS, bearing mostly supportive andnutritional roles. The advent of modern physiological techniques, most notablythose of the patch-clamp and fluorescent calcium dyes, has dramatically changedthis image of glia as ‘silent’ brain cells.

1.3 Changing concepts: Glia express molecules ofexcitation

The first breakthrough discovery was made in 1984 when groups led by HelmutKettenmann and Harold Kimelberg discovered glutamate and GABA receptors incultured astrocytes and oligodendrocytes. Several years later, in 1990 Ann Cornell-Bell and Steve Finkbeiner found that astroglial cells are capable of long-distancecommunication by means of propagating calcium waves. These calcium waves canbe initiated by stimulation of various neurotransmitter receptors in the astroglialplasma membrane.

Detailed analysis of the expression of these receptors performed during the lasttwo decades demonstrated that glial cells, and especially astrocytes, are capableof expressing practically every type of neurotransmitter receptor known so far.Moreover, glial cells were found to possess a multitude of ion channels, which

12 CH01 INTRODUCTION TO GLIA

can be activated by various extracellular and intracellular stimuli. Thus, glial cellsare endowed with proper tools to detect the activity of neighbouring neurones.

1.4 Glia and neurones in dialogue

Neurotransmitter receptors and ion channels expressed in glial cells turned out tobe truly operational. It has now been shown in numerous experiments on variousregions of the CNS and PNS that neuronal activity triggers membrane currentsand/or cytosolic calcium signals in glial cells closely associated with neuronalsynaptic contacts.

Finally, glial cells can also feed signals back to neurones, as they are able tosecrete neurotransmitters, such as glutamate and ATP. This discovery resultedfrom the efforts of several research groups, and has led to the concept of muchcloser interactions between two circuits, neuronal and glial, which communicatevia both chemical and electrical synapses.