The Perineuronal Net: A Weapon for a Challenge

Theories and data do not always fit and sometimes are sources of conflicts among scientists. This is the case of a morphological structure, the perineuronal net, which was denied on the basis of an ideological conflict between two giants of neurosciences: Camillo Golgi and Santiago Ramón y Cajal. The perineuronal net is a reticular structure enveloping many neurons. Originally reported by Golgi in 1893 and 1898 and confirmed by several authors before the turn of the century, the perineuronal net was used by Golgi to support the reticular theory of the organization of the nervous system. Ramón y Cajal, the paladin of the neuronal theory who had also observed this anatomical structure, denied its existence suggesting that it was a fixation artifact. After Cajal's statements, only a few Italian scientists continued to work in this field, and after the 1930s the perineuronal net was forgotten. Only the recent advances in histochemical and immunocytochemical technology confirmed the existence of this structure opening new fields in functional neuroanatomy and neuropathology.     

The impossible interview with the man of the hidden biological structures. Interview by Paolo Mazzarello

This paper presents an "impossible interview" to Professor Camillo Golgi, placed in time in December 1906. The Italian Professor Golgi from Pavia has been awarded the Nobel Prize for Physiology or Medicine ex aequo with the Spanish anatomist Santiago Ramón y Cajal. Both scientists have obtained the award for their work on the anatomy of the nervous system. However, they have opposite views on the mechanisms underlying nervous functions. Golgi believes that the axons stained by his "black reaction" form a continuous anatomical or functional network along which nervous impulses propagate. Ramón y Cajal is the paladin of the neuron theory, a hypothesis questioned by Golgi in his Nobel lecture of Tuesday, December 11. After the ceremony, an independent journalist has interviewed Professor Golgi in the Grand Hotel in Stockholm. Excerpts about his education, his main scientific discoveries, and his personal life are here given (reconstructing the "impossible interview" on the basis of Golgi's original writings).     

Colgi, Cajal and the Neuron Doctrine

Camillo Golgi and Santiago Ramon y Cajal shared the Nobel Prize in 1906 for their work on the histology of the nerve cell, but both held diametrically opposed views about the Neuron Doctrine which emphasizes the structural, functional and developmental singularity of the nerve cell. Golgi's reticularist views remained entrenched and his work on the nervous system did not venture greatly into new territories after its original flowering, which had greater impact than is now commonly credited. Cajal, by contrast, by the time he was awarded the Nobel Prize, was already breaking new ground with a new staining technique in the field of peripheral nerve regeneration, seeing the reconstruction of a severed nerve by sprouting from the proximal stump as another manifestation of the Neuron Doctrine. Paradoxically, identical studies were going on simultaneously in Golgi's laboratory in the hands of Aldo Perroncito, but the findings did not seem to influence Golgi's thinking on the Neuron Doctrine.     

Cajal's views on the Nobel Prize for physiology and medicine (October 1904)

In this report we present and discuss an unpublished letter written by Santiago Ramon y Cajal in October 1904 in relation to his possible nomination for the Nobel Prize for Physiology and Medicine. This letter shows that Cajal was aware of his previous nominations for the Prize. He was convinced that these nominations had not been successful because neither anatomy nor histology were among the sciences included in the Nobel Statutes' definition of Physiology or Medicine. He gives a list of the merits he thought might be used for a new nomination, which included only works concluded during the previous five years.     

Gustaf Retzius and Camillo Golgi

Gustaf Retzius (1842-1919), the Swedish anatomist and anthropologist, and Camillo Golgi were contemporaries. They met on several occasions and came in closer contact when Golgi, together with Ramon y Cajal, was awarded the Nobel Prize in Physiology or Medicine in Stockholm in 1906. Retzius came from an illustrious family. His father was professor of anatomy at Karolinska Institutet and Gustaf himself made a fast career. At 35, he was appointed to a professorship in histology, especially created for him at Karolinska Institutet, and later he became professor of anatomy in the same institution. Retzius was exceedingly productive, and published more than 300 scientific papers, most of which dealt with the nervous system and sensory organs. The majority of these were included in his magnificent volumes Biologische Untersuchungen, Neue Folge (Biological Investigation, New Series), which appeared from 1890 to 1921, and in Das Gehororgan der Wirbelthiere ("The Acoustic Organ of Vertebrates", 1881 and 1884), which may be his internationally better know contribution. Much of his work, especially on invertebrates, was based on Ehrlich's methylene blue method, but he also used the Golgi method early on. Particularly his studies of the innervation of the sensory organs became of great importance for the support of the neuron doctrine. His standing internationally was reflected in his membership in many of the most prominent academies abroad, as well as in invitations to him to give a "Croonian Lecture" in 1908 and "The Huxley Lecture" in 1909.     

