The Concept of Neuroglia - the State of the Art Circa 1900

Glial cells were first defined by Rudolf Virchow in 1856. About 40 years later, glial research had developed into a field distinct from the mainstream study of neurons as the central elements governing brain function. By that time, substantial knowledge about the properties of glial cells had accumulated, exemplified by five important publications by four distinguished investigators: Gustav Retzius, Michael von Lenhossek, Carl Weigert, and Hans Held. These treatises broadly summarized what was known about glial cells, comparing findings from leeches to humans. Practically speaking, these articles represent the foundation of our current knowledge. All five contributions were published in German, which at the time was one of the dominant languages for scientific exchange. This article summarizes and comments on their findings and thus provides insight into what was known about glial cells at that time. More importantly, in the Supporting Information, we provide English translations and original scans of these five publications, making them accessible to an international readership.