The discovery of the Golgi apparatus

The existence of the cell organelle which is now known as Golgi apparatus or Golgi complex, or simply as 'the Golgi", was first reported by Camillo Golgi in 1898, when he described in nerve cells an 'internal reticular apparatus' impregnated by a variant of his chromoargentic staining. It soon became clear that the newly-identified cytoplasmic structure occurred in a variety of cell types. However, the reality of the organelle was questioned for decades, until it was finally ascertained with electron microscopy. The Golgi apparatus was destined to become a protagonist of the research in cytology and cell biology pursued in the second half of the twentieth century.     

Pavlov and Cajal: Two different pathways to a Nobel Prize

Ivan Pavlov (1849–1936) and Santiago Ramón y Cajal (1852–1934) were two contemporary scientists who not only had a great impact on Russian and Spanish science but also on the international stage. Both shared several common features in their life and work, yet they followed fundamentally different paths during their training as scientists. While Pavlov received his laboratory training under the guidance of Ilya Tsion (1843–1912), Cajal did not receive any formal training within a particular laboratory nor did he have a mentor in the traditional sense, rather he was mainly self-taught, although he was supported by key figures like Maestre de San Juan (1828–1890) and Luis Simarro (1851–1921). In this article, we compare the scientific training of these two Nobel Prize laureates and the influences they received during their scientific lives.     

The impossible interview with the man of the neuron doctrine. Interview by Edward G Jones

This paper follows the form of that by Mazzarello that precedes it (Mazzarello, 2006) and presents an imaginary interview with Santiago Ramón y Cajal in December 1906. A few days earlier Cajal had been awarded the Nobel Prize for Physiology or Medicine, an award that he shared equally with Professor Camillo Golgi. Golgi had been recognized for his work as a pioneer into investigations of the nervous system, primarily on account of his discovery of the "black reaction" of silver chromate impregnation of whole nerve cells and their processes. Cajal had been recognized for his implementation of that method and for laying with it the foundations of what was to become modern neuroanatomical science. Paradoxically, the two awardees had been led by their researches to diametrically opposed views of the organization of the nervous system. Golgi believed in a continuous network of axons that formed the basis of all the integrative properties of the nervous system, while Cajal had provided the information that led to the formulation of the neuron doctrine that saw the nervous system as being made up of chains of discontinuous cells joined by polarized functional contacts that we now call synapses. The paper takes the form of an interview with Professor Cajal in the Grand Hotel Stockholm. His responses to questions posed by the imaginary interviewer are all taken from Cajal's own writings.     

Luis Simarro Lacabra [1851–1921]: From Golgi to Cajal Through Simarro,via Ranvier

Knowledge of cerebral structure and function in its modern form can be traced to the neurone doctrine based largely on the work of Santiago Ramón y Cajal [1852–1934] and his lifelong exploitation of the Golgi method. Cajal openly acknowledged his debt to the neuropsychiatrist Luis Simarro Lacabra [1851–1921] who introduced him to the method in 1887, and recalled that the sight of the silver-impregnated nerve cells was the turning point which led him to abandon general anatomy and concentrate on neurohistology. Simarro, who dissipated his free time in trying to improve not only the scientific but also the political world around him, was able to produce exciting Golgi preparations of the cerebral cortex after he returned from voluntary exile in Paris from 1880 to 1885. Certainly it was there that he learned the methods of experimental histology from Louis-Antoine Ranvier [1835–1922] whose laboratory exercises, in the guise of lectures, he attended assiduously.     

Ramón y Cajal erroneously identified as Camillo Golgi on a souvenir postage stamp

Focusing on a philatelic oddity that erringly identifies a picture of Santiago Ramón y Cajal as that of Camillo Golgi, this brief article examines official and unofficial stamp issues honoring the two great neuroanatomists, one from Spain and the other from Italy, who were early Nobel Prize winners in Physiology or Medicine.     

Cajal's brief experimentation with hypnotic suggestion

Spanish histologist Santiago Ramón y Cajal, one of the most notable figures in Neuroscience, and winner, along with Camillo Golgi, of the 1906 Nobel Prize in Physiology or Medicine for his discoveries on the structure of the nervous system, did not escape experimenting with some of the psychiatric techniques available at the time, mainly hypnotic suggestion, albeit briefly. While a physician in his thirties, Cajal published a short article under the title, "Pains of labour considerably attenuated by hypnotic suggestion" in Gaceta Médica Catalana. That study may be Cajal's only documented case in the field of experimental psychology. We here provide an English translation of the original Spanish text, placing it historically within Cajal's involvement with some of the key scientific and philosophical issues at the time.     

Cajal's Brief Experimentation with Hypnotic Suggestion

Spanish histologist Santiago Ramón y Cajal, one of the most notable figures in Neuroscience, and winner, along with Camillo Golgi, of the 1906 Nobel Prize in Physiology or Medicine for his discoveries on the structure of the nervous system, did not escape experimenting with some of the psychiatric techniques available at the time, mainly hypnotic suggestion, albeit briefly. While a physician in his thirties, Cajal published a short article under the title, “Pains of labour considerably attenuated by hypnotic suggestion” in Gaceta Médica Catalana. That study may be Cajal's only documented case in the field of experimental psychology. We here provide an English translation of the original Spanish text, placing it historically within Cajal's involvement with some of the key scientific and philosophical issues at the time.     

Golgi,Cajal and the Neuron Doctrine

Camillo Golgi and Santiago Ramón y Cajal shared the Nobel Prize in 1906 for their work on the histology of the nerve cell, but both held diametrically opposed views about the Neuron Doctrine which emphasizes the structural, functional and developmental singularity of the nerve cell. Golgi's reticularist views remained entrenched and his work on the nervous system did not venture greatly into new territories after its original flowering, which had greater impact than is now commonly credited. Cajal, by contrast, by the time he was awarded the Nobel Prize, was already breaking new ground with a new staining technique in the field of peripheral nerve regeneration, seeing the reconstruction of a severed nerve by sprouting from the proximal stump as another manifestation of the Neuron Doctrine. Paradoxically, identical studies were going on simultaneously in Golgi's laboratory in the hands of Aldo Perroncito, but the findings did not seem to influence Golgi's thinking on the Neuron Doctrine.     

Some aspects of the history of the law of dynamic polarization of the neuron. From William James to Sherrington, from Cajal and van Gehuchten to Golgi

William James was the first to suggest that propagation of impulses in the nervous system proceeds in one direction, from sensory to motor neurons, but not viceversa. His law of forward direction preceded the formulation of the law of dynamic polarization of van Gehuchten and Cajal, which assumed that nerve impulses are conducted cellulipetally along dendrites and cellulifugally along axons, based on different anatomo-functional properties of these neuronal components. Golgi did not accept the law of dynamic polarization because he believed that dendrites are involved in the nutrition of the neuron rather than in impulse propagation, and that impulses can travel in any direction in the axonal components of the diffuse nerve network. Sherrington in turn experimentally demonstrated that intraneuronic conduction is reversible, whereas, in accord with James's law, propagation of impulses along neuronal chains is irreversible, due to the valve-like action of synapses. The story of the law of dynamic polarization shows that neither Golgi nor Cajal paid much heed to Sherrington's findings and to neurophysiological studies in general, probably because they felt that histology alone could provide the key for understanding the general functioning of the nervous system. It is argued here that this attitude was detrimental to the progress of the neurosciences, because a multidisciplinary approach based on different techniques is inevitably called for in order to develop a plausible theory of the nervous system.     

Some Aspects of the History of the Law of Dynamic Polarization of the Neuron. From William James to Sherrington,from Cajal and Van Gehuchten to Golgi

William James was the first to suggest that propagation of impulses in the nervous system proceeds in one direction, from sensory to motor neurons, but not viceversa. His law of forward direction preceded the formulation of the law of dynamic polarization of van Gehuchten and Cajal, which assumed that nerve impulses are conducted cellulipetally along dendrites and cellulifugally along axons, based on different anatomo-functional properties of these neuronal components. Golgi did not accept the law of dynamic polarization because he believed that dendrites are involved in the nutrition of the neuron rather than in impulse propagation, and that impulses can travel in any direction in the axonal components of the diffuse nerve network. Sherrington in turn experimentally demonstrated that intraneuronic conduction is reversible, whereas, in accord with James's law, propagation of impulses along neuronal chains is irreversible, due to the valve-like action of synapses. The story of the law of dynamic polarization shows that neither Golgi nor Cajal paid much heed to Sherrington's findings and to neurophysiological studies in general, probably because they felt that histology alone could provide the key for understanding the general functioning of the nervous system. It is argued here that this attitude was detrimental to the progress of the neurosciences, because a multidisciplinary approach based on different techniques is inevitably called for in order to develop a plausible theory of the nervous system.     

Evolution of the concept of "extracellular matrix' in the brain

The question as to whether an extracellular matrix exists between cells in the adult brain has been debated since the end of the last century. In the early years, zones containing neuropil and glial processes were mistakenly believed to represent this substance. But Golgi's discovery of the "perineuronal net" paved the way for future study of the true extracellular matrix. In the 1950s, application of histochemical techniques established the existence of interstitial material between nerve cells. Unfortunately the similarity between the pericellular distribution of this material and Golgi's "pericellular nets" was overlooked. The detection of an extracellular volume fraction in the central nervous system furnished further indirect proof for the existence of an extracellular matrix in the brain. However, the repeated failure of electron microscopy to reveal a substantial space between cell processes undermined the acceptance of the concept of "extracellular matrix" in the central nervous system. Nowadays this concept has, however, been firmly established.     

Evolution of the Concept of ‘Extracellular Matrix’ in the Brain

The question as to whether an extracellular matrix exists between cells in the adult brain has been debated since the end of the last century. In the early years, zones containing neuropil and glial processes were mistakenly believed to represent this substance. But Golgi's discovery of the “perineuronal net” paved the way for future study of the true extracellular matrix. In the 1950s, application of histochemical techniques established the existence of interstitial material between nerve cells. Unfortunately the similarity between the pericellular distribution of this material and Golgi's “pericellular nets” was overlooked. The detection of an extracellular volume fraction in the central nervous system furnished further indirect proof for the existence of an extracellular matrix in the brain. However, the repeated failure of electron microscopy to reveal a substantial space between cell processes undermined the acceptance of the concept of “extracellular matrix” in the central nervous system. Nowadays this concept has, however, been firmly established.     

Mesoglia & microglia--a historical review of the concept of mononuclear phagocytes within the central nervous system

More than a century and a half has elapsed since the first accounts of mesodermal phagocytic elements were proposed within the central nervous system. Over the intervening decades, body and substance were added to this concept through the advancement of histological techniques at the disposal of the researcher and the acute and keen-minded skills of the pathologist. Notable among these pioneering efforts were the contributions of W. Ford Robertson, Santiago Ramon y Cajal, Pio del Rio-Hortega and Wilder Penfield amongst an entire cavalcade of other noteworthy figures. The term 'mesoglia' and 'third element of the nervous system' was bestowed upon these cells towards the beginning of the twentieth century to account for their separate origins from neurons and macroglia. It was later amended by del Rio-Hortega in 1919, to 'microglia' in order to further discriminate between true mesodermal elements and oligodendrocytes, previously regarded as a component of 'mesoglia'. This particular contention sparked much controversy among del Rio-Hortega's peers and resulted in an escalation of fruitful research throughout Europe that eventually declined up to the outbreak of the Second World War. The post-war years were a period of the 'dark ages' that cast doubt on the very existence and nature of microglia, until the 'renaissance' of research was once again rejuvenated in the 1960s, by a new cohort of intrigued minds: Cammermeyer, Blinzinger, Kreutzberg and others who saw in the 'third element' the potential that is now commonly ascribed to microglia: the intrinsic immune effector cells of the CNS. It is now universally accepted that microglia are involved as the first line of rapid defence in any pathology of the nervous system, and as such, present a diagnostic tool for the neuropathologist. Although our knowledge of microglia stems from an extensive body of work conducted over the last two decades, much of the earlier work (pre-1960s) has remained somewhat obscure. This is partly accountable due to the limited availability of translated works, and additionally to the lack of a compendium of these articles. This paper will present a comprehensive overview of the pioneering research on mononuclear phagocytes within the central nervous system, which has direct bearing on our present-day understanding of the concept of microglia.     

MESOGLIA & MICROGLIA – A Historical Review of the Concept of Mononuclear Phagocytes Within the Central Nervous System

More than a century and a half has elapsed since the first accounts of mesodermal phagocytic elements were proposed within the central nervous system. Over the intervening decades, body and substance were added to this concept through the advancement of histological techniques at the disposal of the researcher and the acute and keen-minded skills of the pathologist. Notable among these pioneering efforts were the contributions of W. Ford Robertson, Santiago Ramon y Cajal, Pio del Rio-Hortega and Wilder Penfield amongst an entire cavalcade of other noteworthy figures. The term ‘mesoglia’ and ‘third element of the nervous system’ was bestowed upon these cells towards the beginning of the twentieth century to account for their separate origins from neurons and macroglia. It was later amended by del Rio-Hortega in 1919, to ‘microglia’ in order to further discriminate between true mesodermal elements and oligodendrocytes, previously regarded as a component of ‘mesoglia’. This particular contention sparked much controversy among del Rio-Hortega’s peers and resulted in an escalation of fruitful research throughout Europe that eventually declined up to the outbreak of the Second World War. The post-war years were a period of the ‘dark ages’ that cast doubt on the very existence and nature of microglia, until the ‘renaissance’ of research was once again rejuvenated in the 1960s, by a new cohort of intrigued minds: Cammermeyer, Blinzinger, Kreutzberg and others who saw in the ‘third element’ the potential that is now commonly ascribed to microglia: the intrinsic immune effector cells of the CNS. It is now universally accepted that microglia are involved as the first line of rapid defence in any pathology of the nervous system, and as such, present a diagnostic tool for the neuropathologist. Although our knowledge of microglia stems from an extensive body of work conducted over the last two decades, much of the earlier work (pre-1960s) has remained somewhat obscure. This is partly accountable due to the limited availability of translated works, and additionally to the lack of a compendium of these articles. This paper will present a comprehensive overview of the pioneering research on mononuclear phagocytes within the central nervous system, which has direct bearing on our present-day understanding of the concept of microglia.     

Book Review

Gustaf Retzius (1842–1919), the Swedish anatomist and anthropologist, and Camillo Golgi were contemporaries. They met on several occasions and came in closer contact when Golgi, together with Ramón y Cajal, was awarded the Nobel Prize in Physiology or Medicine in Stockholm in 1906. Retzius came from an illustrious family. His father was professor of anatomy at Karolinska Institutet and Gustaf himself made a fast career. At 35, he was appointed to a professorship in histology, especially created for him at Karolinska Institutet, and later he became professor of anatomy in the same institution. Retzius was exceedingly productive, and published more than 300 scientific papers, most of which dealt with the nervous system and sensory organs. The majority of these were included in his magnificent volumes Biologische Untersuchungen, Neue Folge (Biological Investigations, New Series), which appeared from 1890 to 1921, and in Das Gehörorgan der Wirbelthiere (“The Acoustic Organ of Vertebrates”, 1881 and 1884), which may be his internationally better known contribution. Much of his work, especially on invertebrates, was based on Ehrlich's methylene blue method, but he also used the Golgi method early on. Particularly his studies of the innervation of the sensory organs became of great importance for the support of the neuron doctrine. His standing internationally was reflected in his membership in many of the most prominent academies abroad, as well as in invitations to him to give a “Croonian Lecture” in 1908 and “The Huxley Lecture” in 1909.